shock and thermal cycling synergism effects on

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Shock and Thermal Cycling Synergism Effects onReliability of CBGA AssembliesReza Ghaffarian, Ph.D.Jet Propulsion LaboratoryCalifornia Institute of Technology

Pasadena, CaliforniaReza. [email protected] 18-354-2059

AbstractBall GridArrays BGAs)areno wpackages of choiceespeciallyorigher' I/O countsorommercialapplications nd are alsobeing onsidered or use inmilitaryandaerospace.ThermalcyclingcharacteristicsofBGA ssemblieshavebeenwidely eported ncluding 'thoseyhe PL -le d consortium.hermalyclingrepresents heon-offenvironmentalcondition ormostelectronic products and therefore is key factor that definesreliability. As aesults, muc h datavailableoraccelerated hermal ycle onditions, but very imiteddata on vibration and shock representative of aerospaceapplications.TestvehicleswithdaisychainplasticandceramicBGAs (CBGA s) ranging rom256 o625 I/Ocountereubjectedoandom vibratiodshockrepresentativeofaspacecraft aunchenvironment. Theeffect of board rigidity on behavior was also investigatedby adding trips to or bonding of board to an Aluminumplate . This paper compares accelerated hermal cycles-to-failure data under four tem perature ranges before andafter thermal random vibration for CBGAs with 361 and62 5 I/Os. Stress and strain projections by finite elementanalysis are also presented.

TABLE F CONTENTS1. ACCELERATEDNVIRONMENTAL TEST2- TEST PROCEDURES3- TEST RESULTS4- CONCLUSIONS5- REFERENCES6- ACKNOWLEDGMENTS7-BIOGRAPHY

1- ACCELERATEDNVIRONMENTAL TESTIntroductionThereremanyurposeferformingcceleratedenvironmentalerificationndestingrogram forelectronics assemblies including:0 Qualification of design for n-service conditions

0 Simulationof n-service estconditionor oprojectDefinitionfanufacturingariablesndheir

0 Screening ormanufacturingdefects0 Demonstration of quality and reliability of a design0 Demonstration of suitability for the intended useForelectronics in commercialapplications,commonly,thermalcycling estsareperformed o imulateordoffcondition. However, most electronic systems are exposedto other environments ncluding mechanical fatigue andrandom vibration. Vibration occurs during transportationand mechanical fatigue by repeated use of key punchingforportable lectronics.Occasionalhigh hock ouldoccur due to accidental drops. Drop test and mecha nicalfatigue re tarted to be onsidered for qualificationelectronic ssemblies speciallyor ewer hipcalepackage assemblies.In dditionomuchharsherhermal equirement oraerospace and military applications, generally assembliesare required o meet sever dyna mic loads and vibrationfatigue ycling.Thereforeheres trong eedounderstand ehaviororuchtress onditions. Verylimiteddataareavailable orvibrationandespeciallyshockehaviorfBG Assemblies. To understandbehavior for space environment, several test vehicles withplasticandceramicBGAswere ubjected odynamictesting epresentative of a aunch, 3 minutes at 3 axisvibration/shock. Each test vehicle had four package typeswhichwerecommerciallyavailableandhad I/Os from250 to 625 including both plastic and ceramic BGAs.

Synergismfhermalycleepresentativef thefollowing nvironmental onditionswere onsidered: '0 Prelaunchhermalyclingueomanufacturing,repair, screening, storage, and transportation0 Dynamicestingepresentative of launchenvironmenthichncludeinusoidalibration,transient vibration, pyroshock, and acoustic0 Thermalyclingepresentative of internal andexternal temperature change of a spacecraft

lifeeffects
mailto:[email protected]:[email protected]


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Effects of several variables w ere conside red.a) Therma l cycling before vibratiordshock and afterb) Increase in rigidity of board by addition of thin rigidstrips i n one caseandbondin g to a rigid plate inanother casec)Change in thermalhistorycondition by exposure othermal cycles before vibration/shock

Thisaperresentsxperime ntal results aswell asanalyses or wo CBGA assemblies (361 and625 I/Os)subjected to either or both thermal cycling and vibration.Refer to the JPL BGA Packaging G uideline (Reference 1)for details on design, package daisy chain, board layout,manufac turing processes, nspection results, and hermalcyclingehaviorferamicnd plastic packag eassemblies.

