Surface Mount Technology

Surface-mount technologySurface-mount technology

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N.K.SharmaN.K.Sharmawww.logextechno.comwww.logextechno.com

What is SMT Surface mount What is SMT Surface mount technologytechnology

Virtually all of today's mass produced electronics circuitry is manufactured using surface mount technology (SMT). However it was not until the 1980 that surface mount technology became widely used. However once SMT started to be used, the change from conventional leaded components to surface mount devices, SMDs took place quickly in view of the enormous gains that could be made using SMT

Why SMT?Why SMT?Mass produced electronic circuit boards need to be manufactured in a highly mechanised manner. The traditional leaded electronic components do not lend themselves to this approach. Although some mechanisation was possible, component leads need to be pre-formed, and when they were inserted into boards automatically problems were often encountered as wires did not fit properly slowing production rates considerably.It was reasoned that the wires that had traditionally been used for connections were not actually needed for printed circuit board construction. Rather than having leads placed through holes, the components could be soldered onto pads on the board instead. This also saved the need to drilling as many holes in boards.

As the components were mounted on the surface of the board, rather than having connections that went through holes in the board, the new technology was called surface surface mount technology or SMTmount technology or SMT. The idea for SMT was adopted very quickly because it enabled greater levels of mechanisation to be used, and it considerably saved on manufacturing costs.

To accommodate surface mount technology, SMT, a completely new set of components was needed. New SMT outlines were required, and often the same components, e.g. ICs were sold in both traditional leaded packages and SMT packages. Despite this, the gains of using SMT proved to be so large that it was adopted very quickly.

Introduction to SMDIntroduction to SMDSMD or Surface Mount Electronic Components for SMT are no different from through-hole components as far as the electrical function is concerned. Because they are smaller, however, the SMCs (surface mount components) provide better electrical performance. Not all components are available in surface mount for electronics at this time; hence the full benefits of surface mounting on PCB are not available, ad we are essentially limited to mix-and-match surface mount assemblies. The use of through-hole components such as pin grid array for high end processors and large connectors will keep the industry in mixed assembly mode for the foreseeable future.

Surface-mount technology (SMT) This is a method for constructing electronic circuits in which the components (SMC, or Surface Mounted Components) are mounted directly onto the surface of printed circuit boards (PCBs). Electronic devices so made are called surface-mount devices surface-mount devices or SMDs. In the industry it has largely replaced the through-hole technology construction method of fitting components with wire leads into holes in the circuit board.

Surface-mount technology (SMT) This is a method for constructing electronic circuits in which the components (SMC, or Surface Mounted Components) are mounted directly onto the surface of printed circuit boards (PCBs). Electronic devices so made are called surface-mount devices surface-mount devices or SMDs. In the industry it has largely replaced the through-hole technology construction method of fitting components with wire leads into holes in the circuit board.

What are SMT components?SMDs, i.e. SMT components, by their nature are very different to the traditional leaded components. They can be split into a number of categories:

Passive SMDsTransistorsIntegrated circuits

Passive SMDs The majority of passive SMDs are either resistors or capacitors for which the package sizes are reasonably standardised. Other components including coils, crystals and others tend to have more individual requirements and hence their own packages.

Resistor and capacitor packages have a variety of packages. These have designations that include: 1812, 1206, 0805, 0603, 0402, and 0201. The figures refer to the dimensions in hundreds of an inch. In other words the 1206 measures 12 hundreds by 6 hundreds of an inch. The larger sizes such as 1812 and 1206 were some of the first that were used.

The connections to the printed circuit board are made through metallised areas at either end of the package.

The majority of passive SMDs are either resistors or capacitors for which the package sizes are reasonably standardised. Other components including coils, crystals and others tend to have more individual requirements and hence their own packages.

Resistor and capacitor packages have a variety of packages. These have designations that include: 1812, 1206, 0805, 0603, 0402, and 0201. The figures refer to the dimensions in hundreds of an inch. In other words the 1206 measures 12 hundreds by 6 hundreds of an inch. The larger sizes such as 1812 and 1206 were some of the first that were used.

The connections to the printed circuit board are made through metallised areas at either end of the package.

Transistors and diodes:

These components are often contained in a small plastic package. The connections are made via leads which emanate from the package and are bent so that they touch the board. Three leads are always used for these packages. In this way it is easy to identify which way round the device must go

Transistors and diodes:

These components are often contained in a small plastic package. The connections are made via leads which emanate from the package and are bent so that they touch the board. Three leads are always used for these packages. In this way it is easy to identify which way round the device must go

PLCC - J leads - Line Diagram Actual Sample PLCC (Top View) (Bottom View)

Plastic leaded chip carriers (PLCCs) are almost a mandatory replacement for plastic DIPs, which are not practical above 40 pins because of excessive real estate requirements. The PLCCs come in a lead pitch of 0.050 inch with J leads that are bent under the packages. These packages have an equal number of J leads on all four sides. The J leads in PLCC provide the compliance needed to take up the solder joint stresses and thus prevent solder joint cracking. The outline configuration of PLCC is given below.

Plastic leaded chip carriers (PLCCs) are almost a mandatory replacement for plastic DIPs, which are not practical above 40 pins because of excessive real estate requirements. The PLCCs come in a lead pitch of 0.050 inch with J leads that are bent under the packages. These packages have an equal number of J leads on all four sides. The J leads in PLCC provide the compliance needed to take up the solder joint stresses and thus prevent solder joint cracking. The outline configuration of PLCC is given below.

Integrated circuits:

There is a variety of packages which are used for integrated circuits. The package used depends upon the level of interconnectivity required. Many chips like the simple logic chips may only require 14 or 16 pins, whereas other like the VLSI processors and associated chips can require up to 200 or more. In view of the wide variation of requirements there is a number of different packages available.

For the smaller chips, packages such as the SOIC (Small Outline Integrated Circuit) may be used. These are effectively the SMT version of the familiar DIL (Dual In Line) packages used for the familiar 74 series logic chips. Additionally there are smaller versions including TSOP (Thin Small Outline Package) and SSOP (Shrink Small Outline Package).

Integrated circuits:

There is a variety of packages which are used for integrated circuits. The package used depends upon the level of interconnectivity required. Many chips like the simple logic chips may only require 14 or 16 pins, whereas other like the VLSI processors and associated chips can require up to 200 or more. In view of the wide variation of requirements there is a number of different packages available.

For the smaller chips, packages such as the SOIC (Small Outline Integrated Circuit) may be used. These are effectively the SMT version of the familiar DIL (Dual In Line) packages used for the familiar 74 series logic chips. Additionally there are smaller versions including TSOP (Thin Small Outline Package) and SSOP (Shrink Small Outline Package).

The VLSI chips require a different approach. Typically a package known as a quad flat pack is used. This has a square footprint and has pins emanating on all four sides. Pins again are bent out of the package in what is termed a gull-wing formation so that they meet the board. The spacing of the pins is dependent upon the number of pins required. For some chips it may be as close as 20 thousandths of an inch. Great care is required when packaging these chips and handling them as the pins are very easily bent.

Other packages are also available. One known as a BGA (Ball Grid Array) is used in many applications.

The VLSI chips require a different approach. Typically a package known as a quad flat pack is used. This has a square footprint and has pins emanating on all four sides. Pins again are bent out of the package in what is termed a gull-wing formation so that they meet the board. The spacing of the pins is dependent upon the number of pins required. For some chips it may be as close as 20 thousandths of an inch. Great care is required when packaging these chips and handling them as the pins are very easily bent.

Other packages are also available. One known as a BGA (Ball Grid Array) is used in many applications.

Fine pitch components Fine pitch packages are also available with leads on all four sides of the package known as quad flat packs (QFPs). The QFP uses a gull-wing lead form, which complicates the automatic handling of the packages since they can not be supplied in plastic tubes like the dual-in-line (DIP) package. Because of this, each package is housed in its own protective compartment and shipped in trays known as “waffle packs” or matrix trays. The major disadvantage of gull wing packages is that they are susceptible to lead damage and distortion of lead planarity during shipping, handling, and placement. Loss of lead planarity in a fine pitch package may be overcome to certain extent if hot bar soldering is used instead of reflow soldering. Also the lower package thickness compounds the thermal problems and therefore the boards should be suitably designed to allow good thermal conductivity through heat spreaders if necessary. Placement and inspection accuracies required will be more demanding for fine pitch components due to closer lead spacing.

Fine pitch components Fine pitch packages are also available with leads on all four sides of the package known as quad flat packs (QFPs). The QFP uses a gull-wing lead form, which complicates the automatic handling of the packages since they can not be supplied in plastic tubes like the dual-in-line (DIP) package. Because of this, each package is housed in its own protective compartment and shipped in trays known as “waffle packs” or matrix trays. The major disadvantage of gull wing packages is that they are susceptible to lead damage and distortion of lead planarity during shipping, handling, and placement. Loss of lead planarity in a fine pitch package may be overcome to certain extent if hot bar soldering is used instead of reflow soldering. Also the lower package thickness compounds the thermal problems and therefore the boards should be suitably designed to allow good thermal conductivity through heat spreaders if necessary. Placement and inspection accuracies required will be more demanding for fine pitch components due to closer lead spacing.

