SMT Troubleshooting Guide - SMTnet.com › library › files › upload › SMT-troubleshooting-guid… · SMT Troubleshooting Guide Easy-to-use advice for common SMT assembly issues

SMTTroubleshootingGuide

Easy-to-use advicefor common SMTassembly issues.

www.alpha.cooksonelectronics.com

Table of Contents

Bridging 3

Insufficient Fill 7

Insufficient Solder 8

Random Solder Balls 9

Solder Spattering 11

Mid-Chip Solder Balls12

Tombstoning 14

Voiding 15

BGA Head-on-Pillow 17

Grainy Joints 19

Cookson ElectronicsTroubleshooting Guide

With this easy-to-use

Troubleshooting Guide, you can

learn to troubleshoot common

SMT issues. After using it a few

times, it will become an essential

companion for you and anyone

in your company responsible for

operating an SMT line.

This Guide offers troubleshooting

advice for common SMT assembly

issues by process defect. If your

issue is not resolved after follow-

ing the steps to help identify the

possible root cause and solution,

please contact your Cookson

Electronics representative who

will be able to provide you with

further assistance.

2 www.alpha.cooksonelectronics.com

Cookson Confidential –For Authorized Use Only

Bridging

Possible Causes: PCB

Description

SMD pads will contributeto coplanarity issue result-ing in poor gasketingduring printer setup.

Recommendations

Highly recommendedto remove solder maskbetween adjacent padsespecially for fine-pitchcomponents

Possible Causes: Stencil

Description

Dirty stencil with pasteunderneath will contam-inate the bare board onthe next print, attributinga potential bridge.

Stencil tension

Aperture Design

Recommendations

• Verify zero print gapset up.• Ensure minimum printpressure.• Increase wipe frequency.•Use different cleaningchemicals.

Ensure stencil tension istight. Poor stencil tensionwill make it impossibleto have a good setup forconsistent print definition.

For fine pitch component,it is highly recommendedto have the openingslightly smaller thanlanding pad size to improvestencil to PCB gasketing.

Definition: Solder connecting, in most cases, miscon-necting two or more adjacent pads that come intocontact to form a conductive path.

3www.alpha.cooksonelectronics.com

Bridging

Possible Causes: Screen Printer

Description

Poor gasketing – pasteoozes out beneath stencilduring printing, increasingchance of wet solderpaste bridges

Misaligned print willchallenge the paste topull back to pads duringmolten stage, increasingthe potential for bridging.

Smearing and bridgingphenomenon on the nextprinted board after stencilcleaning operation

Poor print definition withdog ears especially onfine-pitch components

Dented squeegee bladescould result in unevenprint pressure.

Recommendations

• Zero print gap betweenstencil and PCB•Check paste smearunderneath stencil.• Check sufficient stenciltension.

Ensure print accuracy andconsistency for both printstrokes.

• Verify stencil is dry aftercleaning and before nextprint.• Standard cleaning modeis wet/vacuum/dry.

•Check board support.•Adjust separation speedto achieve minimum dogears.

NB: Different pastechemistry requiresdifferent separationspeed to minimizedog ears.

Check squeegee bladescondition.


Bridging

Possible Causes: Component Placement

Description

Placement inaccuracy willnarrow the gap betweenpads, increasing thechance of bridging.

Excessive componentplacement pressure willsqueeze paste out ofpads.

Recommendations

• Verify componentplacement pressure.•Use X-ray to verify BGAplacement.•Usemicroscope for QFPs.

• Verify actual componentheight against dataentered in the machine•Component placementheight should be ±1/3of paste height.

Possible Causes: Reflow Profile

Description

Extended soak will inputmore heat to the pasteand result in paste hotslump phenomenon.

Recommendations

Adopt a straight ramp tospike profile, without soakzone if possible.


Possible Causes: Solder Paste

Description

Dry paste phenomenon –irregular print shape andinconsistent print volume

Paste oozes out of pads,may form connection withadjacent pads.

