EliminatinginterfacialsegregationandembrittlementofbismuthinSnBi/CujointbyalloyingCusubstrateH.
F.
Zou,Q.
K.
ZhangandZ.
F.
Zhang*ShenyangNationalLaboratoryforMaterialsScience,InstituteofMetalResearch,ChineseAcademyofSciences,Shenyang110016,ChinaReceived3March2009;revised3April2009;accepted4April2009Availableonline12April2009ThesegregationofBiattheCu/Cu3SninterfacesoftheSnBi/Cucoupledramaticallydecreasesthiscouple'smechanicalprop-erties.
Here,wedeliberatelyaddAg,Al,SnandZnelementsintotheCusubstratetoeliminatetheinterfacialsegregationandembrittlementoftheSnBi/Cucouple.
ExperimentalresultsconrmedthatthereisalwaysaperfectCu3Sn/CualloyinterfacewithoutBisegregation,andexcellentmechanicalpropertiesarethusmaintained.
Themostimportantndingisthattheinterfacialembrit-tlementofSnBi/Cualloyjointswassuccessfullyeliminatedevenafterprolongedaging.
2009ActaMaterialiaInc.
PublishedbyElsevierLtd.
Allrightsreserved.
Keywords:Biinterfacialsegregation;Embrittlement;Pb-freesolder;Interfacialstrength;SolderingSolderingisaveryimportantprocedureintheelectronicpackagingeld[1].
Lead-freesolders,thoughpromising,presenttwoseriousdefects:(i)theirmeltingpointsaremuchhigherthanthoseoftraditionalSnPbsolders[1];(ii)BisegregationattheCu/Cu3SninterfacealwaysoccurswiththeSnBi/Cucouple[2].
Thesegrega-tionofimpuritiesalwaysseriouslydeterioratesthephys-icalandmechanicalpropertiesofthematerial[3,4],asdoesBisegregation.
Thisiswhy,intheelectronicpack-agingeld,useoflead-freesoldersisstillnotwidespreadeventhoughlegislationtolimittheuseofSnPbhasbeenextantinmanycountriesformanyyears[1].
Inordertoextendtheapplicationoflead-freesolder,onemust:(i)decreaseitsmeltingpoint;or(ii)restricttheBisegregationattheSnBi/Cuinterface.
Researchre-sultsconcerningtheformerarenotpromising[1,5].
Aboutthelatter,LiuandShang[6]andZhuetal.
[7]havetriedtoemployelectrodepositionofAgorNithinlmsontotheCusubstratetosuccessfullypreventtheinterfacialembrittlementofSnBi/Cuinterconnectsevenafterlongagingtimes.
Butthisapproachcannotbeusedoncertaininterfaceswhereitisdicultorimpossibletoelectricallydepositthinlmsontheinterfaces.
Historically,thepoorductilityofpolycrystallineNi3Alhasbeensuccessfulim-provedbyaddingB[8],andthisapproachhasinfusednewlifeintotheresearchandapplicationofintermetallicsinthepasttwodecades[9–12].
Inthiscontext,therefore,thedeliberateadditionofAg,Al,Sn,ZnelementsintoCusubstratewasexpectedtorestraininterfacialBisegre-gation.
AmpleexperimentalevidencehasindicatedthatalloyingCusubstratecansuccessfullyeliminateBisegre-gationattheSnBi/Cuinterface,leadingtotheimprove-mentofinterfacialmechanicalproperties.
Therefore,intheelectronicpackagingeld,thisstrategymightenableSnBisoldertobewidelyusedandinthefuturereplaceSnPbsolder.
Moreimportantly,themethodprovidesanewvisualizationtounderstandandeliminateotherinter-facialsegregation(suchasP,SorSbinsteelsandBiinpoly-Cu).
Inthisstudy,pureCuandCualloys(Cu–2.
3Al,Cu–2.
5Ag,Cu–18.
7Ag,Cu–10ZnandCu–3Sn)wereusedassubstrates.
EutecticSn–58wt.
%Bialloywasemployedassolder.
Cu,CualloysandSnBialloywerecutandgroundwith800#,1000#,2000#SiCpaperandthencarefullypolishedwiththe2.
5and1.
5lmpol-ishingpastes.
Allthepreparedsampleswerekeptinanovenataconstanttemperatureof200°Cfor6min.
Onegroupofsampleswasisothermallyagedat120°Cfordierenttimestorevealtheinterfacialmicrostruc-tureoftheSnBicouples.
SometensilesamplesofSnBi/CuandSnBi/Cu–Xjoints(X=Al,Ag,ZnandSn)wereisothermallyagedat120°Cfordierentnum-bersofdaysinordertoinvestigatetheinterfacialmechanicalpropertiesofthejoints.
