Using Surface-Mount Components in an Embedded Systems Lab · Using Surface-Mount Components in an Embedded Systems Lab Erik Brunvand School of Computing, University of Utah [email protected]

Using Surface-Mount Components inan Embedded Systems Lab

Erik BrunvandSchool of Computing, University of Utah

[email protected]

Abstract—Embedded systems classes and labs can often benefitfrom having students design their own systems including printedcircuit boards (PCBs). These boards can be the basis of eithercomplete small microcontroller systems or add-on boards toexisting platforms. However, modern circuit components are veryoften available only in tiny surface-mount technology (SMT)packages intended for automatic assembly and reflow soldering.This paper describes how we enhanced an embedded systems labin a cost-effective way to enable students to develop, assemble,and solder custom PCBs that contain SMT components. Thisallows an enriched hands-on experience, and an expansion ofthe scope and complexity of the student projects.

I. INTRODUCTION

Embedded systems courses often include a hands-on com-ponent where students work directly with embedded systemhardware. This may be with existing development boards, orit may include having the students design their own printedcircuit boards (PCBs) either as complete, small microcontrollersystems, or as add-on boards for existing platforms. In the past,students may have prototyped systems using integrated circuitspackaged in dual in-line (DIP) packages, through-hole compo-nents such as resistors and capacitors, and with breadboards,wire-wrap, or custom PCBs as the prototyping substrate. Thesedays the components that both professionals and students aremore likely to encounter are tiny surface-mount technology(SMT) packages intended primarily for automatic (robotic)assembly and reflow soldering. In fact, modern componentssuch as microcontrollers and other complex integrated circuitsare unlikely to be available in anything but an SMT package.

These SMT components are much smaller than through-hole components and result in higher density on a circuitboard, but are essentially impossible to use without designinga custom PCB, and are delicate and cumbersome to solder byhand. A typical old-school DIP package has leads on 2.54mm(0.1 inch) centers that are designed to go through holes ona PCB. These, and the through-hole versions of passive partssuch as resistors and capacitors, are easily used in any of thepreviously mentioned prototyping substrates. A typical SMTpackage, on the other hand, is designed to sit on the surfaceof a PCB, not to have leads go through holes in the board, andthe leads are more likely to be on 0.8mm to 0.5mm centers.Passive devices are also designed to sit on the PCB’s surfaceand come in standard sizes that range from roughly the sizeof a grain of rice to something close to a fleck of pepper.Figure 1 is an example of such a board, designed, assembled,

Fig. 1. A custom PCB designed, assembled, and soldered by undergraduatestudents Dan Willoughby and Zach Toolson. The main component is an ARMCortex M4 in an LQFP 64 package with 0.5mm pitch on the pins. The silverpackage on the upper part of the board is a TI CC3000 network processor ina package that has all its pins on the bottom of the package. Resistors andcapacitors are 0603 and the LEDs are 0805 size. The board was designedusing KiCad [1] and is shown actual size (1.86 x 1.93”).

and soldered by undergraduate students in a senior projectclass at the University of Utah.

SMT components are designed to be soldered to the PCBusing reflow soldering [2]. This is a technique where aviscous solder paste consisting of a mix of solder and flux issqueegeed onto the board through a stencil. The componentsare assembled onto the board by nestling them into the smallpuddles of solder. The board, thus assembled, is put into areflow oven where the temperature is carefully raised andlowered according to a schedule that matches the characteris-tics of the solder paste (see Figure 2 for an example reflowschedule). During the reflow phase of the temperature profile,the solder/flux mixture melts and thus adheres the componentsto the board.

This is a very efficient way of assembling small SMT com-ponents on a custom PCB, but one that is not easily supportedin an academic lab without some specialized equipment. Inthis paper I will describe how we enhanced a student lab atthe University of Utah to support SMT assembly and reflowsoldering of student-designed PCBs in a cost-effective way.This has greatly expanded the scope of projects that studentscan attempt during a semester. I will include a list of thespecific equipment we used. Certainly there are many otherchoices, but this will serve as just one example. This will alsobecome quickly out of date, but I hope that providing specific

Fig. 2. Example reflow soldering temperature profile. [2]

examples and current prices will ground the informationsomewhat. I will also describe the procedure we use with thatequipment that has achieved good results with student projects.My hope is that the description will be detailed enough so thatothers can use this as a model for enhancing their own labs,and enable a new generation of students to include hands-ondesign, prototyping, and assembly of custom electronics backinto their curricula.

