PRINTED CIRCUIT BOARDS Basically consists of a planar (flat) substrate which has electronic components mounted on it that are interconnected by conductive.

PRINTED CIRCUIT BOARDS

Basically consists of a planar (flat) substrate which has electronic components mounted on it that are

interconnected by conductive tracks.

The PCB

PRINTED CIRCUIT BOARDS

Advantages Large number of components can be fitted and connected together on a small, flat substrate – aided by advancements in component reduction, thin line widths of interconnects.

Multi layer possibilities allows more complex circuitry without taking up more room.

Mass production – high volume throughput, reduces cost tocustomer.

The PCB

PCBs for a computer

Populated PCB showing conductive traces, through-hole paths into the opposite side and element montage

One side of a motherboard PCB, showing conductive traces, vias and solder points for through-hole components on the opposite side

The PCB

PCB applications for high frequency telecom and sensor

modules

PCB became commonplace in consumer products aided strongly by US releasing invention for common use in 1948In the 1950’s every electronic component generally had wire leads and PCBs had holes drilled for each wire of each componentThe components were then soldered to the PCB, this is through-hole construction. This was done automatically by passing the board over a ripple, or wave, of molten solder in a wave-soldering machineThrough-hole mounting is still useful in attaching physically-large and heavy components to the board, but wires and holes are wasteful, it costs money to drill holes, and wires are merely cut off

The rise of PCB in the 1950’s

In the 1960’s Surface Mount Technology SMT developed:components made with small contact pads are physically held by solder to the conductorsSolder paste is generally applied by screen printing process and components mounted on, solder paste also acts as temporary adhesivePCB are passed through an IR oven to cure the solder

SMT

• smaller components • no need to drill holes through abrasive boards • simpler automated assembly • small errors in component placement are corrected

automatically, as the molten solder pulls the component into place by surface tension

• components can be fitted to both sides of the circuit board

Advantages of Surface Mount

Subtractive technique of copper foil bonding to whole substrate and removing unwanted metal became generally adopted. Copper foil is typically tenths of microns in thickness.Substrates developed are compatible with this technology and these are generally matted glass fibres with epoxy based – e.g. FR4, FR5. Copper foil is generally pressed while heated onto the substrate with ‘adhesive’.Substrates developed which are made of epoxy and glass fibres to which this process can be done – Bonding aided by Tg of the epoxy. Substrates such as FR4 became prominent.

Patterned Conductive Interconnects

Copper Foil Bonding

Unwanted copper is etched away with ferric chloride, and photolithography exploited and developed to produce patterns.

Light sensitive photoresists are used as patternable barriers for etch and design technology developed to produce circuitry as exposable medium for transfer into photoresist.

Copper Patterning

Copper clad substrate is covered with photoresist and a photomask, clear acetate with dark emulsion as opaque regions, is placed over photoresist during exposure.After exposure, photoresist is developed, copper is left exposed in regions to be removed.Exposed copper is etched away with ferric chloride andphotoresist is then removed.

Photolithography with a Photomask

A basic photomask for a simple PCB can be made today with an inkjet transparency and a printer. Circuit design is made on pc.

Photoresist covered, copper clad FR4 can be bought from RS.

A light box can be easily made for exposing, but in industry much more sophisticated equipment is used, with much better resolution and quality.

Photomask acetates are designed on CAD packages and printed.

Photolithography with a Photomask

Techniques also developed since the 1960’s to screen print resist onto copper layer – no exposure undertaken.

Resolution not as good as photolithography in terms of resolvedfeature sizes but a good method for mass production of low cost boards.

A milling tool can be used to ‘cut’ away the unwanted copper to leave a desired pattern.

This requires sophisticated plotting equipment in either X,Y or X, Y, Z axis control.

This is another Subtractive Method

Alternatives for Patterned Resist

Simple schematic for producing monolayer PCB

Trends in PCB manufacturing

Trends in PCB manufacturing

Trends in PCB manufacturing

Trends in PCB manufacturing

Fabrication of the Laminate

After cooling the sheets are trimmed, inspected for adequate quality, cut into smaller sheets about 1 meter square and vacuum sandwiched between clear plastic sheeting.

This provides protection for the copper surface, particularly from oxidation by air

Boards are rejected at inspection stage if they have warp and twist, imperfection in the copper surface, or poor bonding between the copper and the laminate.

PCB Substrates

Fabrication of the Laminate

The final properties of the laminate depend upon the materials used and process control during manufacture. In addition to electrical properties such as dielectric strength and constant, dissipation factor, insulation resistance, resistivity (both surface and volume), there are physical characteristics. These include flexural strength, punching and drilling qualities, flame resistance, and water absorption

PCB Substrates

One important non-electrical characteristic is the maximum temperature at which boards can be operated. This is also relevant for the curing of printed epoxies and thick film pastes on the boards.

Resin Reinforcement Max T 'C

Phenolic Paper 100

Glass 250

Epoxy Glass 120

Polyester Glass 120

Silicone Glass 250

PTFE Glass 200

PCB Substrates

T dependence of TCE in z direction

of laminates made with different resins

Ceramic substrates for hybrid circuits

Laminates with layers for CTE

Metal core substrates

Metal core substrates

TCE

Hysteresis of Cu clad Invar

Thermal conductivity

TCE

Dielectric constant

Copper is the dominant metal for interconnection use.

