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Micro-mechanical andfracture characteristics of

Cu6Sn5 and Cu3Snintermetallic compoundsunder micro-cantilever

bending

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Citation: LIU, L. ... et al, 2016. Micro-mechanical and fracture characteristicsof Cu6Sn5 and Cu3Sn intermetallic compounds under micro-cantilever bending.Intermetallics, 76, pp. 10-17.

Additional Information:

• This paper was accepted for publication in the journal Inter-metallics and the definitive published version is available athttp://dx.doi.org/10.1016/j.intermet.2016.06.004

Metadata Record: https://dspace.lboro.ac.uk/2134/21824

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Please cite the published version.

1IntroductionDuetoenvironmentalandhealthconcerns,variouslead-freesolderalloyshavebeenproposedtoreplaceleadcontainingsolders.Currently,Sn-basedsolderalloyswithadditionssuchasCu,Agarewidely

employedinelectronicpackageindustry[1–3].ComparingtoSn

Pbsolderalloys,Sn-richsolderalloyshavehighermeltingtemperaturesandhigherSncontents.Thus,itcanreactrapidlywithacommonmetallicsubstrate(Cu),formingthickCu

Snintermetalliccompounds(IMCs)attheinterfacesduringreflowingandservice,whicharousessomereliabilityissues[4–7].

Althoughan initial formationofIMCscanestablishagoodmetallurgicalbondbetweensolderandsubstrateaftersolderingprocess,excessivegrowthofIMCsformedduringageingwoulddeterioratethe

interfacialintegrity,duetotheirbrittlenatureandmismatchofphysicalproperties(e.g.elasticmodulusandcoefficientofthermalexpansion)withsoldersandsubstrates[4,8–10].Withanincreasingtrendtowards

miniaturizationofmicroelectronicproducts,volumeratiooftheIMCsinsolderjointstendstobehigher,whichaffectstheirmechanicalintegritysignificantly[11,12].Especiallyforthree-dimension(3D)integration,

solder jointsconsistingof IMCsfullyactasentire interconnectionsthroughtransient liquidphasebondingoreutecticbondingprocesses[13–15].Hence, it iscrucial toobtainacomprehensiveunderstandingon

mechanicalpropertiesofCu

SnIMCsformedinSn

Micro-mechanicalandfracturecharacteristicsofCu6Sn5andCu3Snintermetalliccompoundsundermicro-cantileverbending

LiLiua

ZhiwenChena,b

ChangqingLiua,∗

C.Liu@lboro.ac.uk

YipingWub

BingAnb

aWolfsonSchoolofMechanical,ElectricalandManufacturingEngineering,LoughboroughUniversity,Loughborough,LE113TU,UK

bSchoolofMaterialsScienceandEngineering,HuazhongUniversityofScienceandTechnology,Wuhan,430074,China

∗Correspondingauthor.

Abstract

ThisstudyfocusesonthefracturecharacteristicsofCu6Sn5andCu3Snmicrobeamsundermicro-cantileverbendingtests.Thesemicrobeamswerefabricatedbyfocusedionbeam(FIB)fromtheSn-rich

solderjointsagedat175°Cfor1132.5h,andthentestedusingananoindenterwithaflattip.ExperimentalresultsshowthatbothCu6Sn5andCu3Snmicrobeamsunderwentelasticdeformationbeforetheir

failure.Fromfractographicanalysis,bothcleavage fractureand intergranular fracturecanbe identified fromthe testedCu6Sn5microbeams,whileonly intergranular fracturewas found inCu3Snmicro

beams.Furthermore,basedontheexperimentalresults,finiteelementanalysiswascarriedouttoevaluatethetensilefracturestrengthandstrainofCu6Sn5andCu3Snmicrobeams.ForCu6Sn,thetensile

fracturestrengthwasestimatedtobe1.13±0.04Paandtheaveragetensilestrainwas0.01.ThetensilefracturestrengthandstrainofCu3Snwereevaluatedtobe2.15±0.19GPaand0.016,respectively.

Keywords:Intermetallics;Fracture;Focusedionbeammachining;Finite-elementmodelling;Mechanicaltesting

Cusolderjoints.