II. EXAMPLE COURSES OVERVIEW

At the University of Utah there are (at least) two coursesthat deal directly with hands-on embedded systems hard-ware: CS/ECE 5780 Embedded Systems and CS/ECE 4710Computer Engineering Senior Project. The embedded systemscourse is typically taken in the spring semester of the junioryear in our Computer Engineering program [3]. The catalogdescription for this course is:

This class is focused on the principles andpractices of modern embedded systems design. Inclass, we will focus on computer architecture beyondthe CPU, fundamentals of the hardware/softwareinterface, techniques for sensing and controlling thephysical world, and a few other topics. In lab,we will focus on the MSP430, ARM Cortex-M3,Microsemi FPGAs, and other supporting hardware,to learn how to design, build, and program embeddedsystems. Labs during the first half of the course willfocus on essential topics. The second half of thecourse will focus on the design and implementationof non-trivial, open-ended projects involving bothhardware and software.

Until recently, this class used primarily pre-built FPGAboards (Actel SmartFusion and Xilinx) and pre-assembledadd-on boards. The addition of SMT capabilities in the labmade a distinct shift in the type and complexity of theprojects. As currently structured, the very first lab in theclass is to begin the design a small 1x2” PCB around an

Fig. 3. A PCB designed, assembled and soldered in the student lab byNathan Hansen and Phillip Bradbury. The processor is an ARM Cortex M3in an LQFP 48 package with 0.5mm pitch pins. The smaller chip on the leftis an RF WiFi chip in a VQFN32 package that has pads on the bottom edgeof the package. The board was designed using Altium Designer. [6]

MSP430 microcontroller. The students source parts, typicallyfrom Mouser [4] or DigiKey [5], and then assemble and soldertheir boards. These boards are then used in later portions ofthe course.

The other course where SMT capabilities have had a hugeimpact is our CS/ECE4710 Computer Engineering SeniorProject course. This is the capstone team project coursefor our Computer Engineering program taken by senior un-dergraduates (the number for the course, strangely a lowernumber than the junior-level Embedded Systems course, is ahistorical oddity). The students in this course will have takena semester-long senior project planning course in the spring,and the teams from that course continue in the Fall semesterto implement a student-chosen project. As senior projects inthe Computer Engineering degree program, senior projects arerequired to have a mix of both hardware and software designedand implemented by the students. Again, until recently thehardware components of the senior projects were largely smallextensions of existing embedded system platforms. With theaddition of PCB design skills and SMT assembly and solderingto the lab, six out of seven teams in the most recent (Fall 2014)offering of the course included a student-designed PCB thatincluded SMT parts as part of their project. Some examples ofstudent-designed boards from this class are seen in Figures 1,3, and 4. Figure 5 shows a student working in the lab duringFall 2014. While it is difficult to quantify and compare thequality and complexity of student projects from year to year,visitors to the demo day at the end of Fall 2014 were greatlyimpressed with the projects from this year, many of whichwould have simply not been possible without the ability towork with SMT components on a custom PCB.

III. LAB ENHANCEMENTS

The first requirement for working with custom PCBs andSMT components is to introduce PCB design tools to thestudents. These tools typically start with a schematic view of

Fig. 4. An assembled board with debugging wires attached. This boardwas designed, assembled, and soldered by Brent Mellor and Chase Wilson.The processor chip is a Cyprus PSoC 4 “programmable system on chip” ina TQFP 44 package with 0.8mm pin pitch. The chip above the processor isa CAN-bus controller chip in a 20-pin TSSOP package with a 0.65mm pinpitch. The board was designed using Altium Designer. [6]

Fig. 5. A student working at the soldering station.

a circuit, and then allow the designer to produce a layout of aPCB that implements that schematic. This may involve havingthe designer draw new “footprints” that define how the SMTcomponents will sit on the board to be included in the PCBlayout. We have used three different PCB tools in differentcontexts at the University of Utah, and there are many othersout there to choose from.