BGA (Ball Grid Array)Instead of having the connections on the side of the package, they are underneath. The connection pads have balls of solder that melt during the soldering process, thereby making a good connection with the board and mechanically attaching it. As the whole of the underside of the package can be used, the pitch of the connections is wider and it is found to be much more reliable.

A smaller version of the BGA, known as the microBGA is also being used. As the name suggests it is a smaller version of the BGA.

BGA (Ball Grid Array)Instead of having the connections on the side of the package, they are underneath. The connection pads have balls of solder that melt during the soldering process, thereby making a good connection with the board and mechanically attaching it. As the whole of the underside of the package can be used, the pitch of the connections is wider and it is found to be much more reliable.

A smaller version of the BGA, known as the microBGA is also being used. As the name suggests it is a smaller version of the BGA.

While only a few types of conventional DIP packages meet all the packaging requirements, the world of surface mount packages is vastly more complex. The package types and package and lead configurations available are numerous. In addition, the requirements of surface mount components are far more demanding. SMCs must withstand the higher soldering temperatures and must be selected, places, and soldered more carefully to achieve acceptable manufacturing yield.

While only a few types of conventional DIP packages meet all the packaging requirements, the world of surface mount packages is vastly more complex. The package types and package and lead configurations available are numerous. In addition, the requirements of surface mount components are far more demanding. SMCs must withstand the higher soldering temperatures and must be selected, places, and soldered more carefully to achieve acceptable manufacturing yield.

There are scores of components available for some electrical requirements, causing a serious problem of component proliferation. There are good standards for some components, whereas for others standards are inadequate or nonexistent. Some electronic components are available at a discount, and others carry a premium.

While surface mount technology has matured, it is constantly evolving as well with the introduction of new packages. The electronics industry is making progress every day in resolving the economic, technical, and standardization issues with surface mount components.

There are scores of components available for some electrical requirements, causing a serious problem of component proliferation. There are good standards for some components, whereas for others standards are inadequate or nonexistent. Some electronic components are available at a discount, and others carry a premium.

While surface mount technology has matured, it is constantly evolving as well with the introduction of new packages. The electronics industry is making progress every day in resolving the economic, technical, and standardization issues with surface mount components.

Component shipping methods Prominent component shipping methods are tape and reel, tube, and tray form. Components are also available in bulk if required. But the bulk components are generally used only for very limited, simple components such as resistors and capacitors. ‘Tape and reel’ form is used for all surface mount components such as resistors, capacitors, diodes, SOTs, SOICs, PLCCs, SOJs etc. Because the chip resistors and capacitors are inexpensive, their inventory costs are negligible and therefore, they are available in bulk and ‘tape and reel’ form only. Tube package is used for components like PLCCs, SOICs (SOs in short) when the quantities involved are small. The selection process may also be affected by the pick and place equipment available with the user.

Component shipping methods Prominent component shipping methods are tape and reel, tube, and tray form. Components are also available in bulk if required. But the bulk components are generally used only for very limited, simple components such as resistors and capacitors. ‘Tape and reel’ form is used for all surface mount components such as resistors, capacitors, diodes, SOTs, SOICs, PLCCs, SOJs etc. Because the chip resistors and capacitors are inexpensive, their inventory costs are negligible and therefore, they are available in bulk and ‘tape and reel’ form only. Tube package is used for components like PLCCs, SOICs (SOs in short) when the quantities involved are small. The selection process may also be affected by the pick and place equipment available with the user.

SMD Terms

Many electronic components are not yet available for surface mounting. Due to this reason, SMT must accommodate some through-hole components. Therefore the term “Surface Mount Assembly is incomplete”.

Surface mount components, active and passive, when attached to the substrate, form three major types of SMT assembly – commonly referred to as Type I, Type II and Type III. The process sequences are different in each type, and all the three types need different equipments.

Type III SMT assembly contains only discrete surface mount components (resistors, capacitors, and transistors) glued to the bottom side.

The Type I assembly contains only surface mount components. The assembly can be either single-sided or double-sided.

The Type II assembly is a combination of Type III and Type I. It generally does not contain any active surface mount devices on the bottom side but may contain discrete surface mount devices on the bottom side.

Type I Surface Mounted components on one or both sides of a PCB, it can save up to 40% of PCB space

1. Silk-screen solder paste

2. Place SMDs

3. Reflow solder

Process

Type II Type II Surface mount components on one side of PCB and through-hole components on the other side of the PCB, it can save up to 25% of the PCB space. For low component density, a single side PCB can be used to save cost.

ProcessType II

1. Place through-hole components

2. Turn PCB

3. Apply adhesive

5. Harden adhesive

4. Place SMD

6. Turn PCB

7. Wave solder

Type III Type III Surface mount components on both sides of PCB and through-hole components on one side of the PCB, it can save up to 60% of the PCB space.

1. Silk-screen solder paste

2. Place SMDs

3. Reflow solder

4. Insert through Hole Components

5. Turn PCB

6. Apply adhesive

ProcessType II

7. Place SMDs

8. Harden adhesive

9. Turn PCB

10. Manual Placement for other through-hole components

11. Wave solder

Solder paste and adhesiveSolder paste and adhesive

Solder pastes, or creams, consists of minute pre-alloyed solder flakes suspended in solder flux and activators. The solder flakes consist of tin, lead and sometimes silver. The flux is usually rosin base mixed with various degrees of halides, acids and other oxide reducing agents. In addition, solvents are added to regulate the viscosity and consistency of application. A 63/37 (tin/lead) mixture with melting point of 183 degrees°C is common. A 62/36/2 (tin/lead/silver) mixture with a melting point of 179 degree °C is better. The lower the reflow temperature, the more desirable the paste.

Solder paste and adhesiveSolder paste and adhesive

Solder pastes, or creams, consists of minute pre-alloyed solder flakes suspended in solder flux and activators. The solder flakes consist of tin, lead and sometimes silver. The flux is usually rosin base mixed with various degrees of halides, acids and other oxide reducing agents. In addition, solvents are added to regulate the viscosity and consistency of application. A 63/37 (tin/lead) mixture with melting point of 183 degrees°C is common. A 62/36/2 (tin/lead/silver) mixture with a melting point of 179 degree °C is better. The lower the reflow temperature, the more desirable the paste.

Adhesive is required only for wave solder. Adhesive secures the components to the board in the wave soldering process. Adhesives add to the cost, complexity, and problems of an assembly and should be avoided whenever it is possible. Adhesive usually consists of epoxy which soften when exposed to heat. It is recommended to select adhesive with a long shelf life, adequate staking strength, and easy to be applied. It should be good resists to solvents and water, cure quickly, and not affecting circuit performance.Solder paste or adhesive can be applied onto a PCB by the following methods:

Dispensing Stencil transferPin matrix transfer

Adhesive is required only for wave solder. Adhesive secures the components to the board in the wave soldering process. Adhesives add to the cost, complexity, and problems of an assembly and should be avoided whenever it is possible. Adhesive usually consists of epoxy which soften when exposed to heat. It is recommended to select adhesive with a long shelf life, adequate staking strength, and easy to be applied. It should be good resists to solvents and water, cure quickly, and not affecting circuit performance.Solder paste or adhesive can be applied onto a PCB by the following methods:

Dispensing Stencil transferPin matrix transfer

Dispensing Dispensing

Solder paste or adhesive is dispensed by a syringe-type nozzle in small dot on a PCB pad. Usually nozzles are connected to a pump through tubing. The pump forces solder paste or adhesive out from the solder paste or adhesive reservoir

Stencil transfer Stencil transfer A screen printing process in which the solder paste or adhesive is printed onto a PCB through a screen or stencil. A stencil is an etched mask with openings corresponding to the area of the lands requiring solder paste. The stencil is attached to a frame by a flexible membrane. The frame attaches to a printer that has a squeegee to force solder paste or adhesive onto the lands through the etched openings of the stencil. The result should be sharp well-defined pillars of solder paste or adhesive on each land.

Pin matrix transfer A matrix of pins is dipped into a bath of solder paste or adhesive, and then lowered onto the PCB

Surface mount components are small and can be mounted on either side of the PCB and have gained widespread usage in electronics. The benefits of SMT or Surface Mount Technology are available in both design and manufacturing.

Benefits of Surface Mount Technology (SMT)Benefits of Surface Mount Technology (SMT)

Design Benefits of Surface MountTechnologyDesign Benefits of Surface MountTechnology

Among the most important design-related benefits are significant savings in weight and real estate and electrical noise reduction. A Surface Mount Components can weigh as little as one-tenth of their conventional thru-hole counterparts. This causes a significant reduction in weight of the Surface Mount Assembly (SMA). Because of their smaller size, surface mount components occupy only about one-half to one-third of the space on the printed circuit board. Since all electronic components are not available in surface mount, the actual area savings on a board will depend on the percentage of through-hole components replaced by surface mount components.

