turewww

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.
[1]M.
Abtew,G.
Selvaduray,Mater.
Sci.
Eng.
R27(2000)95.
[2]P.
L.
Liu,J.
K.
Shang,J.
Mater.
Res.
16(2001)1651.
[3]A.
Y.
Lozovoi,A.
T.
Paxton,M.
W.
Finnis,Phys.
Rev.
B74(2006)155416.
[4]V.
J.
Keast,J.
Bruley,P.
Rez,J.
M.
Maclaren,D.
B.
Williams,ActaMater.
46(1998)481.
[5]C.
M.
L.
Wu,D.
Q.
C.
Yu,M.
T.
Law,L.
Wang,J.
Mater.
Res.
31(2002)3146.
[6]P.
L.
Liu,J.
K.
Shang,J.
Mater.
Res.
20(2005)818.
[7]Q.
S.
Zhu,Z.
F.
Zhang,Z.
G.
Wang,J.
K.
Shang,J.
Mater.
Res.
23(2008)78.
[8]K.
Aoki,O.
Izumi,J.
Jpn.
Inst.
Met.
43(1979)1190.
[9]D.
A.
Muller,S.
Subramanian,P.
E.
Batson,S.
L.
Sass,J.
Silcox,Phys.
Rev.
Lett.
75(1995)4744.
[10]C.
T.
Liu,E.
H.
Lee,E.
P.
George,A.
J.
Duncan,Scr.
Metall.
Mater.
30(1994)387.
[11]G.
K.
Dey,A.
Arya,J.
A.
Sekhar,J.
Mater.
Res.
15(2000)63.
[12]D.
A.
Muller,S.
Subramanian,P.
E.
Batson,J.
Silcox,S.
L.
Sass,ActaMater.
44(1996)1637.
[13]H.
Riedel,FractureatHighTemperatures,SpringerVerlag,Berlin,1987.
[14]J.
Y.
Kim,J.
Yu,Appl.
Phys.
Lett.
92(2008)092109.
[15]S.
Q.
Liu,W.
Q.
Sun,ActaMeter.
Sin.
24(1988)376.
[16]G.
Duscher,M.
F.
Chisholm,U.
Alber,M.
Ru¨hle,Nat.
Mater.
3(2004)621.
[17]R.
Schweinfest,A.
T.
Paxton,M.
W.
Finnis,Nature432(2004)1008.
[18]U.
Alber,H.
Mue`llejans,M.
Rue`hle,ActaMater.
47(1999)4047.
[19]F.
Ren,J.
W.
Nah,K.
N.
Tu,B.
S.
Xiong,L.
H.
Xu,J.
H.
L.
Pang,Appl.
Phys.
Lett.
89(2006)141914.
[20]D.
T.
L.
VanAgterveld,G.
Palasantzas,J.
T.
M.
DeHosson,ActaMater.
48(2000)1995.
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.
3Aljointandthejointagedfor17days.
H.
F.
Zouetal.
/ScriptaMaterialia61(2009)308–311311