Tensiletestswere1359-6462/$-seefrontmatter2009ActaMaterialiaInc.
PublishedbyElsevierLtd.
Allrightsreserved.
doi:10.
1016/j.
scriptamat.
2009.
04.
009*Correspondingauthor.
Tel.
:+862423971043;e-mail:zhfzhang@imr.
ac.
cnAvailableonlineatwww.
sciencedirect.
comScriptaMaterialia61(2009)308–311www.
elsevier.
com/locate/scriptamatperformedwithanInstron8871testingmachineatanaveragestrainrateofabout5105s1atroomtem-peratureinair.
Thefracturesurfaceswereobservedbyscanningelectronmicroscopy(SEM)torevealtheirdeformationandfracturemorphologies.
Figure1showstheclose-upbackscatteredelectronSEMimagesoftheinterfacesintheSnBi/CuandSnBi/Cu–X(X=Ag,Al,SnandZn)couplesat120°Cfordierentperiods.
DiscontinuousBiparticlescouldbeeasilyobservedattheSnBi/Cuinterfacewhenthesamplewasagedat120°Cfor12.
5days,asindi-catedbytheredcirclesinFigure1a.
ThisimpliesthatBisegregationwouldtakeplaceattheSnBi/Cuinterfaceafterprolongedaging,ashasbeendetectedinpreviousstudies[2,6,7].
LiuandShang[6],however,consideredthattheBiatomswouldmainlyappearintheformofamonolayerattheCu/Cu3Sninterface,whichisdier-entfromthecurrentexperimentalresults.
Furthermore,somevoidscanbeobservedneartheBiparticles,asdis-playedbythewhitearrowinFigure1a.
SincetheBisegregationattheCu/Cu3SninterfacecanaectthenucleationandgrowthofKirkendallvoidssimultaneously,thefreeenergybarrierforthevoidnucleationcanbeexpressedas[13]:DG4c3fVr2;1wherer,c,fVarethelocalstress,interfaceenergyandageometricalfactorofthevoid,respectively.
Obviously,rincludesthethermalstressandotherresidualstressformedduringtheagingprocedure.
AccordingtotheGibbsisotherm,theinterfacialenergywouldreducebe-causeoftheBisegregationattheCu/Cu3Sninterface.
Inaddition,thethermalstresswouldincreasewithagingtime.
BasedonEq.
(1),theDGwoulddecreaseastheva-lueofcreduces,andincreasethelocalstressr.
There-fore,asteady-statenucleationrateofvoidswouldoccurduringtheagingprocedure.
Thegrowthofvoidswouldacceleratebecausetheatomsonthevoidsurfacediusetothesolder.
Asaresult,ahighconcentrationofvoidswouldformneartheinterfaceandfurthersupportthegrowthofvoidsduetotheKirkendalleect[13,14].
Incontrast,BiparticlesandvoidswerenotobservedattheSnBi/CuSninterfacewhenthecouplewasagedat120°Cevenfor17days,asdisplayedinFigure1b.
ThisindicatedthatBisegregationwouldberestrainedattheSnBi/CuSninterfacesevenafteralongeragingtime.
Basedontheseexperimentalresults,theinterfacesofSnBi/CuAgcoupleswereinvestigated.
AneCu3Sn/CuAginterfacewasobservedirrespectiveofwhethertheAgconcentrationwashigh(18.
7%)orlow(2.
5%)fortheCuAgsubstrate,asillustratedbytheblackarrowsinFigure1candd.
ThecurrentexperimentalresultsfurtherconrmthatBisegregationwasalsoinhibitedintheSnBi/CuAgcouples.
Inaddition,Cu–2.
3AlandCu–10ZnalloyswereusedtorevealwhetherthesecaninhibitBisegregation.
TheseresultsdemonstratethatBisegregationdidnotoccurattheinterfacesoftheSnBi/CuAlandSnBi/CuZncouples,asillustratedbytheblackarrowsinFigure1eandf.
Averyimportantquestion,however,needstobecon-sidered:wherearetheBiatomsfortheseSnBi/Cu–XcouplesItisassumedthattheBiatomsfromtheSnBisoldershoulddiuseintotheCualloytoformamorestablealloysystemcomparedwiththeBisegregationattheCu/Cu3Sninterface.
Inordertoconrmsuchassumption,anacceleratedexperimentwasdesigned.
TheCu–18.
7AgalloywascompletelysaturatedinliquidBiat500°Cfor6h.
ItisclearlyseenthatsomeAgintheCu–18.
7AgalloywasalmostreplacedbyBi,asillus-tratedinFigure2.