Depending on the component mix, the three types of surface mounting provide different levels of benefits.

SMT also provides improved shock and vibration resistance as a result of the lower mass of components. Shorter lead lengths of Surface Mount Components offer benefits of lower parasitic that reduces propagation delays and reduces package noise.

Depending on the component mix, the three types of surface mounting provide different levels of benefits.

SMT also provides improved shock and vibration resistance as a result of the lower mass of components. Shorter lead lengths of Surface Mount Components offer benefits of lower parasitic that reduces propagation delays and reduces package noise.

Manufacturing Benefits of Surface Mount TechnologyManufacturing Benefits of Surface Mount Technology

In addition to design benefits, SMT also provides many manufacturing benefits. These benefits include reduced board cost, reduced material handling cost, and a controlled manufacturing process. Routing of traces is reduced, size of the board is reduced, and number of drilled holes is also reduced. A smaller board with fewer drilled holes will naturally cost less. If the functions on the surface mount board are not increased, the increased inter package spacings made possible by smaller surface mount components and a reduction in the number of drilled holes may also reduce the number of layer counts in the printed circuit board. This will again lower the board cost.

Assembly techniques

Where components are to be placed, the printed circuit board has flat, usually tin-lead, silver, or gold plated copper pads without holes, called solder pads. Solder paste, a sticky mixture of flux and tiny solder particles, is first applied to all the solder pads with a stainless steel or nickel stencil using a screen printing process. After screen printing, the boards then proceed to the pick-and-place machines, where they are placed on a conveyor belt. The components to be placed on the boards are usually delivered to the production line in either paper/plastic tapes wound on reels or plastic tubes. Some large integrated circuits are delivered in static-free trays. Numerical control pick-and-place machines remove the parts from the tapes, tubes or trays and place them on the PCB.

The boards are then conveyed into the reflow soldering oven. They first enter a pre-heat zone, where the temperature of the board and all the components is gradually, uniformly raised. The boards then enter a zone where the temperature is high enough to melt the solder particles in the solder paste, bonding the component leads to the pads on the circuit board. The surface tension of the molten solder helps keep the components in place, and if the solder pad geometries are correctly designed, surface tension automatically aligns the components on their pads.

There are a number of techniques for reflowing solder.

One is to use infrared lamps; this is called infrared reflow. Another is to use a hot gas convection.

Another technology which is becoming popular again is special fluorocarbon liquids with high boiling points which use a method called vapor phase reflow. Due to environmental concerns, this method was falling out of favor until lead-free legislation was introduced which requires tighter controls on soldering. Currently, at the end of 2008, convection soldering is the most popular reflow technology using either standard air or nitrogen gas.

Each method has its advantages and disadvantages. With infrared reflow, the board designer must lay the board out so that short components don't fall into the shadows of tall components. Component location is less restricted if the designer knows that vapor phase reflow or convection soldering will be used in production. Following reflow soldering, certain irregular or heat-sensitive components may be installed and soldered by hand, or in large scale automation, by focused infrared beam (FIB) or localized convection equipment.

If the circuit board is double sided then this printing, placement, reflow process may be repeated using either solder paste or glue to hold the components in place. If glue is used then the parts must be soldered later using a wave soldering process.

If the circuit board is double sided then this printing, placement, reflow process may be repeated using either solder paste or glue to hold the components in place. If glue is used then the parts must be soldered later using a wave soldering process.

After soldering, the boards may be washed to remove flux residues and any stray solder balls that could short out closely spaced component leads. Rosin flux is removed with fluorocarbon solvents, high flash point hydrocarbon solvents, or low flash solvents e.g. limonene (derived from orange peels) which require extra rinsing or drying cycles. Water soluble fluxes are removed with deionized water and detergent, followed by an air blast to quickly remove residual water.

After soldering, the boards may be washed to remove flux residues and any stray solder balls that could short out closely spaced component leads. Rosin flux is removed with fluorocarbon solvents, high flash point hydrocarbon solvents, or low flash solvents e.g. limonene (derived from orange peels) which require extra rinsing or drying cycles. Water soluble fluxes are removed with deionized water and detergent, followed by an air blast to quickly remove residual water.

However, most electronic assemblies are made using a "No-Clean" process where the flux residues are designed to be left on the circuit board [Benign]. This saves the cost of cleaning, speeds up the whole process, and reduces waste.

Finally, the boards are visually inspected for missing or misaligned components and solder bridging. If needed, they are sent to a rework station where a human operator corrects any errors. They are then sent to the testing stations to verify that they operate correctly.

However, most electronic assemblies are made using a "No-Clean" process where the flux residues are designed to be left on the circuit board [Benign]. This saves the cost of cleaning, speeds up the whole process, and reduces waste.

Finally, the boards are visually inspected for missing or misaligned components and solder bridging. If needed, they are sent to a rework station where a human operator corrects any errors. They are then sent to the testing stations to verify that they operate correctly.

SMD ADVANTAGESSMD ADVANTAGES

1) PCBs area much smaller then by conventional through - hole components.2) Since the both layers of the PCB could be used for assembling the final PCBs area for the same circuits could be decreased by 50 %.3) Simple assembling - no bounding and cutting of the wires.4) Automatic assembling very easy. Low cost of the assembling.5) Small size of components makes very high packing density possible. For the same circuits a volume of a module assembled with SMD could be reduced to 30 % of the device assembled with the conventional technique. Therefore a size of the whole instrument decrease, too.6) Very high resistance to mechanical shock and vibration.7) Low store and transport cost. Low store area and volume.8) Lack of hole’s drilling and metallisation.9) Thin pads.10) For great series low manufacturing cost.

SMD ADVANTAGESSMD ADVANTAGES

1) PCBs area much smaller then by conventional through - hole components.2) Since the both layers of the PCB could be used for assembling the final PCBs area for the same circuits could be decreased by 50 %.3) Simple assembling - no bounding and cutting of the wires.4) Automatic assembling very easy. Low cost of the assembling.5) Small size of components makes very high packing density possible. For the same circuits a volume of a module assembled with SMD could be reduced to 30 % of the device assembled with the conventional technique. Therefore a size of the whole instrument decrease, too.6) Very high resistance to mechanical shock and vibration.7) Low store and transport cost. Low store area and volume.8) Lack of hole’s drilling and metallisation.9) Thin pads.10) For great series low manufacturing cost.

Main advantagesMain advantagesThe main advantages of SMT over the older through-hole technique are:

Smaller components. Smallest is currently 0.4 x 0.2 mm.Much higher number of components and many more connections per component.Fewer holes need to be drilled through abrasive boards.Simpler automated assembly.Small errors in component placement are corrected automatically (the surface tension of the molten solder pulls the component into alignment with the solder pads).Components can be placed on both sides of the circuit board.Lower resistance and inductance at the connection (leading to better performance for high frequency parts).Better mechanical performance under shake and vibration conditions.SMT parts generally cost less than through-hole parts.Fewer unwanted RF signal effects in SMT parts when compared to leaded parts, yielding better predictability of component characteristics.Faster assembly. Some placement machines are capable of placing more than 50,000 components per hour.

SMD LIMITATIONSSMD LIMITATIONSThe real goal of the SMD applications are a maximal packing density and finally volume reduction of the modules and instruments. Exactly this generates for which responsible are not the SMD themselves, but the miniaturisation in general.

1) Using ICs with a high amount of the pins (raster 0.5 to 1.27 mm, max. 148 pins) makes placing of the paths between IC pins impossible.2) Design of SMD layout is very complex. Distances between soldering pads are fix. Dimensions and distances between paths depends from soldering technology used by manufacturer.3) High packing density brings thermal problems. Power dissipation of the power components is transferred directly through a copper layer of the PCB. The high temperature of the layer influence a neighbour components.4) Lack of the general SMD normalisation.5) Not all SMD components are labelled with a clear text. Moreover, very often the components have no labels at all.6) Repair is more complex and difficult than by the conventional components.

SMD LIMITATIONSSMD LIMITATIONSThe real goal of the SMD applications are a maximal packing density and finally volume reduction of the modules and instruments. Exactly this generates for which responsible are not the SMD themselves, but the miniaturisation in general.

1) Using ICs with a high amount of the pins (raster 0.5 to 1.27 mm, max. 148 pins) makes placing of the paths between IC pins impossible.2) Design of SMD layout is very complex. Distances between soldering pads are fix. Dimensions and distances between paths depends from soldering technology used by manufacturer.3) High packing density brings thermal problems. Power dissipation of the power components is transferred directly through a copper layer of the PCB. The high temperature of the layer influence a neighbour components.4) Lack of the general SMD normalisation.5) Not all SMD components are labelled with a clear text. Moreover, very often the components have no labels at all.6) Repair is more complex and difficult than by the conventional components.

Main disadvantagesMain disadvantages

The manufacturing processes for SMT are much more sophisticated than through-hole boards, raising the initial cost and time of setting up for production.

