ipc9708rev4.docx
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ipc9708rev4.docx
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ipc9708rev4
DraftIPC9708Originator:
MudasirAhmadRev1.2
IPC-9708
TestMethodsforCharacterizationofPCBPadCratering
1SCOPE
ThisdocumentprovidestestmethodstoevaluatethesusceptibilityofPrintedCircuitBoard(PCB)materialsanddesigns,tocohesivedielectricfailureunderneathsurfacemount(SMT)attachpads.ThetestmethodscanbeusedtorankorderandcomparedifferentPCBmaterialsanddesignparameters,butdonotdefineacceptancecriteria.
1.2PerformanceClassificationThisspecificationrecognizesthatsurfacemountassemblies(SMAs)willbesubjecttovariationsinperformancerequirementsbasedonenduse.WhilePerformanceClassesaredefinedinIPC-6011,theseperformanceclassificationsarenotspecificastotherequiredreliability.Atthispointintime,theacceptancecriterianeedtobeestablishedbyagreementbetweencustomerandsupplier.
1.3DefinitionofTermsThedefinitionofalltermsusedhereinshallbeasspecifiedinIPC-T-50,exceptasotherwisespecifiedinSection3.
BGABallGridArrayPackage
PCB/PCBPrintedCircuitBoard/PrintedCircuitBoard
ComponentPackagedsemiconductordevice
SolderJoint/BallThesolderinterconnectionbetweenacomponentandPCB
PadCrateringTheformationofacohesivedielectriccrackorfractureunderneaththepadofasurface
mountcomponent,mostcommonlyBGApackages
1.4Interpretation“Shall”isusedthroughoutthisspecificationwheneverarequirementisintendedtoexpressaprovisionthatismandatory;deviationmaybeconsideredifsufficientdataissuppliedtojustifytheexception.
Thewords“should”and“may”areusedwheneveritisnecessarytoexpressnon-mandatoryprovisions.“Will”isusedtoexpressadeclarationofpurpose.
Toassistthereader,theword“shall”ispresentedinboldcharacters.
2APPLICABLEDOCUMENTS
Thefollowingdocumentsareapplicableandconstituteapartofthisspecificationtotheextentspecifiedherein.Sub-sequentissuesof,oramendmentsto,thesedocumentswillbecomeapartofthisspecification.DocumentsaregroupedundercategoriesasIPC,JointIndustryStandard,ITRI,EIAandothersdependingonthesource.
2.1IPC1
IPC-T-50TermsandDefinitionsforInterconnectingandPackagingElectronicCircuits
IPC-D-279DesignGuidelinesforReliableSurfaceMountTechnologyPrintedBoardAssemblies
IPC-TM-650TestMethodsManual2
IPC-S-816SMTProcessGuidelineandChecklist
IPC-9502PCBAssemblySolderingProcessGuidelinesforElectronicComponents
IPC-6012QualificationandPerformanceSpecificationforRigidPrintedBoards
IPC-4101SpecificationforBaseMaterialsForRigidandMultilayerPrintedBoards
2.4.1ElectronicIndustriesAssociation4
JESD22-B117A“BGABallShear,”July2006
Deleted:
0
JESD22-B115SolderBallPull,May2007
3TERMS,DEFINITIONSANDCONCEPTS
3.1General
Mechanicalbendandshocktestsareroutinelyperformedonsurfacemountassembliestoensurethattheycansustainanticipatedproduction,handlingandenduseconditions.ThestrainsandstrainratesappliedtoSMTassembliesduringbendandshocktestingcanleadtoavarietyoffailuremodesinthevicinityofthesolderjoints.Figure1showsthefailuremodesthatareknowntooccurinBGAassembliessubjectedtomechanicalloadingconditions.
Theprevalenceanddistributionofthesefailuremodesdependonseveralfactors,includingthesoldermetallurgyused,thepackagetype,construction,component-to-PCB-padsizeratioandPCBmaterials.Usually,multiplefailuremodesoccurconcurrently,atdifferentstrainandstrainratelevels.
PackageSubstrate
A
B
C
SolderBall
D
E
F
G
PCB
Legend
APackagePadLift/Crater
BPkgBaseMetal/IMCInterfaceFracture
CPkgIMC/SolderInterfaceFracture
DBulkSolderFracture
EPCBIMC/SolderInterfaceFracture
FPCBSolderpad/IMCInterfaceFracture
GPCBPadLift/Crater
Figure1:
ExampleFailureModesOccurringinaBGAPrintedCircuitBoardAssembly
Tomitigatethefailuremodesoccurringatmarginalstrainlevels,itiscrucialtodecouplethefailuremodesandidentifytheweakestlinkintheassembly.Unfortunately,failuremodescannotbeeasilydifferentiatedinhighspeedmonotonicbendtests,wherethetestdurationisfairlyshortandmultiplefailuresoccurinrapidsuccession.
