HOW THE MOLD COMPOUND THERMAL EXPANSION OVERRULES THE SOLDER COMPOSITION...
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HOW THE MOLD COMPOUND THERMAL EXPANSION OVERRULES THE SOLDER COMPOSITION CHOICE IN BOARD LEVEL RELIABILITY PERFORMANCE AUTHORS: B. VANDEVELDE, L. DEGRENDELE, M. CAUWE, B. ALLAERT, R. LAUWAERT, G. WILLEMS SPEAKER: M. TSURIYA (iNEMI)
Transcript of HOW THE MOLD COMPOUND THERMAL EXPANSION OVERRULES THE SOLDER COMPOSITION...
HOWTHEMOLDCOMPOUNDTHERMALEXPANSIONOVERRULESTHESOLDERCOMPOSITIONCHOICEINBOARDLEVELRELIABILITYPERFORMANCE
AUTHORS:B.VANDEVELDE,L.DEGRENDELE,M.CAUWE,B.ALLAERT,R.LAUWAERT,G.WILLEMS
SPEAKER:M.TSURIYA(iNEMI)
OUTLINE
¡ Introduction
¡ Experimental Setup
¡ Results ofthe Thermal Cycling Experiment
¡ Cross-sectional Analysis
¡ Conclusions
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OUTLINE
¡ Introduction
¡ Experimental Setup
¡ Results ofthe Thermal Cycling Experiment
¡ Cross-sectional Analysis
¡ Conclusions
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USEOFLOW-CTEMOLD COMPOUND
¡ Drivingfactors:¡ toreduce thestressonthesiliconchip(CPI:chip-package-interaction)¡ tolowerthemoistureuptake:SiO2 based fillercanreduce theamountofhygroscopicresinto
decrease theMSLanditislessexpensive thanresinfiller¡ Tousehalogen-free “green” compounds which isachieved byhighamountofSiO particles
¡ Asaconsequence¡ theamountofSiO2 particlesintheepoxymatrixincreased uptoafillercontentof85%.¡ Thisleaded toareductionoftheCTEofthemoldcompoundbelow10ppm/°C(CTEofSiO2 is
0.5ppm/⁰C).
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DRIVING FACTORSAND CONSEQUENCES
USEOFLOW-CTEMOLD COMPOUND
¡ Formostpackages, themoldcompoundeasilytakesmorethan50%ofthetotalvolumeofthepackage
¡ So,themoldCTEhasahugeimpactontheaverageCTEofthecomponent
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LOW-CTEMOLD COMPOUNDS
USEOFLOW-CTE“GREEN”MOLD COMPOUNDS
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THECHANGE-OVER TOOKPLACEBETWEEN 2005-2010
0%10%20%30%40%50%60%70%80%90%
Q1 2005
Q3 2005
Q1 2006
Q3 2006
Q1 2007
Q3 2007
Q1 2008
Q3 2008
Q1 2009
Q3 2009
(datafromaleadingsemiconductorsupplier)
Use ofGreenMouldCompounds for plasticpackagesThe introduction ofthe “green” mold compoundsisan indication for the introduction ofthe uselow-CTE versions inthat period. Inrecentyears,also greenmold compoundwithhigher CTEareused through changing the resinmaterials.
USEOFLOW-CTEMOLD COMPOUND
¡ FEMSimulation,byassumingthesamepropertiesforPCB.¡ Testsample:
¡ UsePBGA27x27mm,1.27mmpitch¡ 6versionsofPBGAwithdifferentMoldCTE
¡ TestBoards:¡ Use4differentthicknessofPCBboard(incl.oneflexboard)
¡ SimulationResult:¡ At0.8mmPCBboardassembly,6versionsPBGAfailurerateareno
different.
¡ MoldCTEofthePBGAbecomesveryimportantwhenthePCBislessflexible
¡ AslocalbendingofthePCBbelowthecomponentcanlesscompensatefortheincreasedCTEmismatchbetweenPBGAandPCB.