Manual prototype assembly or component-level repair is more difficult (more so without a steady hand and the right tools) given the very small sizes and lead spacings of many SMDs.

SMDs can't be used with breadboards (a quick snap-and-play prototyping tool), requiring a custom PCB for every prototype. The PCB costs dozens to hundreds of dollars to fabricate and must be designed with specialized software. For prototyping around a specific SMD component, a less-expensive breakout board may be used.

Main disadvantagesMain disadvantages

The manufacturing processes for SMT are much more sophisticated than through-hole boards, raising the initial cost and time of setting up for production.

Manual prototype assembly or component-level repair is more difficult (more so without a steady hand and the right tools) given the very small sizes and lead spacings of many SMDs.

SMDs can't be used with breadboards (a quick snap-and-play prototyping tool), requiring a custom PCB for every prototype. The PCB costs dozens to hundreds of dollars to fabricate and must be designed with specialized software. For prototyping around a specific SMD component, a less-expensive breakout board may be used.

Reworking defective SMD componentsReworking defective SMD components

Defective surface mount components can be repaired in two ways:

by using soldering irons (depends on the kind and number of connections) or using

a professional rework system. In most cases a rework system is the first choice because the human influence on the rework result is very low. Generally, two essential soldering methods can be distinguished: infrared soldering and soldering with hot gas.

Reworking defective SMD componentsReworking defective SMD components

Defective surface mount components can be repaired in two ways:

by using soldering irons (depends on the kind and number of connections) or using

a professional rework system. In most cases a rework system is the first choice because the human influence on the rework result is very low. Generally, two essential soldering methods can be distinguished: infrared soldering and soldering with hot gas.

Benefits and disadvantages of different soldering Benefits and disadvantages of different soldering methodsmethods

Infrared soldering:Infrared soldering:During infrared soldering, the energy for heating up the solder joint will be transmitted by long or short wave electromagnetic radiation.

BenefitsBenefits Easy setup No compressed air required No component-specific nozzles (low costs) Fast reaction of infrared source (depends on used system)

Benefits and disadvantages of different soldering Benefits and disadvantages of different soldering methodsmethods

Infrared soldering:Infrared soldering:During infrared soldering, the energy for heating up the solder joint will be transmitted by long or short wave electromagnetic radiation.

BenefitsBenefits Easy setup No compressed air required No component-specific nozzles (low costs) Fast reaction of infrared source (depends on used system)

DisadvantagesDisadvantages Central areas will be heated more than peripheral areas Temperature can hardly be controlled, peaks cannot be

ruled out Covering of the neighboured components is necessary No component-specific nozzles but covering neighbored

components (to prevent damage) needs additional time for every board

Surface temperature depends on the component's reflection characteristics: dark surfaces will be heated more than lighter surfaces

The temperature additionally depends on the surface shape. Convective loss of energy will reduce the temperature of the component

No reflow atmosphere possible

DisadvantagesDisadvantages Central areas will be heated more than peripheral areas Temperature can hardly be controlled, peaks cannot be

ruled out Covering of the neighboured components is necessary No component-specific nozzles but covering neighbored

components (to prevent damage) needs additional time for every board

Surface temperature depends on the component's reflection characteristics: dark surfaces will be heated more than lighter surfaces

The temperature additionally depends on the surface shape. Convective loss of energy will reduce the temperature of the component

No reflow atmosphere possible

Conventional hot gas solderingConventional hot gas solderingDuring hot gas soldering, the energy for heating up the solder joint will be transmitted by a gaseous medium. This can be air or inert gas (nitrogen).

BenefitsBenefits Simulating reflow oven atmosphere Switching between hot gas and nitrogen (economic use) Standard and component-specific nozzles allow high reliability

and reduced process time Allow reproducible soldering profiles Efficient heating, large heat amounts can be transmitted Even heating of the affected board area Temperature of the component will never exceed the adjusted

gas temperature Rapid cool down after reflow, resulting in small-grained solder

joints (depends on used system)

Conventional hot gas solderingConventional hot gas solderingDuring hot gas soldering, the energy for heating up the solder joint will be transmitted by a gaseous medium. This can be air or inert gas (nitrogen).

BenefitsBenefits Simulating reflow oven atmosphere Switching between hot gas and nitrogen (economic use) Standard and component-specific nozzles allow high reliability

and reduced process time Allow reproducible soldering profiles Efficient heating, large heat amounts can be transmitted Even heating of the affected board area Temperature of the component will never exceed the adjusted

gas temperature Rapid cool down after reflow, resulting in small-grained solder

joints (depends on used system)

DisadvantagesDisadvantages

Thermal capacity of the heat generator results in slow reaction whereby thermal profiles can be distorted (depends on used system)

A rework process usually undoes some type of error, either human or machine-generated, and includes the following steps:

Melt solder and component removal Residual solder removal Printing of solder paste on PCB, direct component

printing or dispensing Placement and reflow of new component.

DisadvantagesDisadvantages

Thermal capacity of the heat generator results in slow reaction whereby thermal profiles can be distorted (depends on used system)

A rework process usually undoes some type of error, either human or machine-generated, and includes the following steps:

Melt solder and component removal Residual solder removal Printing of solder paste on PCB, direct component

printing or dispensing Placement and reflow of new component.

The process sequence for Type I SMT is shown below. For a single sided type I, solder paste is printed onto the board and components are placed The assembly is reflow soldered and cleaned (if needed). For double-sided Type I, the board is turned over, and the process sequence just described is repeated.

Type II assemblies go through the process sequence of Type I SMT followed by the sequence for Type III. In general practice, only passive chip components and low pin count gull wing components are exposed to solder wave immersion.

The most commonly used techniques for mounting SMDs (Surface Mounted Devices) to PC boards are Infrared (IR) and Vapor Phase (VP) reflow. IR and VP reflow are preferred over wave soldering. Wave soldering typically involves increased heating rate, higher temperatures and increased flux exposure. The dynamics of the reflow process are influenced by the type of equipment used. The variables involved must be understood to properly control the board level interconnection of SMDs.

The primary phases of the reflow process are: fluxactivation, melting the solder particles in the solder paste, wetting the surfaces to be joined, and solidifying the solder into a strong metallurgical bond.

Optimum fusing of the component leads with the solder paste on the board is achieved when the leads attain the melting temperature of the plated solder alloy. To avoid thermal shock of the SMDs the maximum heating and cooling rates (i.e., ramp rate) should be controlled.

IR ReflowThe IR reflow technique involves thermal energy supplied via lamps radiating at a given range of wavelength. This heating approach in its basic form is essentially a line-of-sight surface heating technique. Therefore, the amount of thermal energyabsorbed varies with board size, component size, component orientation, and materials used. The surface temperature of the board is not uniform throughout and board edges tend to run 10°C to 20°C higher than the center. If not properlyplanned, component overheating is possible.

PCB assembly process overviewPCB assembly process overview

PCB assembly process involved in building a surface mount technology (SMT) board using pick and place techniques.Within a printed circuit board electronics assembly / production or manufacturing process there are a number of individual stages. However it is necessary for them all to work together to form an integrated overall process. Each stage of assembly and production must be compatible with the next, and there must be feedback from the output to the input to ensure that the highest quality is maintained. In this way any problems are detected quickly and the process can be adjusted accordingly.

PCB assembly process involved in building a surface mount technology (SMT) board using pick and place techniques.Within a printed circuit board electronics assembly / production or manufacturing process there are a number of individual stages. However it is necessary for them all to work together to form an integrated overall process. Each stage of assembly and production must be compatible with the next, and there must be feedback from the output to the input to ensure that the highest quality is maintained. In this way any problems are detected quickly and the process can be adjusted accordingly.

The various stages in the PCB assembly process including adding solder paste to the board, pick and place of the components, soldering, inspection and test. All these processes are required, and need to be monitored to ensure that product of the highest quality is produced. The PCB assembly process described below assumes that surface mount components are being used as virtually all PCB assembly these days uses surface mount technology.

The various stages in the PCB assembly process including adding solder paste to the board, pick and place of the components, soldering, inspection and test. All these processes are required, and need to be monitored to ensure that product of the highest quality is produced. The PCB assembly process described below assumes that surface mount components are being used as virtually all PCB assembly these days uses surface mount technology.

Solder paste:Solder paste: Prior to the addition of the components to a board, solder paste needs to be added to those areas of the board where solder is required. Typically these areas are the component pads. This is achieved using a solder screen.

The solder paste is a paste of small grains of solder mixed with flux. This can be deposited into place in a process that is very similar to some printing processes.

Solder paste:Solder paste: Prior to the addition of the components to a board, solder paste needs to be added to those areas of the board where solder is required. Typically these areas are the component pads. This is achieved using a solder screen.

The solder paste is a paste of small grains of solder mixed with flux. This can be deposited into place in a process that is very similar to some printing processes.