The copper foil is normally specified by weight i.e. 1/2 oz/ft2

(152.5 g/m2), 1 oz/ft2 (305 g/m2); they correspond to foil thicknesses of 17.5 and 35 microns respectively.

It is usually produced by electrolytic deposition on a flat mandril, it is about 99.8% pure and has a tolerance on thickness of about ± 10%.

Copper Foil

Copper is an excellent electrical conductor

Electrical resistivity is approx 1.6 ohm cm

A not so expensive metal

Soft and easily workable

Easily processed and patternable by photolithography, using relatively benign chemicals

Why Copper?

Desired thickness of copper foil will be dependant on required circuitry, i.e. resistance of tracks etc.

Problems exist on high frequency communication boards with respect to the skin effect in a ‘thick’ conductor.

Growing interest in an ability to deposit thinner copper and other metals onto PCB substrates to try and alleviate this problem.

Copper Foil

Copper can be easily electrodeposited onto a PCB Laminate – problems however!

ADHESION!

The adhesion is far superior from bonding copper foil sheets by the combination of heat, pressure and adhesive.

Common adhesion tests include measurement of Peel Strength – ‘how easy is it to peel the copper foil off’ and Pull off Strength – how easy is it to pull up the copper foil from the substrate

Copper Foil

Some Typical Adhesion Values for Bonded Foil

EP–GC–Cu, 152g/m2

Pull off Strength = 60NPeel Strength = 1.1kN/m

PF-CP-Cu, 152g/m2

Pull off Strength = 45NPeel Strength = 1.05kN/m

Copper Foil

Typical Resistance of 305gm-2 foil on laminate 3.5 m

Often tend to speak about a sheet resistance measured in ohms per square

R = l / A

Rs = / t

SEE 4 POINT PROBE ANALYSIS FOR FURTHER DETAILS ON MEASUREMENT

Copper Foil

Reproducibility is extremely important in terms of producing ‘identical’ laminates, i.e. adhesion parameters must be consistent, along with resistivity of copper, change in resistivity of copper as board is bowed or flexed, thickness of copper, dielectric constant of laminate. Boards are therefore produced within specific tolerances.

The produced laminate boards must be robust enough to withstand abuse during processing stages and in later life on the finished product. Abuse can include:Changes in temperature – sometimes rapid and hot and coldFlexing – the boards may have to be flexed, particularly during processing stages.Resistance to chemicals, e.g. solvents, fluxes, acids etc.Physical abuse – to a certain extent being dropped!

Reproducibility and robustness of Boards

Copper layer can be patterned into circuitry

Generally done by photolithography as discussed previously

On a double sided laminate, both layers are patterned and aligned on top of each other for subsequent interconnection.

Alignment or Fiducial Marks are used for this

Copper Foil

Single Sided PCB Board

Top layer is patterned in alignment with bottom layer.Holes are drilled through the board and then copper iselectrodeposited to interconnect top and bottom layers.

Double Sided PCB Board

On multilayer boards, the individual, copper patterned laminates are stacked on top and bottom of each other. Insulating layers between the conductors can also be added.

Often begin with double sided board in middle and work outwards.

Again alignment marks are used to position each layer on top of the next.

Laminates then pressed, heated and bonded in a similar process to before.

Holes then drilled in appropriate places for subsequent electrodeposition of copper to interconnect layers.

Multilayer Boards

Multilayer Boards

Because of T used in processing the fabrication of multilayer boards it should be obvious now that the thermal expansion of each of the laminates and copper must be closely matched.

Any mismatch in thermal expansion will cause a mismatch in alignment of each layer.

Laminate materials have therefore been designed with this factor in mind.

Importance of Thermal Expansion

More and more interest being shown to flexible PCB’s

Applications include flexible interconnections. Advanced applications include small screens, e.g. mobile phones – and large ‘roll up’ screens

Common substrate choices include polyester, polyimide etc.

Typical problems – flexing substrate can cause metal tracks and components etc. to ‘peel’ off

Resistance of metal conductors can also be different when substrate is flexed

FLEXIBLE PCB

Flexible circuits can reduce weight of PCB

Flexing can allow easier installation and servicing

Production can be inexpensive

FLEXIBLE PCB

Flexible Displays can take up less space (rolled up), they are more portable

Active elements can be deposited by vapour deposition (amorphous silicon) – also printed organic materials

FLEXIBLE DISPLAYS

Substrates such as polyester, polyimide etc. tend to have smooth surfaces.

Adhesion of metal for conductive tracks, solder etc. is generally very poor on a smooth substrate.

Substrate is roughened typically by chemical means – e.g. immersion in hot permanganate.

Metal can be deposited by evaporation (for thin layers) or foil can be bonded to substrate or deposited by ACD electroless and ECD electrodeposition.

ACD electroless metal deposition on plastics developed since 1960s

FLEXIBLE PCB

Old solutions never die

• Wire bonding – extended to 3D packaging

• Lamination: – extended to high end quick turn around designs with

> 20 layers and < 10 m resolution.

©Amkor Technology, Inc., 2002