Literatureeview .Cycles-to-failure after vibration of CBG As and column(CCG A) were presented by M. Cole of IBM (Reference2). Balls and columns were 90 Pb/lO Sn with 0.89 mm(.035 inch) in diameter for CBGA and for CCGA0.5 mm(.02 nch) in diameter2.2mm (.050 or .OS7 inch) inheight.Packageswereassembled on FR-4 boards withsingle est specimens mounted on 11 0 x 90 mm board,clamped by screws at 101 x 75 mm locations. Mil-STD8 1OE were used to generate impact and vibration data fortheir test vehicle.No failure ofCBGAsan dCCGAs wasobservedaftervibrationwithheatsink of73 g.CC GAs with heatsinkweights o f 100 and 150 gram s failed whereas CBGAs didnot.Crackswerenduced in CBG As in the utecticsolder either in package or board sites when subjected in20-2000HZwith7.73grms. twas eportedhat orCBGAs,racknitiationswereimilarohosefacceleratedhermalyclingATC),utith nodeformationypically resent in ATC .Also,hermalmismatch nduceboth hearand ensile,bu tvibrationinduce primarily tensile and not cause deformation .It was observed that CBGA assemblies with heatsinks oflower weight that than 150 gram s which were subjectedto shock and vibration did not show any degradation inthermal atigue life, no statisticaldifferencesbetweenthose with initial shock and vibration and hose withoutan y a priori test were observed.In anotherstudy(Reference 3) , CBGAs with 256 IiOs,625, and 1089 I/Os were subjected to thermal cycling andvibration evaluation. The 1089 was fabricated with ballsrather commonly supplied n column since they were sedonmetalmatrix estrainingcore ather than polymericboard such as FR-4 . For restrained board with a CT E (6'ppm/"C) close to the ceramic package 7.2 ppmPC), local

mismatch between solder to package/board considered toplay a primary role than global mismatch. Assemblies onrestrained board were subjected to therma l cycling in therange of -55"C/125OC (a 5 "C/min ramp and a 30 minutedwellateachextreme)and heyreportednofailures o1,500 cycles.They also performed vibrations on three CBG As at threelevels:0 Level I was theilitaryvionicsquipmentworkmanship vibration level of Environmental StressScreening (ESS)0 Level I1 was n nveloped ubassembly ased nseveral avionics programs0 Level 111 was the same as level I1 with power spectraldensity (PSD ) increased by 3 dB .Test results are listed in Table 1 . All assemblies passedthe level I whereas only CBGA with 254 I/O passed theLevel I1 vibration.Finiteelementprediction or evel Iagreed with the expe rimental results, but for other levelsha d a mixed agreement.

2. TESTPROCEDURESTest VehicleThe test ehicles in thisnvestigationncluded bothplastic and ceramic packages on either FR-4 or polyimideprinted circuit board (PWB) with six layers, 0.062 nchthick.Ceramicpackageswith 625 I/Os an d36 1wereincluded in our evaluation. Solder balls for CB GA s hadhighmeltemperatureomposition90Pb/lOSn)ndabout 0.035 inch diameters.Thehighmeltballswereattachedohe eramic ackagewith utectic older(63Sn/37Pb). At reflow, package side eutectic solder andthe PWB side eutectic paste will be reflowed to providethe electro-mechanical interconnects.TheCBG As had internaldaisy hainswhich made aclosed loop with daisy chains on the PW B enab ling themonitoringof older oint failure hrough ontinuouselectrical monitoring. Daisy chains for CB GA 625 werein the ring form center to peripheral in order to identifyfailuresites with increased in therma lcycles. The firstfailure is know n to occur from the periphe ral ring in thecorner solder joints with the maximum distance to neutralpointDNP). Tomprovessembly reliability, th esupplier had removedackagenternalaisyhainconnectionsamong a fewcornerballs;excluding hemfrom assemblyailureetectionuringlectricalmonitoring.Thismeans hatcycles to first failuredatacannot be directlyorrelated to packageiagonaldimension which usually are assumed to be eq ual to themaximum DNP.


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Thermal Cycling ConditionsFour differenthermal ycle rofileswere used.These were:Cycle A : Thecycle A condition ranged rom -30 oI00"C and had an increaseldecrease heating rate of 2to S"C/min and dwell of about 20 minutes at the hightemperature oassurenearcompletecreeping.Theduration of each c ycle wa s 82 m inutes.