Inshocktesting,theentireassemblyissubjectedtoincrementalG-levelsandgrossfailuresaredetectedonlyifachangeinstrainlevelorelectricalresistanceisdetected.Theonsetofpadcratering(failuremodeG)maynothaveanelectricalsignature.AnexampleofapadcrateringfailuremodeisshowninFigure2.[1-6]
SeveralfactorscouldplayaroleintheincreasedprevalenceofthisfailuremodeforPb-freeassembliesthaninSnPbassemblies.
a.Pb-freesoldersaregenerallystifferthanSnPbsolders.Consequently,theycantransfermoreoftheappliedglobalstraintothePCB.
b.Phenolic-curedPCBmaterialstypicallyusedinPb-freeassembliesaremorebrittlethanconventionaldicy-curedFR4materials.
c.ThehigherreflowtemperatureswhichPb-freeassembliesaresubjectedtoleadtohigherstrainsinthe
DraftIPC9708Originator:
MudasirAhmadRev1.2
assembly.Thesestrainscouldeventuallyrelaxovertime,butifmechanicalstrainisappliedshortlyafterreflow,padcrateringcouldoccuratlowermechanicalstrainlevels.
SolderBall
Figure2:
ExampleofaPadCrateringFailure
Tomitigatethisfailuremode,itiscriticaltodeterminetheimpactofeachofthefollowingcontributingfactorsindependently:
soldermetallurgy,PCBmaterialsandreflowconditions.
OnewaytodecoupleandquantifytheeffectofthePb-freesolderjointsrelativetothePCBistodevelopatestmethodthatcanquantitativelymeasureandcomparetheeffectsofdifferentPCBmaterialsanddesignvariablesalone.Suchatestmethod,usedinconjunctionwiththestandardmonotonicbendandshocktestscanprovidethefollowing:
a.Relative-strengthcomparisonofdifferentPCBmaterialsandPCBpaddesignvariables
b.QuantitativeguidetoimprovePCBmaterials,stack-upanddesignvariablestobetterwithstandrequiredstrainsandstrainrates.
c.QuantitativerelationshipbetweenPb-freesolderjointstiffnessandthepropensityforpadcratering.
d.Consistent,repeatablemethodforcheckingsuspectPCBsagainstknownbaselinePCBs.
AvarietyoftestmethodscanbeusedtodeterminethepadcrateringsusceptibilityofaPCBmaterial,aslongasthetestmethodsareeasytoimplementandhavetheresolutionnecessarytoconsistentlydiscerncriticaldifferencesinthePCBmaterialanddesignvariables(resincontent,padconstructionetc.).
Commontestmethodspresentlybeingexploredincludeapin-pulltest,aball-pulltest,aball-sheartestandbasePCBresinfracturetoughnesstests.Eachtestmethodhasitsuniquechallengesandbenefits.Recommendationsandguidelinesareprovidedinthisdocumenttoaddressthedesignrulesandmethodologiesapplicabletoeachofthetestmethods,basedonthedatacurrentlyavailable.GeneralstandardtestmethodsforballpullandsheararealsoavailableviaJEDEC.[7,8]
3.1TestCoupons
IfdirectcomparisonsbetweentwoPCBmaterialsareneeded,identicalfinishedpaddiameters,solder-maskopenings(seeFigure3)andboardthicknessmustbeused.Astandardizedtestcouponcanbeusedforalltestmethodslistedinthisguideline.Padconstructionsonpartsofproductionboardscanalsobeanalyzed.
Theuseofisolatedpads(withoutanyconnectingtraces)ispreferredonatestcoupontominimizevariationsduetotraceconstructiononpadcrateringsensitivity(peakloadvalues).
Non-standardtestpatternsmaybeusedforcomparativePCBPadCraterSensitivitytestsbetweensimilarformfactors,ortomeasureimpactofprocess&CudesignchangesonacommonPCBresinmaterial.Non-standarddatawillcontainmorevariationandwillbeimpactedbypadsize,tracedesignandothertestboarddesignfeatures.
Thetestboardshallhaveatleast3fiducials(registrationmarkers)and4alignmentholes.
Thethicknessofthetestboardcouponshouldbeatleast2.35mm[93mils]-toreduceanyeffectsoflocalizedbendinginthepulltestfixture.Nocoppertracesarerequiredintheinnerlayersofthetestboard.
PackageSubstrate
CopperPad
PackageSubstrate
SolderMask
SolderBall
PWBPad
PCB
PCB
SolderMaskDefined(SMD)
Non-SolderMaskDefined(NSMD)
Figure3:
DefinitionofPadConstruction
Alltestboardsshouldbeserializedpercouponandperpanel.Testboardsshallbefabricatedinpanelformandshallnotbedepanelizeduntilaftersolderpasteassembly.Aclearanceofapproximately0.100”to0.250”shallbekeptbetweenthepadpatternandtheedgeofthedepanelizedPCBcoupon.
Theactualpadsizeandsoldermaskopeningsizeshallbemeasuredonatleast3locationsoneachcoupon,todeterminethevariationinfinishedpadsizeandsoldermaskopening.Therecommendedetchtoleranceis+/-0.0254mm[1mil]or10%,whicheverisless,whenmeasuredatthebaseoftheBGApad.
Whencomparingacrossdifferenttestsplits,thesamePCBsurfacefinishshallbeusedacrossthetestsplits,exceptwheredifferentsurfacefinishesaretobecompared.
3.2PadCrateringTestMethodComparison
Eachtestmethodhasitsbenefitsandtrade-offs.Someofthebenefitsandtrade-offsofthethreetestmethodsoutlinedinthisdocumentaresummarizedinTable1.
TestMethod
Pros
Cons
1.
Canbe
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