Recommendations

• Paste expiry•Operating temperaturewithin supplier’s recom-mendations. Checktemperature insideprinter. Normal require-ment around 25°C,50%RH•Do not mix using newand old paste.

•Operating temperaturewithin supplier’s recom-mendations• Verify with another batchof paste to confirm prob-lem is batch-related.• Perform cold and hotslump test result usingIPC-TM-650 Method2.4.35.

Bridging


Insufficient Fill


Description

Paste scooping effectespecially on large pads

Recommendations

Segment the largeopening into smallerapertures.

Description

Paste does not roll intoaperture

Recommendations

• Reduce print speed.• Increase print pressure.•Adopt lower squeegeecontact print.• Ensure paste is notexpired or dry.• Ensure sufficient boardsupport.• Reduce squeegeepressure.

300 µm

Definition: Amount of solder paste deposited onPWB at printer station is much less than stencilopening design.



Insufficient Solder

Definition: Amount of solder paste deposited on PWBat printer station is much less than stencil openingdesign or, after reflow, insufficient solder to form a filletat the component leads.

Description

Solder paste adheres onthe stencil aperture walls

Recommendations

•Area ratio > 0.66•Aspect ratio > 1.5•No burr on stencilaperture edge


Description

Print definitions

Recommendations

• Verify print setup•Reduce print speed toprovide sufficient timefor paste to roll intoaperture.


Description

Mismatch in CTE betweencomponent and PCB cancause solder wicking effectwhich may look likeinsufficient solder on pads.

Recommendations

•Attach thermocouple oncomponent and PCB.•Apply soak profile tominimize delta T beforereflow zone.• Set bottom zones to behigher temperature ifpossible, to keep PCBhotter than componentleads.


Description

Solder paste viscosity

Recommendations

Check paste conditionssuch as dry pastephenomenon by verifyingif paste rolls or skids alongprint direction.



Random Solder Balls

Definition: After reflow, small spherical particles withvarious diameters are formed away from the mainsolder pool.

Description

Paste stuck under thestencil will be transferredonto the solder mask ofthe next PCB.

Recommendations

• Verify zero print gap setup.•Check minimum printpressure used.•Check cleaning efficiencysuch as wet/dry/vacuum.•Check wipe frequency.


Description

Fast ramp-up rate orpreheat rate will notallow sufficient time forthe solvent to vaporizeoff gradually.

Recommendations

Slow preheat rate isrecommended, typically< 1.5°C/sec from roomtemperature to 150°C.


Description

Trapped moisture mayresult in explosivevaporization.

Recommendations

Especially for lower gradePCBs such as FR2, CEM1,tends to absorb moisture.Bake 120°C for 4 hours ifnecessary.

Possible Causes: PCB Moisture


Description

Especially for water-soluble solder paste whichis hygroscopic, it tends tohave limited stencil lifebecause of moistureabsorption.

Recommendations

•Minimize exposure time•Printer temperature andhumidity to be withinrecommendation• Try new lot of solder pasteto verify paste integrity.•Use coarser powder sizeif possible as fine powdersize has more oxides andtends to slump morereadily.


Random Solder Balls


Solder Spattering

Definition: Solder Spatter phenomenon is very similarto solder balling, but the concern is usually about solderdeposited onto Au fingers.

Description

Handling of boards

Bare boardscontamination

Recommendations

•Do not mix clean andwashed boards.•Open fresh PCBs frompackage when readyto run.• Ensure working area iscleaned thoroughly andnot contaminated withsolder paste remains.

Inspect bare PCBs tocapture and filter solderfound on bare PCB beforeprinting station.


Description

Ineffective cleaning ofstencil wipe will transfersmall particles of solderonto the top surface ofthe next bare board.

Recommendations

• Ensure wipe frequency isset correctly.•Use effective solvent,preferably SC10.•Use printer machinecamera to inspect theeffectiveness of stencilcleaning.


Description

Control the flux out-gassing rate to minimizeexplosive solder scatteron Au pads.