• Eagle CAD: Eagle is widely used by hobbyists and issupported by hobbyist web sites such as Sparkfun andAdafruit who publish Eagle footprint libraries for mostparts that they sell. Eagle has a freeware version fornon-profit use that restricts designs to two signal layersand a 4x3.2 inch board area. This may be completelysufficient for student lab needs. All but one of the boardsdesigned in my Fall 2014 Senior Project class wouldhave met these specifications. A multi-user license for anaccredited educational institution has costs that depend

on the number of licenses and features desired, but asan example Eagle standard (99 schematic sheets, 6 signallayers, 6.4x4 inch board area) for 10 users would cost$1,230 as of April 2015. [7]

• KiCad: This is an open-source community-supported toolthat runs on a wide variety of platforms. At least one ofmy student teams used KiCad because of the open-sourcenature, but the support, especially in terms of footprintsfor non-common parts, is less than with Eagle. [1]

• Altium Designer: This is a professional-grade PCB tooldesigned for large high-volume professional users. It isconsiderably more expensive than Eagle, but is widelyrespected as one of the best professional tools. Althoughthere is no specific university program, universities maybe able to negotiate an educational discount as we haveat the University of Utah. Footprint support is limited,although the “footprint wizard” is very efficient, and thereare third-party companies that provide device footprintsat a cost.

Even with the ability to make SMT-capable PCBs, somerestrictions on what types of SMT components are usedcan increase the success rate. For example, in two-terminalcomponents (resistors, capacitors, LED’s, etc.) we recommendusing 1206, 0805, and if necessary, 0603 components. Smallercomponents are certainly possible, but dramatically increasethe soldering difficulty (at the extreme, a 01005 componentis barely visible at 0.4 mm x 0.2 mm (0.0157 in x 0.0079in)). In three-terminal packages, an SOT-23 is a good usablesize if possible. For larger components, we recommend thatstudents to not go below a 0.5mm lead pitch, and to not messwith ball grid array packages that have a set of issues all theirown, especially for rework. Packages that have been used withgood success include SOIC, SSOP, TSSOP, QFP, LQFP, andTQFP. A good overview of SMT packages can be found onWikipedia [8].

Once the PCBs are designed, they must be fabricated.Partly because of the small SMT parts, the circuit traces aretypically too fine for easy fabrication in-house. Also, withdenser designs using many small components, at least a two-layer board is usually required. Fortunately, there are manycost-effective ways to get high-quality small-volume PCBsfabricated. These typically have minimum trace widths/spacingof 6-8 mils in 1oz copper. Some fabrication houses that mystudents have used include:

• OSH Park: This is a “panel sharing” site where userdesigns are aggregated until a full PCB panel is ready tofab. This amortizes the setup costs for PCB fabricationamong the users and keeps costs low. The cost is $5.00/in2

for a two-layer board and $10.00/in2 for a four-layerboard with 6mil minimum width traces and three copiesof the board are included in that price. Turnaround isaround 12-14 days. OSH Park takes Eagle files directly,or Gerber files from other PCB design tools. [9]

• Circuit Graphics: This fabrication facility also has apanel sharing option at $3.25/in2 per copy of the board.

Circuit Graphics has an 8mil trace minimum and a five-day turnaround. [10]

• Advanced Circuits: This fabrication facility has an op-tion for a single larger two-layer board of up to 60in2 for$33 (four-layer boards are $66). Boards can have 6miltraces, and there is a five-day turnaround. [11].

A. Equipment List

A photo of the side of the student lab devoted to SMTassembly and soldering is shown in Figure 6. The followingis the list of equipment that we installed in our lab to supportassembly and soldering of SMT components on student-designed PCBs:

• Stencil definition: In order to apply the solder paste onlyto the pads on the board where components will be sol-dered, a solder mask needs to be cut. This is a stencil thathas holes only where the solder paste should be applied.The PCB design programs described in Section III allhave a specific layer in the board description (Gerber file)that defines these places on the board. What is needed isa way to convert this Gerber solder-mask description tosomething that can be cut out of mylar and be used as astencil. We use “gerber2graphtec,” a free and open-sourceconversion program that converts the Gerber definitioninto a description that can be used with a hobbyist vinylcutter. [12]

• Stencil cutting: To actually cut the mylar for the stencil,we use a hobbyist vinyl cutter that is geared to scrap-booking and other crafts. Specifically we have installed aSilhouette CAMEO cutting machine [13]. This consumer-grade cutter costs approximately $270 and easily cuts the4mil mylar that we use for stencils [14] (see Figure 7).