Using the solder screen, placed directly onto the board and registered in the correct position , a runner is moved across the screen squeezing a small mount of solder paste through the holes in the screen and onto the board. As the solder screen has been generated from the printed circuit board files, it has holes on the positions of the solder pads, and in this way solder is deposited only on the solder pads.

Using the solder screen, placed directly onto the board and registered in the correct position , a runner is moved across the screen squeezing a small mount of solder paste through the holes in the screen and onto the board. As the solder screen has been generated from the printed circuit board files, it has holes on the positions of the solder pads, and in this way solder is deposited only on the solder pads.

Pick and place: Pick and place:

During this part of the assembly process, the board with the added solder paste is then passed into the pick and place process. Here a machine loaded with reels of components picks the components from the reels or other dispensers and places them onto the correct position on the board.

The components placed onto the board are held in place by the tension of the solder paste. This is sufficient to keep them in place provided that the board is not jolted.

Pick and place: Pick and place:

During this part of the assembly process, the board with the added solder paste is then passed into the pick and place process. Here a machine loaded with reels of components picks the components from the reels or other dispensers and places them onto the correct position on the board.

The components placed onto the board are held in place by the tension of the solder paste. This is sufficient to keep them in place provided that the board is not jolted.

In some assembly processes, the pick and place machines add small dots of glue to secure the components to the board. However this is normally done only if the board is to be wave soldered. The disadvantage of the process is that any repair is made far more difficult by the presence of the glue, although some glues are designed to degrade during the soldering process.

The position and component information required to programme the pick and place machine is derived from the printed circuit board design information. This enables the pick and place programming to be considerably simplified.

In some assembly processes, the pick and place machines add small dots of glue to secure the components to the board. However this is normally done only if the board is to be wave soldered. The disadvantage of the process is that any repair is made far more difficult by the presence of the glue, although some glues are designed to degrade during the soldering process.

The position and component information required to programme the pick and place machine is derived from the printed circuit board design information. This enables the pick and place programming to be considerably simplified.

What is a pick and place machine?

Pick and place machines are used in the manufacture if electronic circuit boards for the placement of Surface Mount Technology (SMT) components onto boards. They pick the components, typically from specially prepared reels or other packaged forms of components, and place them onto printed circuit boards. The pick and place machines are pre-programmed with the information about component positions so that they know where to place the components. This programme is normally developed directly from the printed circuit board design information.

What is a pick and place machine?

Pick and place machines are used in the manufacture if electronic circuit boards for the placement of Surface Mount Technology (SMT) components onto boards. They pick the components, typically from specially prepared reels or other packaged forms of components, and place them onto printed circuit boards. The pick and place machines are pre-programmed with the information about component positions so that they know where to place the components. This programme is normally developed directly from the printed circuit board design information.

Soldering:Soldering:Once the components have been added to the board, the next stage of the assembly, production process is to pass it through the soldering machine. Although some boards may be passed through a wave soldering machine, this process is not widely used for surface mount components these days. If wave soldering is used, then solder paste is not added to the board as the solder is provided by the wave soldering machine. Rather than using wave soldering, reflow soldering techniques are used more widely.

Soldering:Soldering:Once the components have been added to the board, the next stage of the assembly, production process is to pass it through the soldering machine. Although some boards may be passed through a wave soldering machine, this process is not widely used for surface mount components these days. If wave soldering is used, then solder paste is not added to the board as the solder is provided by the wave soldering machine. Rather than using wave soldering, reflow soldering techniques are used more widely.

InspectionInspection

After the boards have been passed through the soldering process they are often inspected. Manual inspection is not an option for surface mount boards employing a hundred or more components. Instead automatic optical inspection is a far more viable solution. Machines are available that are able to inspect boards and detect poor joints, misplaced components, and under some instances the wrong component.

InspectionInspection

After the boards have been passed through the soldering process they are often inspected. Manual inspection is not an option for surface mount boards employing a hundred or more components. Instead automatic optical inspection is a far more viable solution. Machines are available that are able to inspect boards and detect poor joints, misplaced components, and under some instances the wrong component.

Test:Test: It is necessary to test electronic products before they leave the factory. There are several ways in which they may be tested.

Feedback: To ensure that the manufacturing process is running satisfactorily, it is necessary to monitor the outputs. This is achieved by investigating any failures that are detected. The ideal place is at the optical inspection stage as this generally occurs immediately after the soldering stage. This means that process defects can be detected quickly and rectified before too many boards are built with the same problem.

Test:Test: It is necessary to test electronic products before they leave the factory. There are several ways in which they may be tested.

Feedback: To ensure that the manufacturing process is running satisfactorily, it is necessary to monitor the outputs. This is achieved by investigating any failures that are detected. The ideal place is at the optical inspection stage as this generally occurs immediately after the soldering stage. This means that process defects can be detected quickly and rectified before too many boards are built with the same problem.

Reflow soldering equipment / machineReflow soldering equipment / machine

A reflow soldering equipment / machine is the second major equipment after pick-and-place machine in any SMT line.

There are many reflow soldering methods and each method has advantages and disadvantages. Equipment cost, maintenance cost and yield are some of the leading decision factors.

The most widely used reflow processes in electronics are: vapor phase and infrared.

Reflow soldering equipment / machineReflow soldering equipment / machine

A reflow soldering equipment / machine is the second major equipment after pick-and-place machine in any SMT line.

There are many reflow soldering methods and each method has advantages and disadvantages. Equipment cost, maintenance cost and yield are some of the leading decision factors.

The most widely used reflow processes in electronics are: vapor phase and infrared.

Reflow soldering is the most widely used method of soldering used within PCB assembly whether it is used for mass production or for prototype PCB assembly with surface mount components. The technology uses two main stages. First a solder paste is applied to the board, and then secondly the board is heated to enable the solder to melt. This stage in itself has several steps that are needed to ensure that the board is heated and cooled correctly.

Using reflow soldering technology it is possible to reliably solder surface mount components, and particularly those with very fine pitch leads. This makes it ideal for use with the components used in mass produced electronics products.

Reflow soldering is the most widely used method of soldering used within PCB assembly whether it is used for mass production or for prototype PCB assembly with surface mount components. The technology uses two main stages. First a solder paste is applied to the board, and then secondly the board is heated to enable the solder to melt. This stage in itself has several steps that are needed to ensure that the board is heated and cooled correctly.

Using reflow soldering technology it is possible to reliably solder surface mount components, and particularly those with very fine pitch leads. This makes it ideal for use with the components used in mass produced electronics products.

PreparationThe first stage in reflow soldering for PCB assembly is to apply solder paste and components to the board. These stages are covered in more detail in a separate page on this section of the website.Solder paste: In essence solder paste is applied to the board. The paste is only applied to the areas that require soldering. While boards have solder resist layers added to them, it is necessary to only add solder paste to those areas where the solder is actually required. This is achieved by having a solder mask and solder paste "machine". This only allows the solder paste to be added to those areas of the board where it is needed. Once added the solder paste has been added to the board, it can move on to the next stage.

PreparationThe first stage in reflow soldering for PCB assembly is to apply solder paste and components to the board. These stages are covered in more detail in a separate page on this section of the website.Solder paste: In essence solder paste is applied to the board. The paste is only applied to the areas that require soldering. While boards have solder resist layers added to them, it is necessary to only add solder paste to those areas where the solder is actually required. This is achieved by having a solder mask and solder paste "machine". This only allows the solder paste to be added to those areas of the board where it is needed. Once added the solder paste has been added to the board, it can move on to the next stage.

Solder paste basics

The solder particles are a mixture of solder. Traditionally this used to be tin and lead, but with the legislation being introduced around the world, there is a move to lead free solders. These may be made from a variety of mixtures. One is 99.7% tin and 0.3% copper, whereas there are other mixtures that include other metals including tin.

Solder paste basics

The solder particles are a mixture of solder. Traditionally this used to be tin and lead, but with the legislation being introduced around the world, there is a move to lead free solders. These may be made from a variety of mixtures. One is 99.7% tin and 0.3% copper, whereas there are other mixtures that include other metals including tin.

Solder paste storageIn order to ensure that the solder paste is suitable for proving the highest performance for PCB assembly it is necessary to ensure that it maintains the required properties. To achieve this it is imperative that the solder paste is stored correctly. It should always be stored in an airtight container to prevent oxidation.

Additionally, the solder must be stored at low temperatures. Not only does this reduce the rate of any oxidation there may be, but it also reduces the rate at which the flux degrades. In view of the way in which solder paste can degrade, it also has a defined shelf life and it should not be used after its end date. If old solder is used there is a distinct risk of a much higher defect rate, and the cost of any rework.

Solder paste storageIn order to ensure that the solder paste is suitable for proving the highest performance for PCB assembly it is necessary to ensure that it maintains the required properties. To achieve this it is imperative that the solder paste is stored correctly. It should always be stored in an airtight container to prevent oxidation.

Additionally, the solder must be stored at low temperatures. Not only does this reduce the rate of any oxidation there may be, but it also reduces the rate at which the flux degrades. In view of the way in which solder paste can degrade, it also has a defined shelf life and it should not be used after its end date. If old solder is used there is a distinct risk of a much higher defect rate, and the cost of any rework.

