Predicative reliability assessment for electronics packagingIMAPS.docx
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Predicative reliability assessment for electronics packagingIMAPS.docx
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PredicativereliabilityassessmentforelectronicspackagingIMAPS
PredictiveReliabilityAssessmentandFailureRateModelsforElectronicPackages
LiyuYang1,JosephBernstein2,DonGajewski1,JamesWalls1
1FreescaleSemiconductorInc.,1300N.AlmaSchoolRd.,Chandler,AZ85224,USA
2ReliabilityEngineering,UniversityofMaryland,CollegePark,MD20724,USA
B07626@
Abstract
Traditionalpackagetechnologyevaluationprocesswouldnotcharacterizethefailuremechanismsaswellfailureratesverywell.Inmostofthecases,thefailurerateobtainedfromtestingwasnotwellcorrelatedtothatinthefieldapplication.Packagetechnologiesweretypicallycertifiedusingstandardacceleratedstresstests,suchasthosefoundinMIL-883orJEDECstandards.Thetestconditionsanddurationsaredefinedbasedoneffectivefieldlifetime,accelerationfactors,ortherecommendationsfromthespecifications.Inaddition,thetestswerenotdesignedtogeneratefailures,instead,thedesiredtestingresultswerezerofailures.Thisapproachgaveverylittleinsightsintothefailuremechanismsandfailureratesinthefieldaswelllifetimeofthetechnologyinterested.
Anewphysics-basedapproachforpackagereliabilitylifetimepredictionandfailureratemodelingwouldbediscussedinthispaper.Thisapproachwouldyieldacompletereliabilitymodel,includingtheaccelerationfactor,lifetimeprojectionsatoperatingconditionsandfailureratecorrelations.Themodelswillbemoreobjectiveandhelpfultomeetresponsetorapidlydevelopingmarketdemandsandquickestimationofpackagereliabilityundervariousconditions.
Casestudiesofthereliabilitywoulddemonstratetheneedsforthenewphysics-basedpackagingtechnologyqualificationapproach.Thedisadvantagesofthetraditionalreliabilitytestingapproachandtheneedsoftoday’srequirementsforlowcost,quick-to-marketandspecializedapplicationswerediscussed.Additionally,itpointedoutthatfailureswereoftenhappenedrandomlyataconstantfailurerate.Keyfailuremechanismsinthepackagewereoftendefectdriven,andseldomshowedincreasingfailurerateduringthetestduration.Morestudieswillbecarriedoutinthefuturetobuildthedatabaseneededforpackagereliabilitymodelpredictions.
KEYWORDS:
Package,reliabilitymodels,acceleratedfactor,failurerate,failuremechanisms.
Introduction
Reliabilityisdefinedastheprobabilityofoperatingaproductforagiventimeperiodunderspecifiedconditionswithoutfailure.Ingeneral,thestatisticalanalysisoffailuretestsresultsandtheuseofdistributionfunctionstodescribepopulationthatfailisessentialpartofthereliabilityanalysis.Itshouldbepointedoutthatitwasnotfeasibletopredictthelifetimeofanindividualelectroniccomponent,insteadanaveragelifetimecanbeestimatedbasedonthetreatinglargepopulationofcomponentsprobabilistically.Keyquantitativemeasureofreliabilityisthefailurerate,whichistherateatwhichadeviceorcomponentcanbeexpecttofailunderknownuseconditions.Inordertoestimatethefailurerate,accelerationfactorandnumberoffailureswereneededinordertocalculateaccuratefailurerates.
Inpractice,reliabilityassessmentofelectronicpackagesoftenmeanstestingpackagesusingestablishedJEDECorsimilarteststandards,suchastemperature/humiditytestsortemperaturecyclingtests.Ifatestwasconductedandnofailureseenatthecompletition,wewillclaimthatthepackagespassreliabilitytestingandthetechnologyiscertified.Iftherewerefailuresseenduringthetesting,therootcauseswouldbeinvestigatedandnewsampleswouldbebuiltfornextroundreliabilitytesting.Testswouldbesuccessfulonlyiftherearezerofailuresortheriskisfeltlowbasedontheuseconditionandtherearemeasurestocontaintherisk.Inadditiontoit,therearehugevariationsofsamplesizeusedinthereliabilitytestingpractice.Asamplesizeof45,or77and230willtellusdifferentreliabilityinformationevenallofthemyieldzerofailuresduringthetests.Thisreliabilityapproachisverydifferentfromthedefinitionofareliabilitystudy.
Moreover,typicalreliabilitytestapproachdoesnotfollowthroughtocalculatethefailurerateinfielduseconditions.Theobjectiveistounderstandthefailuremechanismandbuildbettersamplesinordertoachievezerofailuresduringthefinalreliabilitytesting.Insomecases,acceleratedfactorsmightbecalculatedbasedonassumedfailuremechanismanditsactivationenergywhichmightnotbesuitableforcurrentfailuremechanisms,sotheaccuracyoftheAFcouldbeaquestion.