Recommendations

For SAC 305, set slowramp rate of 0.3-0.4°C/secfrom 217-221°C.



Mid-Chip Solder Balls (MCSB)

Definition: After reflow, large solder ball(s) is/arelocated on the side of the chip components, betweenthe terminations and away from the pads.

Description

Solder dissociation anddoes not adhere on soldermask.

Recommendations

• Remove solder maskbetween pads.•Gap between pad andsolder mask is recom-mended to maintain atleast 75µm~100µm,preferably >120µm.• Solder mask may not becentralized around pad.

>120µm

60%

20%

20%


Description

Excess paste squeezesunderneath componentbody tends to dissociatewith the main body ofsolder during reflow.

Recommendations

Home plate or U-shapedesign may help to reducethe amount of pastepotentially squeezedunder the componentbody, onto the mask.

NB: Aperture reductionmay not be suitable forcomponent size smallerthan 0603. Besides, LFalloy has higher surfacetension and does not pullback after reflow.

Possible Causes: Stencil Design


Description

Excessive placementpressure will squeezepaste on pad

Recommendations

• Verify actual componentheight against dataentered in the machine.•Component placementheight should be ±1/3 ofpaste height.

Possible Causes: Component Placement

Description

Paste smearing on soldermask

Recommendations

• Printer set up for zeroprint gap, verified bypaste height consistencywithout dog ears• Print alignment accuracy


Description

Extended soak will inputmore heat to the pasteand result in paste slumpphenomenon.

Recommendations

Adopt a straight ramp tospike profile, without soakzone if possible.


Mid-Chip Solder Balls (MCSB)


Tombstoning

Definition: A tombstone, sometimes called Manhattaneffect, is a chip component that has partially or com-pletely lifted off one end of the surface of the pad.

Description

Component body mustcover at least 50% of bothpads.

Unequal pad sizeespecially with groundpad on one side

Recommendations

If component terminationsare not covering >50% ofpads, high tendency to haveimbalance wetting force,resulting in tombstoning.Feedback to supplier foralteration if possible.

Unequal size meansdifferent solder volume,increasing potential forunequal wetting force. Ifdue to design limitation,use a gradual soak ramprate just before reachingliquidus point, e.g.,SAC305, soak @190-220°C for 30-45 sec.

Possible Causes: Pad Design

Description

Skew placement willcreate imbalance wettingforce on both pads.

Recommendations

Check other componentsplacement accuracy.Re-teach fiducials if allcomponent shifted, elseedit that specific locationmanually.

Possible Causes: Placement Accuracy & Pressure

Description

Extend soak zone can aidin balancing the wettingforce on both pads beforepaste reaching to moltenstate

Recommendations

Focus at 30°C before alloyliquidus temperature, e.g.,for SAC305, liquidus @220°C, ensure soak at190~220°C for minimumof 30 seconds



Bad design

Good design

Definition: Voids in solder joints are empty spaceswithin the joint, increasing concern about voiding,especially on BGAs and large pads such as LGAs.Two main contributors of voiding are(i) outgassing of flux entrapped(ii) excessive oxidation.

Voiding

Description

Micro-via holes on padstrapped flux and airpockets

Recommendations

• Typically via holes <6milswill be more difficult tovaporize the flux or airtrapped.• Plug the blinded viaholes before printing.•Double print helps topack more solder pasteinto via holes.•Use finer powder size.•Avoid printing paste ontop of via holes, insteadaperture openingdesigned around it.


15

BGA

LGA

Passive Component


Voiding


Description

Flux entrapped withoutsufficient time to outgas

Oxidation ratepredominates

Recommendations

• Establish soak zone from170~220°C for 60-90sec.•Also make sure profileset between 130~220°Cfor 180 sec.

•Adopt short profileconcept to preserve fluxactivity, no soak zone.•Use nitrogen if possible.

Description

For large pads such asLGA, massive soldervolume has a lot of fluxto vaporize during reflow.Any trapped flux will resultin voids.