How to use solder paste

When solder paste is used in mass PCB assembly as well as prototype PCB assembly there are a number of stages that are undertaken. First solder paste is applied to the printed circuit boards. The solder paste is only applied to the areas where solder is required. This is achieved using a solder paste stencil that only allows the solder paste through in certain areas.

How to use solder paste

When solder paste is used in mass PCB assembly as well as prototype PCB assembly there are a number of stages that are undertaken. First solder paste is applied to the printed circuit boards. The solder paste is only applied to the areas where solder is required. This is achieved using a solder paste stencil that only allows the solder paste through in certain areas.

Once the solder paste has been applied to the printed circuit board, it is then passed into the pick and place machine where the components are added. The solder paste has sufficient tension that it holds the components in place. However care should be taken not to knock the board at this stage otherwise the components may move or fall off. Additionally the board should be soldered within a few hours of being placed, otherwise the solder paste may deteriorate.

Once the solder paste has been applied to the printed circuit board, it is then passed into the pick and place machine where the components are added. The solder paste has sufficient tension that it holds the components in place. However care should be taken not to knock the board at this stage otherwise the components may move or fall off. Additionally the board should be soldered within a few hours of being placed, otherwise the solder paste may deteriorate.

Prototype PCB Soldering Demo 1.flv

Pre-heatThe boards need to be brought steadily up to the required temperature. If the rate is too high, then the board or the components may be damaged by the thermal stress. Additionally if the board is brought up to temperature too quickly then areas may not reach the required temperature because of the thermal mass. If the board is brought u to temperature too slowly then the board may not reach the required temperature.The temperature rise rate that is often used for infra-red reflow soldering is between 2 and 3 C per second, although rise rates down to 1C per second may be used on some occasions.

Pre-heatThe boards need to be brought steadily up to the required temperature. If the rate is too high, then the board or the components may be damaged by the thermal stress. Additionally if the board is brought up to temperature too quickly then areas may not reach the required temperature because of the thermal mass. If the board is brought u to temperature too slowly then the board may not reach the required temperature.The temperature rise rate that is often used for infra-red reflow soldering is between 2 and 3 C per second, although rise rates down to 1C per second may be used on some occasions.

Thermal soak

Having brought the board up to temperature it next enters what is often termed a thermal soak area. Here the card is maintained at temperature for two reasons. One is to ensure that any areas that are not adequately heated because of shadowing effects come up to the required temperature. The other is to remove the solder paste solvents or volatiles and to activate the flux.

Thermal soak

Having brought the board up to temperature it next enters what is often termed a thermal soak area. Here the card is maintained at temperature for two reasons. One is to ensure that any areas that are not adequately heated because of shadowing effects come up to the required temperature. The other is to remove the solder paste solvents or volatiles and to activate the flux.

Reflow

The reflow area is the area of the soldering process where the highest temperature is reached. It is here that the solder is caused to melt and create the required solder joints. The actual reflow process involves the flux reducing the surface tension at the junction of the metals to accomplish metallurgical bonding, allowing the individual solder powder spheres to combine and melt.Very careful control of the temperature and time is required to ensure that the process provides optimal quality.

Reflow

The reflow area is the area of the soldering process where the highest temperature is reached. It is here that the solder is caused to melt and create the required solder joints. The actual reflow process involves the flux reducing the surface tension at the junction of the metals to accomplish metallurgical bonding, allowing the individual solder powder spheres to combine and melt.Very careful control of the temperature and time is required to ensure that the process provides optimal quality.

Infrared reflow soldering Infrared reflow soldering

This method consists of heating the SMD with the heat generated from an infrared panel heater and soldering it onto the PWB.The radiation efficiency of the infrared ray varies depending on the color and shape of the SMD to be soldered. Temperature variations may occur as the result of uneven coloring on the SMD's surface.

The features of the infrared reflow is described as follows.

Infrared reflow soldering Infrared reflow soldering

This method consists of heating the SMD with the heat generated from an infrared panel heater and soldering it onto the PWB.The radiation efficiency of the infrared ray varies depending on the color and shape of the SMD to be soldered. Temperature variations may occur as the result of uneven coloring on the SMD's surface.

The features of the infrared reflow is described as follows.

1. AdvantagesLow running cost and excellent maintainability Short soldering time

2. Disadvantages

Temperature rise on the leads greatly depends on the package size Great thermal stress The temperature of shadowed regions may not rise because infrared radiation does not reach them.

1. AdvantagesLow running cost and excellent maintainability Short soldering time

2. Disadvantages

Temperature rise on the leads greatly depends on the package size Great thermal stress The temperature of shadowed regions may not rise because infrared radiation does not reach them.

The vertical hot air stream evenly heats the complete PCB. The high air volume guarantees equal heating rates in all the components and the substrate. This technology eliminates the risk of hot spots or heat shadows.

The vertical hot air stream evenly heats the complete PCB. The high air volume guarantees equal heating rates in all the components and the substrate. This technology eliminates the risk of hot spots or heat shadows.

Vapor Phase ReflowVapor Phase Reflow

This process heats an inactive solvent to produce vapor through which the PCB passes through for soldering. The benefits are that the temperature can be maintained uniformly. The risk of oxidation and contamination is minimized due to the use of the inactive solvent.

The resin surface temperature and oven profile must be controlled. The temperature rise as recommended shall be kept to within 1-5°C/minute, and the rise should be as shallow as possible.

Vapor Phase ReflowVapor Phase Reflow

This process heats an inactive solvent to produce vapor through which the PCB passes through for soldering. The benefits are that the temperature can be maintained uniformly. The risk of oxidation and contamination is minimized due to the use of the inactive solvent.

The resin surface temperature and oven profile must be controlled. The temperature rise as recommended shall be kept to within 1-5°C/minute, and the rise should be as shallow as possible.

Vapor Phase Reflow

The vapor phase reflow technique uses vapor from a boiling inert fluorocarbon liquid. The heat of condensation provides a thermal constraint dependent on the liquid selected. A typical material in the industry has a boiling point of 215°C.

PC board temperature exposure should be very uniform.With essentially no temperature gradient across the surface of the board, component location design rules for even heating is not significant compared to IR reflow.

Vapor phase reflow soldering (VPS)Vapor phase reflow soldering (VPS)

This method consists of heating to a boiling temperature a special inert liquid, and then subjecting the SMD to the saturated steam to achieve soldering by latent heat.

The features of VPS is described as follows.

1. Advantage · Low thermal stress · Even heating of components regardless of their shape · Accurate temperature control through use of latent heat · Extremely high heat transfer efficiency makes it possible to lower the heating temperature and shorten the soldering time. · Little oxidation and dirt on the soldered joint because soldering is performed in inert atmosphere.

2. Disadvantages · High running cost

Vapor phase reflow soldering (VPS)Vapor phase reflow soldering (VPS)

This method consists of heating to a boiling temperature a special inert liquid, and then subjecting the SMD to the saturated steam to achieve soldering by latent heat.

The features of VPS is described as follows.

1. Advantage · Low thermal stress · Even heating of components regardless of their shape · Accurate temperature control through use of latent heat · Extremely high heat transfer efficiency makes it possible to lower the heating temperature and shorten the soldering time. · Little oxidation and dirt on the soldered joint because soldering is performed in inert atmosphere.

2. Disadvantages · High running cost

Vapor Phase SolderingVapor Phase SolderingVapor phase, or condensation soldering, is taken place by transferring the latent heating of a boiling vapor to the PCBA surface upon which it condenses. If a liquid is chosen so that its boiling point is the same as the melting point of a solder paste, then no further heat will be required once the liquid is boiled. This can prevent the PCBA from overheated. Also, the process permits a high through-put of several PCBA without changing the process profile, solder paste and with minimized variable control.

Vapor Phase SolderingVapor Phase SolderingVapor phase, or condensation soldering, is taken place by transferring the latent heating of a boiling vapor to the PCBA surface upon which it condenses. If a liquid is chosen so that its boiling point is the same as the melting point of a solder paste, then no further heat will be required once the liquid is boiled. This can prevent the PCBA from overheated. Also, the process permits a high through-put of several PCBA without changing the process profile, solder paste and with minimized variable control.

Temperature Profiles For all soldering processes, temperature control of a PCBA is important, particularly for the reflow soldering process. Figure below shows a typical soldering machine temperature profile. It consists of four temperature regions:

1)Preheat time is the time for a PCBA to be preheated to a temperature just below the soldering temperature. 2)Dwell time is the time for a PCBA subjected to the maximum preheat temperature. This allows all parts of the PCBA to be soaked for enough time at the same dwell temperature. Hence, when final temperature application occurs, all parts of the PCBA can rise to a soldering temperature almost at the same time. 3)Liquid time is the time during which solder is molten. 4)Cooling time is the time during the PCBA temperature is cooled.