Reliabilitystudyforsolderjointwearoutfatiguestudyhasbeenanexception.Alotofdatahasbeenaccumulatedinthesolderjointstudyusingreliabilityapproach.Thesearenotthecaseforotherfailuremechanismsseeninpackagingworld,wherefailuresareusuallyduetoprocessorassemblydefects,thefailuresaremoreseenasrandomfailures.
Theneedsforfuturepackagereliabilitystudywillneedtodefinethesamplessizeandtestconditionsbasedonuseconditionsandcontinuethetestingtoachievecertainfailuresinordertocalculatethefailurerateforcertainfailuremechanisms.Itisalsousefultodevelopfailuremodelswhichcanbeusedforfuturesystemfailureestimationandreliabilityestimation,orestimatethesystemreliabilitybasedonfailurerateofcomponentsofthesystem.
Thefailurerateapproachforpackagereliabilitystudywillhelpussuccessfullyestimatethefailurerateinthefield..Theemphasisistotesttocollectfailuredatainordertounderstandcrucialfailuremechanismsandhelpgeneratefailuredistributionsandreliabilitytimeinthefield.Thefailureratemodelwouldgeneratetremendousamountofdataforfuturecalculation.Accelerationfactorisacriticalingredientinthefailureratecalculation.ThefocuswastogeneratemodelstoestimatetheAFundervarioustestingconditions.Ithastobefailuremechanismoriented.
Literaturereview.
a.Reliabilitytestingapproaches,failuremechanismsandfailuremodels
Onetypeofreliabilityapproachistoreallyinvestigatetheintegrityofthepackageinordertounderstandthefailuremechanismsandfailurelocation.Bychangingthematerialsselectedorthepackagegeometry,ahighintegritypackagecouldbeachieved.Typically,therewasnocommonreliabilityknowledgeused,suchasstatisticalanalysisorfailuredistributions.Theresultsusuallytellsyouthatacertainmaterialsorcertaindesigngeometriesaregoodtoimprovetheintegrity,butthereisnoknowledgeabouttheexpectedreliabilityimprovementandfieldlifeexpectations.[YaominLin,2005]inthereliabilitystudy,theauthorslookedonetypesoffailuremechanismsexpectedtoseeinflipchippackages-underfillcrackingassociatedwithsubstratedielectricandcoppertracecracking.Temperaturecycling(-65C+150C)aresimulatedastheloading.
Reliabilityapproachwasalsousedtoevaluatetheeffectivenessofvariousacceleratedstresstesting,includingcomparingthetestconditions,andestimatetheaccelerationfactorsbetweenvarioustestconditionsanduseconditions.[HelenCui,2005]demonstratedtheapplicationofCoffin-Mansonequationtocalculatetheaccelerationfactorsinthetest.Viacrackingisthemainfailuremechanism.Dwelltimeandrampratearecriticalfeaturesforthedamageontheparts,butCoffin–mansonequationdidnotelaborateittoomuch.Itmightnotbeeffectivetodifferentiatethethedifferencesbetweenthermalshockandthermalcyclingtesting.Activationenergyiscriticalforthefailuremechanismandmodels,butitisoftenassumed.Inthispaper,theauthorcalculatedtheAFbasedonthetestdata.Itwasalsorecommenedtohaveat5readpointtohaveagoodWeibullanalysis.TheBetaparameterofWeibullanalysisisaround1-2.
Solderjointreliabilitystudy:
[J.C.Suhling,2004]studiedthesolderjointreliabilityperformanceofceramicchipresistersundertemperaturecyclingconditions.Failuremechanismsandfailurelocationswereidentified,failuredatawereplottedinWeibullandBetaandcharactisticlifewerecalculated,however,theauthorsdidnotcontinuetoproceedtolookforaccelerationfactorsandactivationenergies.Thereisnofailuremodelsgeneratedinordertohaveabetterunderstandingthefieldfailures.TheBetaforthesolderjointcrackingisaround2-7forvarioussolderalloys,surfacefinishandtestconditions.
b.Accelerationfactorscalculationcriteria(singlefailuremechanismandmultiplefailuremechanisms)
c.Fielddatafailureratesmodelsforfailuremechanisms
Thedifficultyoftheapproachistogetgoodestimationofaccelerationfactors.TheAFcalculationcouldbedifferentifsingleandmultiplefailuremechanismswereused.
Thefailuresseeninelectronicpackagescanbeattributedintothefollowingcategories,whichmightnottypicalcomponenttotalfailures.
ReliabilityTests
Reliabilityisaprimaryconcerntotheuserandismeasuredfromthetimetheproductisputinservice.Itcanbeexpressedas
P(f)istheprobabilityoffailure.Toachievereliability,itisnecessarytounderstandandreduceunreliability.
Indefiningtheprobabilityoffailure,thefocusisonthestatisticaldistributionoffailuresratherthanthephysicalorchemicalphenomenathatcausethefailures.Inthebathtubcurve,theflatmiddlesectionisaconstantfailurerateduetorandomfailures-whoseexpectedtimeperiodofoccurrenceisunpredictableandindependentofprioruse.Wear-outisusuallyalifelimitingphysicalorchemicalprocesswhichisinherentlyrelatedtothedesignofthepa
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