Troubleshooting the Stencil Printing Process - SMTA

Troubleshooting the StencilPrinting ProcessChrys Shea, Shea Engineering ServicesSponsored by:

TopicsPrinting– Solder paste properties– Key elements of the processInspecting– Phase Shift Interferometry– Typical Control Methods– SPI SPC and other applicationsOverall system check– Will identify root cause 80% of the timeInvestigating specific defect modes– For the other 20%

Solder Paste Properties• Solder paste is not solid, it is notliquid, it is thixotropic:– Yields (moves) when pressure is applied to it– Holds its shape when pressure is not applied– Thins down and stiffens up (like peanut butter)• Viscosity – how easily it flowsunder pressure• Rheology – how its viscositychanges as the pressure changes

Key Elements of a Solder PastePrinting SystemPWBStencilSolder PasteSqueegeeMechanical Setup- Support, Alignment, Contact, GasketingSeparation

PWBPads: Flat, RightSize, Free of Debrisor Solder MaskSolder Mask: No taller thanpad, not encroaching onpad (unless SMD)PWB: Minimal warp and bowPositional AccuracyN O T T O S C A L E

StencilApertures: Right Size & Shape, SmoothWalls, Uniform Thickness, Free of Debrisor Solder PastePositional AccuracyN O T T O S C A L E

Solder PasteFormulation, Rheology, ReleaseCharacteristics, Particle Size, PasteTemperature, Ambient Temperature& HumidityN O T T O S C A L E

SqueegeeAngle, Speed, Pressure,Stiffness, Surface EnergyN O T T O S C A L E

A Good Mechanical Setup isEssentialAperture-PadAlignmentAperture-PadGasketingBoard SupportN O T T O S C A L E

GasketingPoor AlignmentPWB ShrinkAperture largerthan padPad smallerthan aperturePaste on bottomof stencilSolder Mask on Mask padonPadHASL DomeN O T T O S C A L E

Stencil SeparationIf printer has stepper motor on Z-axis, separation speed isvery important to print quality:• Steppers induce vibration in the system, which causes pastes tothin out at aperture walls• Some pastes benefit by the localized shear thinning and workbest with slow separation speed• Some pastes thin too much with slow separation, producingpoor print definition and stringy bridges, and work best with fastseparation speed.How does the engineer know? Simple experiment.N O T T O S C A L E

Print Process1) Alignment, gasketing ofapertures to Pads2) Squeegee motion thins pasteso it flows into apertures3) Paste recovers; stiffens up 4) Stencil separates from PWB,paste deposits release fromstencil onto padsN O T T O S C A L E

Results16-mil pitch QFP

Results20-mil pitch QFP

Preventing The Bad and The Uglywith Solder Paste Print Inspection

Phase Shift InterferometryShines a grated light on the subject– Called a Moire pattern– Pattern appears to move as it crossestopography– Like afternoon sunlight shining through blindsTakes photos at known angles– Calculates height by changes inpattern at different angles– Extremely accurate andrepeatable– Used in all types of surfacemeasurements

PSI and Solder Paste“Chops” each deposit into tiny segments – as small as 10 µmCalculates height volume of each segmentCompiles data for each deposit and returns:– Area– Height– Volume– Offsets in X and Y– Solid Model

Solder Paste Inspection CriteriaStarting parameters– Gets theoretical aperture volume from stencil Gerber file– 50% to 150% of theoretical volume– 50% offset in X or YTightening the process– Criteria can be set tighter or looser for each device on PWB(uBGA +/- 20%)– Track effects of changes in process or in controlparameters– Use historical production yield data to optimize forindividual processes

Process ControlWith Automatic Solder Paste InspectionStart with standard SPC– X-bar, Sigma, Cp/Cpk, and Histograms– Optimize the basic process– Compare before and after adjustmentsMine Data to improve yields– Create defect Paretos– Boards with highest fallout, defect types, componentswith highest fallout ratesVerify stencils before putting them into production– Assure all apertures are in right position and cut properly– Avoid recurring defects on the line

Comparing Before & AfterProcess Adjustments

Still Making Defects?Check the easiest, most obviousthings firstInvesting a few minutes in a systemonce-over will find the problem4 out of 5 times

Overall System CheckFirst: Knead the paste, wipe the stencil, print a board, observeIs the right amount on stencil?• Paste bead should be about 1.5cm diameter (5/8”)• Bead size affects fill pressureDoes it roll over the surface of the stenciland release cleanly from squeegee blade?• If no, replace it with fresh solder paste• Check temperature and consistency

Check the ToolingNext: Remove the stencilInspect the stencil• Physical damage• Paste/debris in apertures• Worn out or dirty fiducials• Rips or tears in mounting meshInspect the board support• Is dried paste interfering with PWB?Shuttle a board into position• Tap or press on top to verify support• Check for movement in X & Y• If vacuum, check for leaks