Temperature Profiles For all soldering processes, temperature control of a PCBA is important, particularly for the reflow soldering process. Figure below shows a typical soldering machine temperature profile. It consists of four temperature regions:

1)Preheat time is the time for a PCBA to be preheated to a temperature just below the soldering temperature. 2)Dwell time is the time for a PCBA subjected to the maximum preheat temperature. This allows all parts of the PCBA to be soaked for enough time at the same dwell temperature. Hence, when final temperature application occurs, all parts of the PCBA can rise to a soldering temperature almost at the same time. 3)Liquid time is the time during which solder is molten. 4)Cooling time is the time during the PCBA temperature is cooled.

Vapor phase process is very versatile and it can be applied to assemblies of any shape. Many models of phase process are available in both batch and in-line types.

Vapor phase process is very versatile and it can be applied to assemblies of any shape. Many models of phase process are available in both batch and in-line types.

Reflow Temperature Profile

Industrial Soldering Processes

• Wave Soldering– The PCB is passed over a standing wave of molten

solder– Used for through hole and some SMT parts– May use an inert atmosphere to reduce dross

Industrial Soldering Processes

• Reflow Soldering– The PCB is stenciled with solder paste where SMT

component electrodes will be located– Components are positioned on the board by a

pick-and-place machine– The board is heated in a reflow oven through a

closely controlled temperature profile to melt the solder paste

– If components are on both sides, can glue the bottom components or make two passes by tight temperature control on bottom side

Soldering PrecautionsSoldering Precautions

The soldering process can create a thermal stress on anysemiconductor component. The melting temperature of solderis higher than the maximum rated temperature of the device.The amount of time the device is heated to a high temperatureshould be minimized to assure device reliability. Therefore, thefollowing precautions should always be observed in order tominimize the thermal stress to which the devices are subjected.

1) Always preheat the device.2) The delta temperature between the preheat and

soldering should always be less than 100°C. Failure to preheat the device can result in excessive thermal stress which can damage the device.

Soldering PrecautionsSoldering Precautions

The soldering process can create a thermal stress on anysemiconductor component. The melting temperature of solderis higher than the maximum rated temperature of the device.The amount of time the device is heated to a high temperatureshould be minimized to assure device reliability. Therefore, thefollowing precautions should always be observed in order tominimize the thermal stress to which the devices are subjected.

1) Always preheat the device.2) The delta temperature between the preheat and

soldering should always be less than 100°C. Failure to preheat the device can result in excessive thermal stress which can damage the device.

3) The maximum temperature gradient should be less than 5°C per second when changing from preheating to soldering.

4) The peak temperature in the soldering process should be at least 30°C higher than the melting point of the solder chosen.

5) After soldering is complete, forced cooling will increase the temperature gradient and may result in latent failure due to mechanical stress.

6) During cooling, mechanical stress or shock should be avoided.

3) The maximum temperature gradient should be less than 5°C per second when changing from preheating to soldering.

4) The peak temperature in the soldering process should be at least 30°C higher than the melting point of the solder chosen.

5) After soldering is complete, forced cooling will increase the temperature gradient and may result in latent failure due to mechanical stress.

6) During cooling, mechanical stress or shock should be avoided.

Test criterion Defects1 Presence of solder bridges Short circuits2 Missing solder joints Open circuits

3Deviating diameter of solder joints

Faulty paste print.Insufficient wetting.Tittled component.

4None circular shape of solder joints

Wetting problems.Titled component.

5 Fuzzy edges of solder joints Insufficient reflow

6Grey level deviation(varying solder joint thickness)

Tittled component.Component warpage or popcorning.Open solder joint (Depending on distribution of deviations)

7 Shifted solder joints Misaligned component.

8Bright spots (bubbles) in solder joint.

Voiding - low reliability of solder joint.

Surface mount components are placed on a printed circuit board after deposition of adhesive or solder paste. Generally, the adhesive deposition is done at the placement equipment itself. Thus the main function of most placement equipment is adhesive deposition, and of course component placement. Placement equipment is commonly referred to as “pick-and-place” equipment. The components can be placed on the board by

1. Manual placement 2. Automatic placement

Surface mount components are placed on a printed circuit board after deposition of adhesive or solder paste. Generally, the adhesive deposition is done at the placement equipment itself. Thus the main function of most placement equipment is adhesive deposition, and of course component placement. Placement equipment is commonly referred to as “pick-and-place” equipment. The components can be placed on the board by

1. Manual placement 2. Automatic placement

1. Manual placement of parts Manual placement of surface mount components is not reliable and can be used only for prototyping applications. 1. Most passive components do not have any part markings,

and therefore, the possibility of part mix-up is very high. 2. Placement of components in wrong orientations due to

operator related errors 3. Placement accuracy will again depends on the operator and it is very difficult to obtain good placement accuracy with manual placement. In component placement, there are two main functions, pickup and placement. In manual placement, the components parts are picked up either by tweezers or by a vacuum pipette. For passive components tweezers are adequate, but for multi-leaded active devices, a vacuum pipette is very helpful in dealing with component rotation and is recommended when working with manual placement of components.

1. Manual placement of parts Manual placement of surface mount components is not reliable and can be used only for prototyping applications. 1. Most passive components do not have any part markings,

and therefore, the possibility of part mix-up is very high. 2. Placement of components in wrong orientations due to

operator related errors 3. Placement accuracy will again depends on the operator and it is very difficult to obtain good placement accuracy with manual placement. In component placement, there are two main functions, pickup and placement. In manual placement, the components parts are picked up either by tweezers or by a vacuum pipette. For passive components tweezers are adequate, but for multi-leaded active devices, a vacuum pipette is very helpful in dealing with component rotation and is recommended when working with manual placement of components.

2. Automatic placement of parts Automatic placement machines are required for high volume production as the manual method turns out to be very slow. It is also possible to achieve consistent quality levels with automatic placement equipment. Vision capability is required for placement of fine-pitch components and it may add to the overall cost of the equipment. The following are the important parameters while selecting an automatic placement machine. 1. Placement speed, i.e. the number of components that the machine

should be able to place per minute 2. Requirement of placement of fine pitch or very fine pitch components 3. The variety of components that the machine should handle 4. Types of feeders that the machine should support 5. Other features like CAD data downloading for reduction in programming time 6. Ability to dispense adhesive 7. Need for component testing

2. Automatic placement of parts Automatic placement machines are required for high volume production as the manual method turns out to be very slow. It is also possible to achieve consistent quality levels with automatic placement equipment. Vision capability is required for placement of fine-pitch components and it may add to the overall cost of the equipment. The following are the important parameters while selecting an automatic placement machine. 1. Placement speed, i.e. the number of components that the machine

should be able to place per minute 2. Requirement of placement of fine pitch or very fine pitch components 3. The variety of components that the machine should handle 4. Types of feeders that the machine should support 5. Other features like CAD data downloading for reduction in programming time 6. Ability to dispense adhesive 7. Need for component testing

Pick and place machinePick and place machineWhat is a pick and place machine?

Pick and place machines are used in the manufacture if electronic circuit boards for the placement of Surface Mount Technology (SMT) components onto boards. They pick the components, typically from specially prepared reels or other packaged forms of components, and place them onto printed circuit boards. The pick and place machines are pre-programmed with the information about component positions so that they know where to place the components. This programme is normally developed directly from the printed circuit board design information.

What is a pick and place machine?

Pick and place machines are used in the manufacture if electronic circuit boards for the placement of Surface Mount Technology (SMT) components onto boards. They pick the components, typically from specially prepared reels or other packaged forms of components, and place them onto printed circuit boards. The pick and place machines are pre-programmed with the information about component positions so that they know where to place the components. This programme is normally developed directly from the printed circuit board design information.

SMT Pick-and-Place MachineSMT Pick-and-Place Machine SMT Pick-and-Place Machine for Picking and Placement of Surface Mount Components onto the PCB.

SMT (Surface Mount Technology) needs several types of equipment. Pick-and-Place machine is the heart of surface mount. A pick-and-place machine picks and places electronic components onto the PCB prior to soldering. These machines generally contribute to about 50% of the total cost of a complete SMT production line.

SMT Pick-and-Place MachineSMT Pick-and-Place Machine SMT Pick-and-Place Machine for Picking and Placement of Surface Mount Components onto the PCB.

SMT (Surface Mount Technology) needs several types of equipment. Pick-and-Place machine is the heart of surface mount. A pick-and-place machine picks and places electronic components onto the PCB prior to soldering. These machines generally contribute to about 50% of the total cost of a complete SMT production line.

Some surface mount placement machine (pick-and-place machine) is very versatile and are capable of placing many different components used in electronics, while others are dedicated to a few component types.

Pick-and-Place machines use vacuum pickup tools to hold the components. Few others also use vision-assisted alignment. In general, pick-and-place machines offer better speed, accuracy, and flexibility than through-hole insertion machines.

Some surface mount placement machine (pick-and-place machine) is very versatile and are capable of placing many different components used in electronics, while others are dedicated to a few component types.