• Assembly: To assemble the components onto the board(after the application of solder paste), it is very helpful tohave an inspection microscope. This is a relatively low-powered binocular microscope with a ring light aroundthe lens. We find that it is important to have a longworking distance to the lens and a boom stand to make iteasier to move the scope over the board. There are manymicroscopes that would work - we chose a “Circuit ZoomStereo Microscope” from Amscope [15]. Prices dependon features - our scope was approximately $570.

• Soldering Oven: The reflow soldering action takes placein a reflow oven. Reflow soldering can actually be doneon a hot plate or even a frying pan (there are manyhobbyist web sites that will describe how to do this).However, a reflow oven works much more reliably andcan execute a specific reflow temperature profile like thatin Figure 2. Large commercial high-volume reflow ovenscan be extremely expensive, but we have found that smallinexpensive bench-top “mini reflow ovens” work well forour lab. We have had good success with small ovensfrom SMTmax that cost from $550-$900 depending onsize [16].

• Rework: Even with the most careful schematic design,first-spin PCBs often have issues, and components can

also sometimes be damaged in the assembly process orover time and need to be replaced. It’s also possible thatfor some parts of a design hand-soldering is needed forsome reason. Because of this, good soldering stations aredesirable both for hand-soldering and for rework. We havefound that two types of soldering stations are particularlyuseful.

– Soldering tweezers: Especially for two-terminalcomponents such as SMT resistors, capacitors, andLEDs we have found that soldering tweezers (hottweezers) are extremely useful. This is essentiallya tweezer with a soldering iron in each arm ofthe tweezer. With a fine tip, it’s easy to grab thecomponent by the ends with the hot tweezers andremove it from the board when the solder melts,or to place the component by grasping with thehot tweezers and placing into the solder puddlesalready on the board. There are many hot tweezersto choose from, and nearly all high-end solderingstations have a hot tweezer option. We have foundthat the inexpensive tweezers from Circuit Specialistswork well at only $90 [17].

– Hot air soldering: For point-specific reflow, it ispossible to use a hot-air soldering system. This isessentially a high-temperature small-tipped heat gun.Using a hot-air system one can quickly loosen thesolder on a component that needs replacing, or reflowthe solder to attach a new component. We have useda station from X-tronic that includes both a hot-airsystem and a fine-pitch traditional soldering iron andstarts at around $150 [18].

• Cleaning: After reflow, there is often flux residue onthe boards. When using solder paste with water-washableflux, we have found that a commercial steam cleanermakes a very effective cleaning system for the circuitboards. For example, a hand-held steamer from DBTechcosts around $35 [19].

• Other supplies: Other consumable supplies that areneeded for SMT soldering include solder paste (we likethe water washable type [20]), flux pen for hand-solderingand rework [21], blank mylar for stencils [14], tweezersfor assembling components onto the board [22], blankcircuit boards for holding the board that is to have solderpaste applied, a putty knife for applying the solder pastethrough the stencil, and a small refrigerator for keepingthe solder paste cold during storage.

IV. EXAMPLE AND PROCEDURE

The following is an example of the procedure for assemblingand reflow-soldering a small circuit board. This example usesthe board shown in Figure 1 that was designed and assembledby undergraduate students Dan Willoughby and Zach Toolson.The procedure starts with a designed and fabricated PCB.In this case Dan and Zach used KiCad, but any PCB toolwill work if it produces Gerber output files. Once the board

Fig. 6. Our student lab with SMT equipment. From left to right: Silhouette CAMEO mylar cutter (with host PC), SMTmax reflow oven, inspection microscope(with monitor on wall for camera attachment), hot-air rework station, heat gun for shrink-tubing, hot-tweezers and fine-pitch soldering station.

Fig. 7. Silhouette CAMEO vinyl cutter being used to cut a solder stencilfrom 4mil mylar.

has been fabricated and the components been acquired, theprocedure is as follows:

1) Convert the Gerber solder mask layer in yourPCB description to a “graphtec” format using ger-ber2graphitec [12].

2) Cut a stencil of the converted solder mask on 4milmylar using an automated cutter such as the SilhouetteCAMEO cutter (Figure 7).