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INCREASED CTEMISMATCHWITH THE PCB
0
1000
2000
3000
4000
5000
6000
5 10 15 20
MTTF(cycle
s)MoldCTE(ppm/°C)
0.8mm
1.6 mm
2.4 mm
No PCB flexing
FEMSimulationResultExample:PBGA27x27areaarray1.27mmpitch
LOW-CTEMOLD COMPOUNDS
¡ Highermoldstiffnesscanlead tobrittlefracture (inparticular inshockloadings)
¡ Low-CTE moldcompounds alsohaveahigherstiffnessduetothehigherSiOfillercontent
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OTHER NEGATIVE IMPACTS
Each datapoint onthis graph represents acommercially available mold compound
LOW-CTEMOLD COMPOUNDS
¡ Higher mold stiffness can lead to brittle fracture (inparticular inshockloadings)¡ Moreissueswith head-in-pillow due to packagewarpage
¡ ThedifferentCTEofthe mold changed the packagewarpage behaviour during solder reflow
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OTHER NEGATIVE IMPACTS(2)
LOW-CTEMOLD COMPOUNDS
¡ Higher mold stiffness can lead to brittle fracture (inparticular inshockloadings)¡ HigherCTEmismatchbetween copperwireandmoldCTE leadtowirefatiguefractures
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OTHER NEGATIVE IMPACTS(3)
OUTLINE
¡ Introduction
¡ Experimental Setup
¡ Results ofthe Thermal Cycling Experiment
¡ Cross-sectional Analysis
¡ Conclusions
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EXPERIMENTAL SETUP
¡ Test component:¡ 9x9mmQFNwith64pins¡ The solderable exposedpadisabouthalfofthepackage size.¡ Die sizeisroughly4x4mm2
¡ Itmakesthatthispackagehasaratherlowdietopackageratio.¡ Threeversions aremade:
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PACKAGEDESCRIPTION
MoldCTE LeadTipPlating WettableFlank
QFN#1 7ppm/°C CuExposed Yes
QFN#2 7ppm/°C E’less Tinplating Yes
QFN#3 12ppm/°C CuExposed No
WettableFlank
EXPERIMENTAL SETUP
¡ PCB:¡ 8-layerboardswiththe6innerlayers
completely filledwithcopper¡ tomaximizethestiffnessoftheboards¡ ToobtainaCTEasclosetothatofCuaspossible
¡ The totalthicknessis2.4mm.¡ Assembly:
¡ Twodifferent soldermaterials:SnPb andSn3%Ag0.5%Cu.
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PCBANDBOARDASSEMBLY
2.4mmPCB
EXPERIMENTMATRIX
QFNtypeMold CTELeadTip
Wettable Flank
SolderPaste
SnPb SAC305
QFN17ppm/ ⁰CCuExposed
WettableFlank0to 100°CTC 0to 100°CTC
QFN27ppm/⁰CTinplating
WettableFlank0to 100°CTC 0to 100°CTC
QFN312ppm/⁰CCuExposed
Non -Wettable0to 100°CTC 0to 100°CTC
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EXPERIMENTAL SETUP
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THERMAL CYCLING TESTCONDITIONS
¡ The IPC-9701TC1test condition forsolder jointevaluation wasselected asthemostappropriate test.¡ 0to100⁰Cthermalcycling(air-to-air)¡ Totalcyclingtime=1hour
¡ rampuptime=10minutes¡ dwell-time=20minutes
¡ In-situmeasurement foropens
-20020406080100120
0 20 40 60 80 100 120 140 160 180 200
Tempe
rature
(°C)
Time(min.)
OUTLINE
¡ Introduction
¡ Experimental Setup
¡ Results ofthe Thermal Cycling Experiment
¡ Cross-sectional Analysis
¡ Conclusions
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THERMAL CYCLING TESTRESULTS
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WEIBULL DISTRIBUTIONSQFN type Solder N63% β
QFN #1 SnPb 971 4.4
QFN #2 SnPb 1720 4.8
QFN #3 SnPb 5323 3.0
QFN #1 SAC305 1682 6.3
QFN #2 SAC305 1901 3.7
QFN #3 SAC305 5604 3.0
The results clearly reveal that the mostdominant parameter is the CTE of themold.• For SAC305 assemblies, the difference
between 7 ppm and 12 ppm/C QFN’s isa factor three.
• Solder material and flank wettabilityhavemuch less on the BLR.
OUTLINE
¡ Introduction
¡ Experimental Setup
¡ Results ofthe Thermal Cycling Experiment
¡ Cross-sectional Analysis
¡ Conclusions
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CROSS-SECTIONAL ANALYSIS
¡ Forallanalyzedsamples,oneorevenallfourcorner jointsshowedfractures leading tothedaisychainresistance increase. ¡ Thisconfirmsthatthefractureisinducedbythe
CTEmismatchbetweenQFNandPCB,whichishighestinthe4corners.
¡ The fracturewasalwaysinsidethesolderjoint,andnotattheintermetallic compoundsnorthecopperpaditself.
¡ The solderfracturesareinduced throughlow-cyclefatigueduringthetemperature cycling.