Pick-and-Place machines use vacuum pickup tools to hold the components. Few others also use vision-assisted alignment. In general, pick-and-place machines offer better speed, accuracy, and flexibility than through-hole insertion machines.

Pick-and-place machines for fine pitch (0.5mm) and ultra fine pitch (0.4mm – 0.3mm) placement machines require greater dexterity and precision placement capability. Some of these machines cut and form the leads of the IC package at the time of placement in order to avoid component lead damage due to mishandling. This feature increases process accuracy and precision capabilities.

The number of machines required to adequately assemble all surface mount components varies depending on the type of components being assembled and the through-put desired by the manufacturer. Dedicated SMT placement machine can achieve the greatest through-put.

Pick-and-place machines for fine pitch (0.5mm) and ultra fine pitch (0.4mm – 0.3mm) placement machines require greater dexterity and precision placement capability. Some of these machines cut and form the leads of the IC package at the time of placement in order to avoid component lead damage due to mishandling. This feature increases process accuracy and precision capabilities.

The number of machines required to adequately assemble all surface mount components varies depending on the type of components being assembled and the through-put desired by the manufacturer. Dedicated SMT placement machine can achieve the greatest through-put.

Surface Mount Pick and Place MachineSurface Mount Pick and Place Machine

Before the commencing a production, the machine head is programmed to move a position over a PCB, then to deposit a component onto a board. Also, the alignment must be made between the machine head and the PCB. This is often done using fiducials marks printed on the PCB. The machine is equipped with a vacuum pick-up tool. To pick up a component, the machine head moves to the pre-programmed components which are packaged in tape reels, tubes or trays, and suck a component.

Surface Mount Pick and Place MachineSurface Mount Pick and Place Machine

Before the commencing a production, the machine head is programmed to move a position over a PCB, then to deposit a component onto a board. Also, the alignment must be made between the machine head and the PCB. This is often done using fiducials marks printed on the PCB. The machine is equipped with a vacuum pick-up tool. To pick up a component, the machine head moves to the pre-programmed components which are packaged in tape reels, tubes or trays, and suck a component.

Then the head moves the component to its pre-programmed position on the PCB and lowers it onto the board, where the vacuum is released and the machine head moves to pick the next component. This machine is a sequential type in which the components are picked and placed one at a time. There is a multi-heads machine in which several components are picked and placed at one time.

Then the head moves the component to its pre-programmed position on the PCB and lowers it onto the board, where the vacuum is released and the machine head moves to pick the next component. This machine is a sequential type in which the components are picked and placed one at a time. There is a multi-heads machine in which several components are picked and placed at one time.

1. Requirement of cleaning Cleaning of printed boards is required to remove flux residues and other contaminants which are left behind after soldering . Cleaning or washing of the boards prevent potential electrical failures due to electromigration. Cleaning operations does the removal of the following contaminants: I.) Ionic contaminants ii) Non-ionic contaminants iii) Particulate contaminants Fluxes that are water soluble generally produce ionic(also called polar) contaminants that require aqueous cleaning. Non-ionic(also called non-polar) contaminants produced by rosin fluxes require non-ionic solvents such as trichloroethane.

1. Requirement of cleaning Cleaning of printed boards is required to remove flux residues and other contaminants which are left behind after soldering . Cleaning or washing of the boards prevent potential electrical failures due to electromigration. Cleaning operations does the removal of the following contaminants: I.) Ionic contaminants ii) Non-ionic contaminants iii) Particulate contaminants Fluxes that are water soluble generally produce ionic(also called polar) contaminants that require aqueous cleaning. Non-ionic(also called non-polar) contaminants produced by rosin fluxes require non-ionic solvents such as trichloroethane.

Water soluble fluxes need to be cleaned thoroughly due to corrosive elements that are present in the flux residues, aqueous cleaning is ideal. Because rosin is not soluble in water, when aqueous cleaning is used for rosin fluxes, alkaline chemicals called saponifiers are added to the water. The efficiency of cleaning treatment is significantly increased if the cleaning is assisted by ultrasonic vibration. However, the ultrasonic vibrations are not confined to the cleaning fluid and the surfaces to be cleaned, but are also transferred into the electronic components, which may be damaged . Bonding wires, inside active components between the die and the bonding pads may break if the bonding wires are free and have not been tightly encapsulated in plastic or any other packaging material. High-power, high frequency vibration induce fracture in external leads also. The following set of parameters is generally accepted: § maximum frequency of 40 kHz § maximum time of ultrasonic load 1 to 5 minutes § maximum power of 10 W/ litre § boards in racks, so that they cannot touch each other.

1. Tools for inspecting assemblies: § Magnifying glasses 5 to 10X with halo lamps § Printed workmanship standards to eliminate confusion § 10 to 30X microscope may be required for inspecting difficult parts such as fine pitch components and solder paste quality. § For high volume manufacturing automated visual inspection such as 3 dimensional laser scan or X-ray scan may be justified.

1. Tools for inspecting assemblies: § Magnifying glasses 5 to 10X with halo lamps § Printed workmanship standards to eliminate confusion § 10 to 30X microscope may be required for inspecting difficult parts such as fine pitch components and solder paste quality. § For high volume manufacturing automated visual inspection such as 3 dimensional laser scan or X-ray scan may be justified.

2. Techniques for Inspecting Assemblies: § Boards need to be tilted to examine under components when inspecting for trapped contaminants or J type leads. Also, if rework and touch-up are to be done along with inspection, a board positioning table may be useful. § Dental picks or pointed wood sticks are useful to verify the soldering joints of leaded components when in doubt. Use pick or stick to gently push the top edge of lead to check for joint attachment. This prevents damage to the board and lead. An unsoldered lead or one with cold joint will move when pushed and this needs to be touched-up. § Write down or log all defect types using a common vocabulary. This information should be useful to find common defects and trends so their source can be found and eliminated.

2. Techniques for Inspecting Assemblies: § Boards need to be tilted to examine under components when inspecting for trapped contaminants or J type leads. Also, if rework and touch-up are to be done along with inspection, a board positioning table may be useful. § Dental picks or pointed wood sticks are useful to verify the soldering joints of leaded components when in doubt. Use pick or stick to gently push the top edge of lead to check for joint attachment. This prevents damage to the board and lead. An unsoldered lead or one with cold joint will move when pushed and this needs to be touched-up. § Write down or log all defect types using a common vocabulary. This information should be useful to find common defects and trends so their source can be found and eliminated.

3. Touchup, Rework and repair: o Rework or repair is required to be done on components/ boards that are not meeting the workmanship or performance standards. o Rework tools that are commonly used are soldering irons, desoldering stations using suction, hot air jets, solder wick etc. o Rework on SOs and other high lead count packages needs prior training and some practice on dummies will be helpful. o Since leads and terminations of SMT packages are small, the thermal requirement is less compared to through-hole component pads. The components are to be removed only after ensuring that all leads are desoldered or solder has reflowed. If not done properly, the chances of damage to the pad area is significant. o Before removing the component gently push it to check for complete solder melt. o When the connecting solder melts on each lead or termination of the component, it’s readily removed and replaced with a new one. o SMT rework requires operator training as it requires new techniques and new tools.

3. Touchup, Rework and repair: o Rework or repair is required to be done on components/ boards that are not meeting the workmanship or performance standards. o Rework tools that are commonly used are soldering irons, desoldering stations using suction, hot air jets, solder wick etc. o Rework on SOs and other high lead count packages needs prior training and some practice on dummies will be helpful. o Since leads and terminations of SMT packages are small, the thermal requirement is less compared to through-hole component pads. The components are to be removed only after ensuring that all leads are desoldered or solder has reflowed. If not done properly, the chances of damage to the pad area is significant. o Before removing the component gently push it to check for complete solder melt. o When the connecting solder melts on each lead or termination of the component, it’s readily removed and replaced with a new one. o SMT rework requires operator training as it requires new techniques and new tools.

4. Tools Required: o Alcohol in dispensing bottle o Cotton swabs o ‘R’ , ‘RMA’ flux in a small dispensing bottle o Dental picks or pointed wooden sticks o Long, thin tweezers o Solder wicks in sizes as required o Fine tip temperature controlled soldering iron and spares as

required o Desoldering station with appropriate bits o Manual hot air jet and nozzles and hot air rework station o Solder paste and dispenser o 24 gauge ( 0.015”) flux cored solder wire o Static free work station with wrist strap and proper

grounding o Workmanship guidelines

4. Tools Required: o Alcohol in dispensing bottle o Cotton swabs o ‘R’ , ‘RMA’ flux in a small dispensing bottle o Dental picks or pointed wooden sticks o Long, thin tweezers o Solder wicks in sizes as required o Fine tip temperature controlled soldering iron and spares as

required o Desoldering station with appropriate bits o Manual hot air jet and nozzles and hot air rework station o Solder paste and dispenser o 24 gauge ( 0.015”) flux cored solder wire o Static free work station with wrist strap and proper

grounding o Workmanship guidelines