ThisindicatesthatthereplacementofAgatomsbyBiatomsmakestheCu–Agsystemmoresta-blecomparedwiththebinaryCu–Agalloy.
IthasbeenprovedpreviouslythattheAg–Cu–BialloyformstheAg5-Cu0.
5Bi94.
5(at.
%)eutecticphase[15].
Thus,theexistenceofAginCu–AgalloyassiststhediusionofBiatomsintoCu–Agalloy,formingamorestableternaryCuAgBialloyduringaging.
However,thediusionofBiatomsintotheFigure1.
Interfacialmicrostructuresofsamplesagedat120°C:(a)SnBi/Cufor12.
5days;(b)SnBi/Cu–3Snfor17days;(c)SnBi/Cu–2.
5Agfor12.
5days;(d)SnBi/Cu–18.
7Agfor12.
5days;(e)SnBi/Cu–2.
3Alfor17days;and(f)SnBi/Cu–10Znfor17days.
Figure2.
MicrostructureofCu–18.
7AgalloyaftersaturatinginliquidBifor6h.
H.
F.
Zouetal.
/ScriptaMaterialia61(2009)308–311309pureCusubstrateisverydicultbecausetheCu–Bisys-temiscompletelyimmiscible[16–18].
SinceBisegregationhasbeeneliminated,itisneces-sarytomeasurethemechanicalpropertiesoftheSnBi/Cu–Xjointsafteragingfordierentperiods.
Figure3ashowsthetensilestress–straincurvesoftheSnBi/CuandSnBi/Cu–18.
7Agjoints.
TheseresultsdemonstratethatdierentmechanicalpropertiesareobservedfortheSnBi/CuandSnBi/CuAgjoints.
ThetensilestrengthoftheSnBi/Cuisstronglyaectedbytheagingtime(seecurvesAandC).
Figure3bshowsthatthetensilestrengthofSnBi/Cudramaticallydecreasesfromabout155MPaforas-reowedto31MPaafteragingat120°Cfor8.
25days.
WhentheSnBi/Cujointswereagedat120°Cfor13.
5days,thejointswereveryeasytobreakevenduringgrinding,soitstensilestrengthisdeemedtobeapproximatelyzero.
However,thetensilestress–straincurveoftheSnBi/Cu–18.
7Agjointshowsnoobviouschange(seecurvesBandDinFig.
3a).
Thedecreaseinthetensilestrengthisonlyabout10MPawhentheSnBi/Cu–18.
7Agjointsampleswereagedat120°Cfor11days.
SincealloyingCusubstratecansignicantlyimprovethejointstrength,otherSnBi/Cu–Xjointswerealsoinvestigatedinourexperiment.
Figure3bshowsthedependenceoftensilestrengthonagingtimeforalltheSnBi/Cu–X(X=Al,Ag,SnandZn)joints.
Itcanbeseenthatthetensilestrengthdecreasesonlyslightlywithincreasingagingtimeforallthejoints,whichissignicantlydierentfromtheSnBi/Cujoints,asillustratedbythepanelinFigure3b.
Ifonlythemechanicalpropertiesareconsid-ered,theCu–Agalloysarethebestsubstrate.
However,thegrowthkineticsofintermetallicscompounds(IMCs)andthecostoftheproductneedtobeconsidered.
ExperimentalresultshaveconrmedthattheCu3SnwouldnotformattheinterfaceandthegrowthrateofIMCistheslowestfortheCu–Znalloy(H.
F.
Zou,unpublished).
Inaddition,thepriceofZnisthelowestamongthesemetals[1];therefore,theCu–Znalloycanberecommendedasapotentialsubstratefortheapplica-tionoftheSnBi/Cu–Xcouplejoints.
Figure4showsthefracturemorphologiesofthejoints.
ManysoldersandIMCswereobservedonthefracturesurfacesoftheas-reowedSnBi/Cujoints,whichissimilartootherlead-freesolderjoints[19],asdisplayedinFigure4a.
TheinsetpictureofFigure4aindicates,basedonthecross-sectionoffracturemor-phology,thattheIMClayeradheredtoCusubstrate.
Comparedwiththeas-reowedSnBi/Cujoint,thefrac-turemorphologyoftheSnBi/Cujointagedfor8.
25daysisremarkablydierentfromthatoftheas-reowedsam-ples,asdisplayedinFigure4b.
MostoftheCusubstratewasexposedonthefracturemorphology,asillustratedbytheleftinsetdiagramofFigure4b.
TheCu–SnIMClayersadhereattheSnBisolder,butnotattheCusubstrate,asdemonstratedbytherightinsetdiagramofFigure4b.