3) Prepare the board for the application of solder paste bytaping it down nestled between blank circuit boards. Thismakes an even surface that extends past the board tomake the squeegee process easier. This can be seen inFigure 8. The purple circuit board is surrounded on allsides by yellow blank circuit board material to make asmooth surface for the solder paste squeegee.

4) Carefully place the stencil onto the board, lining up the

Fig. 8. Bare circuit board taped down and ready to receive solder pastethrough the stencil.

openings over the pads on the board. Tape this downtoo. The inspection scope is very useful in this step tomake sure that the openings in the stencil are properlyaligned with the solder pads on the board.

5) Apply the solder paste near one edge of the board, readyto be deposited on the PCB through the stencil with thesqueegee. See Figure 8 again.

6) Use a plastic putty knife to squeegee the solder pasteacross the stencil to apply the paste to the board. Thisis similar to screen printing if that helps describe thestep. We have found that it is best to apply the solder inone steady pass rather than try to move back and forthacross the stencil (see Figure 9).

7) Carefully lift the stencil leaving the solder only on thesolder pads of the PCB.

8) Verify with the scope that each copper pad actually has

Fig. 9. Plastic putty knife being used to apply solder paste through thestencil.

solder on it. You may need to place a small amount ofsolder paste on some pads using a toothpick or tweezers.

9) You may also need to remove extra solder paste that isbridging multiple pads. You don’t actually have to beperfect here, especially for fine-pitch pads. The reflowprocess will tend to draw the solder to the pads andaway from the coated parts of the board. But, if thereare obvious blobs, it’s helpful to remove them with atoothpick or very fine tweezers.

10) Clean the stencil and putty knife with the steam cleanerso that they’ll be ready for the next use.

11) Now use tweezers to carefully place the componentsonto the board, nestling them into the solder paste (Fig-ure 10). The paste is sticky and will hold the componentsinto position. Components should be placed as carefullyas possible, but it’s all right if they’re slightly crooked.When the solder flows during the reflow process it willtend to pull the components into place through surfacetension. The inspection scope is very useful in this step,especially for components with a large number of pins.It’s also helpful to have a copy of the PCB layout handyto remind you where each component goes, and to checkoff components as you go.

12) Once the components are placed, carefully transfer theboard to the reflow oven. The basic reflow schedule islikely quite effective, but you may want to adjust theschedule to match the characteristics of the solder pasteyou are using. Figure 11 shows this board on the oventray ready to be slid into the oven.

13) Fire up the oven to go through the reflow process. If youroven is not vented to the outside (ours is not), place afilter fan such as you would use for soldering behind the

Fig. 10. Components being placed on the circuit board after the solder pastehas been applied.

Fig. 11. Board with placed components about to be soldered in the reflowsoldering oven.

oven to draw the fumes through the filter.14) Allow the oven to cool completely before opening. This

will allow the components and the solder to set properly.In our oven, this means letting the oven cool to at least70 ◦C before opening the oven to remove the board.That’s still pretty toasty, so you can wait longer if youlike.

15) Inspect the resulting soldered board using the scopeto see if there are any obvious issues before applyingpower. If there are issues, use the hot-air or hot tweezersto rework and fix the issues.

16) Clean the board using the steam cleaner.17) Apply power and enjoy your completed, working board.

In Figure 12 the LEDs indicate at least basic function-ality of this particular board.

V. CONCLUSIONS

Prototyping with modern integrated circuits and circuitcomponents essentially requires the ability to deal with SMT

Fig. 12. A working board after reflow soldering and cleaning.

components. Many current parts are, in fact, only availablein SMT-type packages. Fortunately, using these parts with areflow soldering setup is not all that difficult, or expensive.Certainly care must be taken when using parts with fine pitchleads, or that are physically tiny, but our experience is that thisis very feasible in an undergraduate embedded systems lab.A production setup for SMT and reflow would be extremelyexpensive, but a student lab setup such as described here can beinstalled for under $2000 (not including the optional expenseof non-free PCB design software). Even with a few addedextras and upgraded equipment from the basic suggestions inthis paper, the cost is unlikely to exceed $3000 for the capitalequipment.