0 Cycle B: TheCycle B condition anged rom -5 5 to100Cwithon gime uration.The eating ndcooling rates were 2 to 5C per minute with an ovendwell etting f 5minutes t the two extremetemperatures. Th e duration of eachcyclewa s24 6minutes.0 CycleC:Thecycle C condition anged rom 55 o125 "C with2-5"C/min h eating koolin g rate. Dwellat extreme temperatures were ateast 10minutes withduration of 159 minutes for each cycle.0 CycleD:Thecycle D condition anged rom -55 to125"C, the same as condition C, but with very highheatingkooling rate tcouldalso be considereda

thermal shock since it used a three region chamber:hot,ambient,an dcold.Heatingan dcooling rateswere nonlinear and varied between 10 to 15 "C/min.withdwellsatextreme emperaturesofabout20minutes.Theotal ycleasted pproximately 8minutes.The criteria for an open solde r joint pecified in IPC-SM-785, ect. .0,were sed as guidelines to interpretelectricalnterruptions.enerally,nce the firstinterruptionwa sobserved, here were manyadditionalinterruptionswithin 10% of he ycle life. Thiswasespecially true for ceramic packages.

VibrationThreees tehiclesithhreeevels of rigidityrepresenting hree oading conditions were stacked withspacers and subjected to randomvibration. The bottom

- TVwas ondedo n luminum late,hemiddle adcenter nd dge tiffener trips, ndhe op had nostiffeners. The test vehicles were clamped from he twosides of PWB , the sides with no connecto rs as shown inFigure 1 . The block of three stacked est vehicles weremounted on a very stiff thick AI plate on the shaker tablewith their natural frequencies well above the test vehicleassembly range.Initially, a very low vibration spectrum was sweeped odeterminenatural requencies or he test vehicle.Theboard were subjected to a vibration spectrum in the rangeof 200-2000HZ.

3. TESTRESULTSNatural Frequency Meusurement/ProjectionPlots for natural frequencieser eenerated. Asimplifiedinitelementnalysiswa serformedopredict natural frequencies and stresdstrain condition atsolder oint (Reference 4). Ten percent model dampingwassedorheandomesponsealculation.Projections for the lst, 2nd,nd 3rd natural frequencies arecompared to measured values s listed in Table 2.Damage Inducedfor Thermal Cycling AloneBoth board and package interface cracking was observedwith increasing number o f cycles. Figure 2 show s typicalfailures for the two cycling conditions. Failure under theA conditions were generally from the PW B and for theDconditions rom hepackage ites.Failuremechanismdifferences could be explained either by global o r localModeling ndicates hat hehigh tress egions hiftedfrom heboard o hepackage hemselveswhenstressconditions changed from he global o ocal. For he Acycling, with slow heatkooling ramping, which allowedthe system to reach uniform temperature, damages couldindicate a global stress condition. For he D cycle withrapid heatlcooling, damages could indicate a local stresscondition.

' stress conditions.

Damage Induced with apriori vibrationDamage induced by vibration was different in some casesashown in Figures-4 .ppearance of tensiledeformation rom hecenter of highballs asshown issignificantlydifferent rom hoseobserved or hermalcyclingcondition.Similarly to thermalcyclecondition,damages were more dominant for he balls with higherDNPs,speciallyorheorneralls.owever,additiona l microc racks in eutec tic solder joints, differentfrom the norm for thermal cycling, are induced by tensileand shear load during random vibration.Cycles-to-failurefo r Thermal Cycling AloneFigure 5 shows W eibull plots of c ycles to first failures forCBGA 625 and CBG A 361 under four different thermalcycle conditions. To generate plots, cycles-to-failures fora populationwere anked rom low to highand ailuredistribution percentiles were approximated using median.plottingosition,i = (i-0.3)/(n+0.4) ( e g , seeKapur,1977).Then, wo-parameterWeibulldistributionwas used to characterize failure distribution. Th e Weibullcumulativ e failure distribution was used o fit cycles ofailure data. The equation is

F(N) = 1 - exp (-(N/No)"' )where


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F (N ) is the cumulative failure distribution functionN is the number of thermal cyclesNo is a scale parameter that commonlys referred to ascharacteristic life, and is the number of thermal cy cleswith 63.2% failure occurrence.T hem is the hapeparamete r nd or arge m isapproximately inversely proportional to the coefficient ofvaria tion.(C V) by 1.2/CV; that is, as m increases, spreadin cycles to failure decreasesThisequation, in double ogarithm ormat, esults in astraight line. The slope of the line will define the W eibullshapeparameter.The ycles-to-failuredata in log-logwereittedotraightinendhe two Weibullparameters were calculated.Synergism of Vibration and Thermal cyclingCycles-to-failures orhree ssemblieswithevels frigidityan dapriorivibrationcondition are shown inTable 3 . Two test data for cycles-to-failure after vibrationfor the most sever co ndition, .e. the test vehicles with no

3. Marsico,J.W., AIL, privatecommunications.4. Pitarresi, J., University of New York at Binghamton,5. Kapur, K.C., Lamberson,.R.,eliability inEngineering Design, John Wiley & Sons, 1977

private comm unications.