Recommendations

• Reduced amount ofsolder deposit• Total solder volumereduction can be as highas 45%.•With solder mask inbetween, break thelarge aperture intosmall openings.•Without solder mask, cuta large round opening inthe middle.



55% opening

BGA Head-on-Pillow

Definition: Head-on-pillow is an assembly defect inwhich the spheres from a BGA or CSP don’t coalescewith the solder paste on the PCB pad. It is importantto differentiate head-on-pillow from a defect causedsimply by insufficient reflow temperature, which ischaracterized by distinct solder spheres from the pastethat have not been properly melted on the pad andBGA solder sphere. With head-on-pillow the solderingtemperature is sufficient to fully melt the solder sphereand paste deposit, but an impediment to the formationof a proper solder joint exists.

Description

Irregular print definitionacross the pads mayhinder some solder bumplocations to be in contactwith solder paste.

Recommendations

Verify print definition andmeasure print heightconsistency


Description

Increase paste depositionvolume to better com-pensate for substratewarpage.

BGA coplanarity issue

Oxidized BGA solder balls

Recommendations

Increase print volume byusing square aperture vs.round opening, or enlargeoverall deposition volumewithout jeopardizingbridging.

Increase solder volume.

•Use higher activity paste.•Use nitrogen reflow.

Possible Causes: PCB/Component


BGA Head-on-Pillow

Description

Board warpage especiallyfor double reflow boardsor thin PCBs (<1mm thick)

Variance in CTE betweenPCB and BGA

Paste hot-slump effect willaggravate BGA open jointsif there are coplanarityissues.

Long soak profile mayexhaust the flux capacitybefore reflow.

Recommendations

•Critical to minimize timeabove Tg, (typically130°C for FR4 boards)with BGAs mounted.Target to maintain< 2 min if possible.• For second reflow cycle,try to adopt lowerpreheat to reducewarpage occurrence.

Ensure minimum deltatemperature differencebetween the BGAcomponent and the restof the components on theboard. Apply short soakif necessary.

Minimize time from 150°Cto liquidus temperature.

If a long soak is mandatoryfor complex board, usenitrogen cushion the fluxcapacity in overcomingoxidation rate.

Possible Causes: Reflow


Grainy Joints

Definition: Sometimes called “Cold Solder,” it isrecognized by dark, non-reflective, rough surfaces froman alloy that is normally bright and shiny.

Description

Insufficient heat absorbedby the solder

Excessive heat imposed

Cooling rate is too slow

Recommendations

Ensure a TC is properlyattached to this com-ponent. Verify peaktemperature is at least15°C above alloy liquidusand time above liquidus(TAL) > 45 sec.

Adopt a ramp-to-spikeprofile with soak zone tominimize oxidation andflux exhaustion. If soakingis mandatory, use nitrogenreflow whenever possible.

Ensure alloy cooling ratefrom molten solder is3-8°C/sec. Fast coolingrate will result in fine-grainstructure appearance andlooks shiny.

Possible Causes: Reflow

Tem

per

atur

e(°

C) 250

200

150

200

50

0

Ramp

Time (sec.)

Soak Reflow Cool

Normally, soakingzone is used onlyif deltaT is >10°CRamp-to-spike

profile is preferred

Peak temp

TAL Cool down3-8°C/sec.

19

Ramp-to-spike profile


Global Headquarters109Corporate BoulevardSouth Plainfield,NJ 07080USATel: +1-800-367-5460

European HeadquartersForsyth Road, SheerwaterWoking, Surrey GU21 5RZUKTel: +44 (0) 1483 758400

Asia/Pacific1/F, Block A, 21 Tung Yuen St.Yau Tong Bay, KowloonHong KongTel: 852-3190-3100

©2008 Cookson ElectronicsIssued 10/08SM982



Documents

SMT Troubleshooting Guide - SMTnet.com › library › files › upload › SMT-troubleshooting-guid… · SMT Troubleshooting Guide Easy-to-use advice for common SMT assembly issues