We have found that the re-introduction of hands-on systemprototyping and assembly that is enabled by this SMT and re-flow equipment has made a noticeable difference in the qualityand complexity of the projects our students pursue. This allowsstudent projects to develop exactly the board/system they needrather than settle for the board they can buy. It also flexestheir design skills to imagine the complete system with all therequired components to make the board work. Finally, thereis a hard-to-quantify sense of ownership and accomplishmentthat the students get from deploying their own board in theirprojects that has seemed to really raise the level of excitementin the lab. This seems to be an essential “maker” capabilityfor modern embedded system design and prototyping.

ACKNOWLEDGMENTS

I have benefited greatly from the advice of graduate studentAnh Luong who developed a similar SMT/reflow procedurefor his research group at the University of Utah. Also, mystudents in CE/ECE 4710 from Fall 2014, especially PhillipBradbury, Nathan Hansen, Brent Mellor, Zach Toolson, DanWilloughby, and Chase Wilson, bravely used this setup in itspreliminary form and discovered many interesting tweaks andcorrections to our initial flow.

REFERENCES

[1] KiCad, “An open source software suite for Electronic Design Automa-tion (EDA),” http://www.kicad-pcb.org/display/KICAD/KiCad+EDA+Software+Suite.

[2] Wikipedia, “Reflow soldering,” http://en.wikipedia.org/wiki/Reflowsoldering.

[3] University of Utah, “Computer engineering undergraduate program,”http://www.ce.utah.edu/.

[4] Mouser Electronics Inc., “Authorized distributor of semiconductors andelectronic components for design engineers,” http://www.mouser.com/.

[5] Digi-Key Electronics, “Authorized distributor of electronic componentssince 1972,” http://www.digikey.com/us/en/digihome.html.

[6] Altium Limited, “Altium Deisgner,” http://www.altium.com/altium-designer/overview.

[7] CadSoft Computer GmbH, “EAGLE PCB design software,” http://www.cadsoftusa.com/.

[8] Wikipedia, “surface-mount technology,” http://en.wikipedia.org/wiki/Surface-mount technology.

[9] OSH Park, “A community printed circuit board (PCB) ecosystem,” https://oshpark.com/.

[10] Circuit Graphics, “Printed circuit board (PCB) fabrication and panelsharing,” http://circuitboard.com/.

[11] Advanced Circuits, “Printed circuit board (PCB) fabrication,” http://www.4pcb.com/.

[12] P. Monta, “Gerber2graphtec: Cut SMT stencils from gerber files us-ing a Graphtec cutter,” Source on GitHub, https://github.com/pmonta/gerber2graphtec.

[13] Silhouette America, inc., “The Silhouette CAMEO,” http://www.silhouetteamerica.com/shop/machines/cameo.

[14] Stencil Ease International, LLC, “4 mil blank mylar stencil sheets,” http://www.stencilease.com/db/display.asp?input=1858.

[15] AmScope, “3.5X-90X Circuit Zoom Stereo Microscope (SM-4TZ-144),”http://www.amscope.com/.

[16] Omxie Corp., “Mini automatic reflow ovens (models AS-5060 and AS-5001),” http://www.smtmax.com/.

[17] Circuit Specialists, “BlackJack SolderWerks 60 Watt Solder Station withHot Tweezers,” http://www.circuitspecialists.com/bk3060.html.

[18] X-Tronic International Inc., “X-Tronic Model #4040-XTSHot Air Rework Station,” http://www.xtronicusa.com/home/#!/X-TRONIC-4000-SERIES-MODEL-4040-XTS-Hot-Air-Rework-Station/p/9238594.

[19] DBTech, “Multi-Purpose Pressurized Steam Cleaning andSanitizing System with Attachments,” http://www.amazon.com/DBTech-Multi-Purpose-Pressurized-Sanitizing-Attachments/dp/B005SI8YZC/ref=sr 1 6?s=home-garden&ie=UTF8&qid=1415937952&sr=1-6&keywords=steamer.

[20] Chip Quik inc., “Solder paste, no clean, water washable,” http://www.chipquik.com/store/product info.php?products id=430001.

[21] SRA Soldering Products, “SRA #80 Water Soluable Flux Pen and 5 re-fills,” http://sra-solder.com/sra-80-water-soluble-flux-pen-and-5-refills/.

[22] ——, “Swiss-style tweezers set,” http://sra-solder.com/swiss-style-tweezers-set/.