6. ACKNOWLEDGMENTSThe majority of research described n this publication wascarried out by the Jet Propulsion Laboratory, CaliforniaInstitutef echnology,nder aontractwithheNational Aeronautics and Space Administration.I would like to ack nowledg e the in-kind c ontributions ndcooperative fforts f JPL BGA Consortium on testvehicle design and assembly and thosewho contributed toth erogram.lso,hankso Dr. Jamesitarresi,University of New York at Binghamton for performing; analyticalmodeling.

7- BIOGRAPHYrigiditynhancement,weremarked in Figureorcomparisononly. t is difficult o draw a tatistically Dr. Reza Ghaffarian has nearly 20meaningfbl onclusionbecauseof nsufficient am ple years of industrial and academicexperience in mechanical, materials,size. and manufacturing process

kngineering. At JPL, QualityAssurance Ofice, he supportsresearch and development activities. CONCLUSIONS0 A near-therma l shock in the range of -55C to 125C in S M , BGA, and CSP technologies for infusion intoinduced the most damage on CBGA assemblies, up NASAs missions. He has authored nearly 100 technicalto 50 percent reduction compared to a thermal cycle papers andumerous patentable innovations. Hecondition. received his M S . in 1979, Engineering Degree in 1980,0 Assemblieswithhreeeveligidity assedaunch and Ph.D. in 1982 in engineering f i om University ofrandom ibratiodshock ondition. However,ycles- California at Los Angeles (UCLA).to-failures after vibration affected by th e rigidity ofthe board, a significant reduction (>50%) for a lessrigidndminimumeductionorighlyigidassemblies after vibration.0 Failures for hermal cycled CBGAs were either fromboard or package sites in the eutectic solder joints.Both tensile eformation in highmelt alls andtensile nd hear ailures in eutectic older wereobserved for assemblies subjected to vibration.

5 RE FE RE NCE Si . Ghaffarian, R. BallridrrayackagingGuidelines, distributed by Interconnec t Techn ologyResearchnstituteITRI),ugust998,http://www.lTRl.org 2 . Cole, M., Kastberg, E., Matin, G., Shock andVibrationLimits orCBGA and CCGA,SurfaceMount International Proceedin gs, 1996, pp.89-94
http://www.ltrl.org/http://www.ltrl.org/


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Table 1 Vibration and thermal cycling for CBGAsVibration CBGABGABGAesponsenput levelurationlevel

Pass (313)ass (212)ass (414)1.06O1089 I/O25 11056 I/Ogrms) (urns)min)

I1 Pass (013)ass (012)ass (414)70. . ,12.8 1 , \ - ~I

111 NAAass (114)3 18.0720

Table 2 Measured and Finite Elem ent P rojection ofNatural Frequencies an dStresdStrains for CBGA 61

TV Condition Maximum Maximummplitude FrequencyMode 1 (mode 2,3) Strain (pStrain)tress (Psi)g2MZ)Measured Values

Platebonded to A1 8 25IA31553520,4700)EA forVStiffeners I8558 4393 (820,1625)EA for TV withno Stiffener 31 54517 081 (594,795)EA forTVwithfor no Stiffener NIAIA9056 (688,1700)

Table 3 Cycles-to-failures after random vibrationID- Vibration (3 axes)& Thermal Cycles to failureTV Condition Thermal Cycle Type

No Stiffener U nder#50- Failed at 400 cycles50- Vib. + Cycle Bibration #4, Failed between 292& 326 cycles4- Vib. + Cycle B#34- Vib.+ Cycle A #34- No failure to 434

With Stiffeners #33, Failed between 292 and 326 #33- Vib.Cycle A#13- Vib. + Cycle A cycles#13- Failed at 330 cyclestest vehiclesBonded to A1 Plate #3 1 - No apparent change to virgin3 1- Vib.+ Cycle A


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Figure 1 Photos of randomvibration test set up, left photo shows two three-stack of te st v ehicle on vibra tion table andthe right photo shows the enlargedest vehicle with the CB GA361 on the top right corne r

Global CTE Mismatch Local CTE Mismatch-3O"CO 100C - 5 j O C o l25CFigure 2 Cross-sections of ailure sites for CBGA 625 after 50 cycles under A and CBGA 361 under D conditions

Figure 3 Therm ally cycled samples after vibration. Note tensile failure for the corne r ball and m inim um da mage forthc center balls


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damage ata center ball.

100 150 200 25 0 300 350 400 450 500Number of Thermal CyclesFigure 5 Cycles to failure data forwo CBGAs under four thermal conditionswith two assemblies with a priorivibration con dition

shock and thermal cycling synergism effects on

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