Presently,significanteffortshavebeenmadetocharacterisemechanicalpropertiesofCu

SnIMCs insolder joints (i.e.Cu6Sn5andCu3Sn)withvarious techniques.For instance,numericalanalysisprovidesanapproach toestimate theelasticmoduliofCu6Sn5,Cu3SnandAg3Sn [16–18].Moreover, bulk

intermetallicswerepreparedthroughcastingandannealingprocessestoenablemechanicaltestsatamacro-scale[19,20],butresidualporosityandoxidesmayemergefromtheseprocesses,degradingapplicability

oftheresults.Besides,themicrostructureofbulkIMCsamplesareconsiderablydifferentfrominterfacialIMCslayersinsolderjoint.Somepreliminarystudies[19–22]reportedthattheYoung'smodulusofCu6Sn5showsalargedegreeofvariability.Therefore,ithasbeenconsideredthatanin-situmicro-scaletest,e.g.nanoindentation,wouldbeappropriatetoobtainYoung'smodulusandhardnessoftheseIMCsinsolderjoints

atmicro-scale [4,23–25]. Furthermore, someworkswere reported on the fracture characteristics of individual IMCs in solder joints throughmicro-scale tests. For example, the compression and shear fracture

characteristicsofindividualCu6Sn5pillarsformedattheSn-richsolder/Cuinterfacewereinvestigated[26–28].However,thetensilefracturebehavioursofindividualCu6Sn5andCu3SnIMCsattheinterfacesofSn-

basedsolderjointsremainunclear,whichdemandfurtherinvestigations.

Inthispaper,microbeamsofCu6Sn5orCu3SnforcantileverbendingtestswerefabricatedbyFIBmillingatSn99Cu1solder/Cuinterfacesageingat175°Cfor1132.5h.Electronback-scattereddiffraction

(EBSD)wasutilisedtorevealtheinterfacialmicrostructureaswellasthesizesofCu6Sn5andCu3Sngrainsattheinterfaces,subjecttothelocationanddimensionofCu

Sn IMCbeams fabricatedwithFIB.The tensile fracturebehavioursofCu6Sn5andCu3Snmicrobeamswere studied through the results ofmicro-cantileverbending tests, followedby a finite element analysis to

estimatethetensilestrengthofCu6Sn5andCu3Snmicrobeams.Afterthebendingtest,fracturemorphologiesandcompositionofCu6Sn5andCu3Snsampleswereexaminedbyscanningelectronmicroscope(SEM)and

energydispersiveX-ray(EDX)tounderstandthefracturemechanismsofCu

SnIMCs.

2Experimentalandmodellingprocedures2.1Samplespreparation

In this work, polycrystalline Cu sheets (purity: 99.9%, 5 mm thickness) and Sn99Cu1 solder alloys were used as substrates and solder materials, respectively. Firstly, a trench of subsidence, with dimensions of

15mm×15mm×2.5mm(showninFig.1(a)),wasmilledatacorneroftheCusheet.ProperamountofSn99Cu1solderwasplacedwithinthetrenchandthenreflowedat270°Cforapproximately2min.Next,theas-reflowed

sampleswerestoredinavacuumoven,ageingat175°Cfor1132.5htofacilitateafurthergrowthofinterfacialCu

SnIMClayers.Then,thesamplesweregroundandpolishedcarefullytoa0.05μmfinishwithcolloidalsilicatorevealtheinterfacialmicrostructureandtheCu

Sn IMCs formed at the Sn99Cu1/Cu interfaces, which were subsequently cross-sectional milled and characterised through SEM, EDX and EBSD incorporated in a FIB equipment (FEI, Nova 600 Nanolab Dual Beam). Cross-

sectionmillingwasperformedwithGaionof30kvusing500pAaperture,preparingthesurfaceswithlowmillingdamageforagenerationofEBSDpatterns.