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GENERAL FINDINGSCreep strain (-)
CROSS-SECTIONAL ANALYSIS
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QFN#1:MoldCTE:7ppm/°C; Cuexposedtip,Wettableflank
SnPb solder paste Failing after 971 cycles
• This assembly showed the earliest failures.• It is related to the non-optimal wetting resulting in almost all solder at the side,
however, not really functioning as a strong fillet.
CROSS-SECTIONAL ANALYSIS
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QFN#2:MoldCTE:7ppm/°C;E’less Tinplatingontip,WettableFlank
SnPb solder paste Failing after 1720 cycles
¡ Thankstothefillet,thissolderjointcanresistmorecycles
¡ Cross-section showsstrongdamagewhichisduetotherepeated largeexpansion mismatchbetween QFNandboardineach cycle,after ithasalready failedmuchearlier butwhilethetestwasstillongoing.
¡ Thiscanonlybeexplained bythelow-CTEmoldcompoundusedinthisQFNpackage.
CROSS-SECTIONAL ANALYSIS
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QFN#1:MoldCTE:7ppm/°C; Cuexposedtip,Wettableflank
SnAgCu solder paste Failing after 1682 cycles
¡ While theSnPb solderedQFNshowedverylowstand-offheightsatoneside,theSAC305solderedQFN’shasaquiteuniformstand-offheightresultinginaboutdoublelifetime
¡ The fracturesarefoundneartheinterfacewiththeleadofthecomponent, however,stillinthesolderjointitself(noIMCfailure).
¡ Alsohere, quitesomedamagewereseenwhichisrelated tothelarge expansionmismatchbetween QFNandPCB.
CROSS-SECTIONAL ANALYSIS
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QFN#2:MoldCTE:7ppm/°C;E’less Tinplatingontip,WettableFlank
SnAgCu Solder paste Failing after 1901 cycles
¡ Similarconclusion aswithnon-wettableflankQFN
¡ Andasthesolderjointshapeisalsosimilartothenon-wettable version,weindeed canexpect aboutthesamelifetime.
CROSS-SECTIONAL ANALYSIS
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QFN#3:MoldCTE:12ppm/°C;Cuexposed ontip,non-wettableflank
SnPb Solder paste Failing after 5323 cycles
¡ TheQFNwiththe12ppm/°Cmoldmaterialsurvivesaboutthreetimemoretemperature cycles.
¡ The fractureisvisible,butthere isnotsuchalargedamageasseenwiththepreviouscases.
CROSS-SECTIONAL ANALYSIS
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QFN#3:MoldCTE:12ppm/°C;Cuexposed ontip,non-wettableflank
SnAgCu Solder Paste Failing after 5604 cycles
¡ Atinycrackisseenoverthewholelength.¡ BothSnPb andSAC305solderpastecan
surviveslonger than5,000cycleswhen12ppmCTEmoldcompound isusedinencapsulation.
OUTLINE
¡ Introduction
¡ Experimental Setup
¡ Results ofthe Thermal Cycling Experiment
¡ Cross-sectional Analysis
¡ Conclusions
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CONCLUSIONS
¡ The conclusionoftheelaborated testwasthattheimpactofthesoldermaterialandflankwettability(electroless vsCuexposed) wasminimum(lessthan20%ofdifference) whilethemoldcompoundhadahugeimpact. Thecharacteristic lifetimeoftheQFN’swith12ppm/⁰Cwas3timeshigher thantheQFN’swith7ppm/⁰C.¡ Adifference insolder qualitywasseenwithnon-uniform stand-offheightsforSnPbsoldered
versusuniformstand-offheightswithSAC305solder.¡ Thisresultedinatwotimeshigher lifetimefortheSAC305solderedQFN.This wasonly
experienced with the Cuexposed QFN’s.¡ Forhighreliabilityapplicationsandelectronics operatingunder severeconditions, thismold
compound change createsamajorreliability concern andrequiresthoroughevaluation.¡ The impactonelectronics reliabilityisconsiderably greaterthanthatofachange insolderalloy
butasyetdidnotgetasimilarlevelofattention.
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NEXTSTEPS
¡ Perform abroader experimental programto demonstrate the impactofthe mold CTE, andbenchmark its effect to other parameterssuch asD2Pratio,boardstiffness and solder composition
¡ Characterisation techniques to measure the mold CTEofcommercial components¡ Explore mitigation techniques
Inordertogetabroaderandgeneric viewontheimpactofthemoldCTEonBLR, andtocreateawarenesswithintheworld-wide industry,INEMI initiatedaprojectonthistopicwithmajorplayersintheelectronics assemblyindustry.
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INEMINEWCOLLABORATIVE EFFORTS