Check Alignment and SetupFinal: Reinstall the stencilCheck the alignment• Watch the process, includingthe vision finding the fiducial• Confirm alignment• Check contact between stencil& boardInspect the squeegees• Damage or dings, angle• Check pressure balance on bladesRecheck Print Parameters• Speed, Pressure• Snap off Delay, Speed and Distance

Didn’t find the Root Cause?Time to dig a little deeper into the likelycauses of typical print problems…

Solder Paste Print DefectsSolder BridgesPoor Print Definition- Peaks or “Dog Ears”Insufficient Solder Volumes- Poor Aperture Fill- Poor Aperture ReleasePoor GasketingPoor Alignment

Solder Bridges

Solder BridgesIf you suspect…Bad GasketingResidual paste fromprevious printSeparation speed(too fast or too slow)Squeegee pressuretoo highToo much pastePaste is too warmThen investigate:See slides on possible reasons for badgasketing.Check board supportStencil cleaning parametersIncrease wipe frequencyIncrease or decrease separation speed- Different pastes have different optimumsand its usually one or the other – no middleDecreasing the force. Most pastes work wellwith 1 – 1.25 lb/in forceCheck bead on stencil. ½ - ¾ inch is typical(the diameter of a dime or nickel)Compare working temperature and tech datasheet. If printer is getting hot inside, checkexhaust fans

Poor Print DefinitionPeaks and “Dog Ears”

Poor Print DefinitionIf you suspect…Bad GasketingSeparation speed(too fast or too slow)Residual paste fromprevious printMisalignmentSqueegee pressuretoo high or too lowPaste is too warmThen investigate:See possible reasons for bad gasketingCheck board supportIncrease or decrease separation speed- Different pastes have different optimums andits usually one or the other – no middleStencil cleaning parametersIncrease wipe frequencySee section on alignmentAdjust force. Most pastes work well with 1 –1.25 lb/in.Check temperature and tech data sheet

InsufficientsPoor Aperture FillBonus Question: what’s wrong with this aperture design?

Poor Aperture FillIf you suspect…Pause in printingraised paste viscositySqueegee speed toohigh or too lowSqueegee pressuretoo lowNot enough paste onstencilPaste is too coldPaste sticking tosqueegee bladeSqueegee worn ordamagedThen investigate:Knead 4 -1 0 strokes. Clean board used forkneadingCheck print speedIncrease the force. Most pastes work wellwith 1 – 1.25 lb/in.Check bead on stencil. ½ - ¾ inch is typicalCheck temperature and tech data sheetCheck bead on stencil. ½ - ¾ inch is typical.Check paste temperatureInspect blades and replace if necessary

InsufficientsPoor Aperture Release

Poor Aperture ReleaseIf you suspect…Pause in printingraised paste viscosityResidual pastebuilding up inaperturesPaste is too coldSqueegee pressuretoo lowThen investigate:Knead 4 -1 0 strokes. Clean board used forkneadingCheck stencil cleaning parameters, increasefrequency, clean after down timeCheck temperature and tech data sheetIncreasing the force. Most pastes work wellwith 1 – 1.25 lb/in

Poor Gasketing

Poor GasketingIf you suspect…Board SupportBad alignmentSolder mask higherthan padsStencil apertureslarger than PWB padsHot Air Solder Levelfinish creates unevenprinting surfaceLabels, inks, or othersurface featuresprevent stencil fromseating on PWBThen investigate:Check (clean or improve) PWB supportSee section on alignmentCheck solder mask height and compare tospecificationMeasure and compare to specificationMore planar, non-HASL finishes. Consult withPWB vendor on improving doming effect ofHASL process.Proximity of features to defects. Considerchanging locations of those features or halfetchingthe bottom of the stencil toaccommodate them.

Poor Alignment

PoorAlignmentIf you suspect…Board SupportPrinter alignmenterrorStencil mesh torn ortension too loosePWB or stencilpositional errorPWB shrink or stretchThen investigate:Check (clean or improve) PWB supportCheck printer fiducial reading routine. Watchfiducial find on screenCheck for stencil movement at beginning ofprint strokeCorner-to-corner alignment of apertures andpads.Corner-to-corner alignment. If alignmentcannot be achieved, stencil can be scaled tocompensate for PWB error.

Summary - Troubleshooting• Understand the key elements in the solder pasteprinting process• Maintain control of the process– It’s where most of the rework comes from– It’s where the money is in SMT!• When problems arise, first do the 5-minute overallsystem check– 80% chance that you resolve the problem• If specific defects continue to occur, follow logicaltroubleshooting guidelines

Thank YouMany thanks to:• Local SMTA Chapter– Event hosts• Jabil, San Jose, CA– GOS Laboratory engineers and technicians– Hien Ly, Michael Lapitan, Mike Santos• Christopher Associates– Koki– Koh Young Solder Paste Inspection

Questions?Contact Chrys at:chrys@sheaengineering.com609 239-2995Copies of this presentation can be downloaded at:http://www.christopherweb.com/news.aspx#40Sponsored by:www.christopherweb.com714 979-7500