Theseresultsindicatethatthefractureoc-cursalongtheCu/Cu–SnIMCinterfaceratherthanalongtheCu–SnIMC/solderinterface,leadingtotheinterfacialembrittlement.
ConcerningthefracturemodeoftheSnBi/Cujointagedat120°Cover8.
25days,LiuandShangpostulatedthatthedierenceinsizebetweenCuandBiatomsplaysasignicantroleintheembrittlementasinthecaseofBi–CualloybecausetheBiatomislargerthantheSnandCuatoms[2].
ThepreviousresultsreportedthatthesizeeectcausesBiprecipitationandvoidformationintheCumatrixfortheCu–Bialloy.
BiparticlesandvoidswerefoundonthefracturesurfaceoftheCu–Bialloybecauseofthedierentthermalexpansioncoe-cientsandtheweakadhesionbetweentheCumatrixandBiprecipitates[20].
ThisiscompletelydierentfromthesituationfoundinSnBi/Cujoints.
Thus,theembrit-tlementmechanismofSnBi/CujointsshouldbedierentfromthatoftheCu–Bialloysystem.
Asmentionedabove,BisegregationacceleratestheformationofvoidsinSnBi/Cujoints.
Furthermore,thevoidswouldisolatetheCu/Cu3SninterfaceandthendramaticallydecreasetheinterfacialbondingstrengthoftheSnBi/Cujoint,asconrmedinFigure1aandtheleftinsetdiagraminFigure4b.
However,thetensilestrengthsofallSnBi/Cu–Xjointsdecreaseonlyslightlywithincreasingagingtime,asillustratedbythepanelinFigure3b.
AlljointsbrokealongtheSnBi/Cu6Sn5interface,andsomeSnBisolderwasobservedtoadhereonthefracturesurfacefortheas-reowedSnBi/Cu–18.
7Agcouples,asdisplayedinFigure4c.
ThefracturemorphologiesoftheSnBi/Cu–18.
7Agcoupleshownoobviouschangealthoughtheagingtimeincreasesupto11days,asshownbyFigure3.
(a)Tensilestress–straincurvesforSnBijoints.
AandCrepresentedthetensilecurvesofSnBi/Cujointsagedat120°Cfor0,8.
25days,respectively;BandDrepresentedthetensilecurvesofSnBi/Cu18.
7Agjointsagedat120°Cfor0,11days,respectively.
(b)ThedependenceoftensilestrengthonagingtimeforalltheSnBi/CuandSnBi/Cu–Xjoints.
310H.
F.
Zouetal.
/ScriptaMaterialia61(2009)308–311therightinsetdiagraminFigure4c.
ThemainreasonisthatthevoidwouldnotformattheSnBi/Cu–Xinter-facewithoutBisegregation.
TheleftinsetdiagraminFigure4cdemonstratesthatthecrackrstnucleatedalongtheSnBi/Cu–SnIMCinterface.
IrrespectiveofwhethertheSnBi/Cu–2.
3Aljointswereas-reowedoraged,someSnBisolderstillclungtothesubstrate,asdisplayedbythewhitearrowsinFigure4d.
ForotherCualloysubstrates,thefracturemodewasunchangedforbothas-reowedandagedstates.
Theseresultsfur-therconrmtheeliminationofBisegregationandtheinterfacialembrittlementinSnBi/Cu–Xjointsonthemi-cro-scale.
Insummary,alloyingCusubstratecanessentiallyeliminatetheinterfacialembrittlementanddramaticallyimprovethemechanicalpropertiesoftheSnBi/Cusys-temafterprolongedaging.
ThisstrategynotonlypavesthenewwayforthewidescalefutureuseofSnBisolderintheelectronicpackagingeldinthefuture,butalsoprovidesanewmethodtoanalyzeinterfacialsegrega-tioninsystemssuchasBi-dopedCu.
TheauthorswouldliketoacknowledgeQ.
Q.
Duan,P.
Zhang,Y.
Z.
Tian,X.
H.
An,S.
Qu,H.
H.
Su,L.
X.
ZhangandW.
Gao.
Thisworkwasnanciallysup-portedbyNationalBasicResearchProgramofChinaunderGrantNo.
2004CB619306,theNationalOut-standingYoungScientistFoundationunderGrantNo.
50625103.
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Figure4.
FracturesurfacesofSnBi/Cu,SnBi/Cu–18.
7AgandCu–2.
3Al/SnBijoints.
(a)SnBi/Cujointas-reowed;(b)agedat120°Cfor8.
25days—theleftinsetdisplaysamagniedmorphology,therightinsetdisplaysthecross-sectionmorphology;(c)as-reowedSnBi/Cu–18.
7Agjointandthejointagedfor11days;and(d)as-reowedSnBi/Cu–2.
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