MicrobeamsofCu6Sn5andCu3SnforcantileverbendingtestswerethenindividuallyfabricatedbyFIBmillingatselectedlocationsoftheSn99Cu1/Cuinterfaces,asillustratedinFig.1(b)and(c).ThedimensionsofCu6Sn5andCu3Snmicrobeamswere3×3×10μmand1.5×1.5×10μm,respectively,duetotherestraintofavailablethicknessesoftheIMClayersattheinterfaces.ToensureaconsistentcompositionofCu6Sn5orCu3Snmicrobeams,the

FIBmillingmustbeoperatedalongthehorizontal(or inparallelto)directionsofthecorrespondingIMClayersatthe interfaces,asseeninFig.1(b)and(c).A lowbeamcurrentof500pA(voltage:30kV)wasemployed in final

polishingtominimizeaneffectofGaimplantationandre-depositionofmilledmaterialsonthesamplesurfaces[29].

2.2Micro-cantileverbendingtestsMicrocantileverbendingtestswereconductedonCu6Sn5andCu3Snbeamstoacquireload-displacementcurvesusingananoindentationsystem(MicroMaterials,NanoTest600,Wrexham,UK).Aflatcylindricalindenterwith

adiameterof5μmwasusedtominimizelocalizedstressesattheindenter/beamcontactingregions(asshowninFig.1(a)and(b)).Thecentralpointsofthesecontactingregionswereapproximately7.5μmawayfromthebottomofthe

IMCmicrobeams.Theindentationsystemwascarefullycalibratedtoachievethelocationaccuracywithin1μmbeforethetest;threebeamsforeachtypeofIMCsweretestedunderthesameconditionstoensurethereliabilityand

accuracyoftheexperimentalresults.TheparametersandsettingsforthecantileverbendingtestswerelistedinTable1.TheloadingrateofCu6Sn5andCu3Snmicrobeamswassetas0.10mN/sand0.02mN/s,respectively,sincethe

cross-sectional area ofCu6Sn5 beam is approximately 4 times as large as that of Cu3Sn beam. Therefore, deformations of both Cu6Sn5 andCu3Snmicro beams are under similar straining conditions, i.e. strain rate, for direct

comparison.Afterthebendingtests,fracturesurfacesofCu6Sn5andCu3SnmicrobeamswerefurtherexaminedbySEMandEDX.

Table1ParametersandsettingsforcantileverbendingtestsonCu6Sn5andCu3Snmicrobeams.

Parameters Cu6Sn5 Cu3Sn

Pre-definedmaximumdepth 3μm 1μm

Initialload 0.05mN 0.05mN

Loadingrate 0.10mN/s 0.02mN/s

Unloadingrate 0.10mN/s 0.02mN/s

Dwellingtimeatmaximumload 0s 0s

2.3ModellingTounderstandastressdistributionwithintheCu

Sn IMCbeamsduring thenanoindentation tests, finiteelementanalysiswascarriedoutwithAbaqussoftware (version6.12).GeometriesofFE-modelswerebasedon theprepared IMCmicrobeamsofCu6Sn5andCu3Sn before

mechanicaltests,asillustratedinFig.2.Toimproveefficiencyandaccuracyofthenumericalanalysis,themodelsweremeshedwithtwodifferentsizesofquadraticelements(C3D20):afinemeshintheregionsaroundthebottomof

themicrobeamandacoarsemeshfortherestarea(asshowninFig.2).Mechanicalpropertiesoftheindenter[25,27,30-34],theCu

Sn IMCsand the support of themicrobeamsaregiven inTable2.A forcemeasured from the experimental testwas applied on top of the indenter in themodel to simulate a cantilever bendingprocess. The support of beams

(theregionshighlightedinFig.2)werefixedduringthesimulation.Moreover,thesimulationwasbasedonfourassumptions:1)Cu6Sn5andCu3Snarebothisotropicmaterials;2)Theflatendoftheindenterisparalleltothetop

surfaceofthemicrobeamatthebeginningofbendingtests;3)thediamondindenterandtheIMCbeamswerebothsubjecttoelasticdeformationinthetestsaccordingtoreportedresearches[26–28];4)Thedeformationinthearea

beyondthefiniteelementmodelsisnegligible.

Fig.1SchematicsofCu

SnIMCmicrobeamspreparation:(a)Sn99Cu1solderjointsbeforeFIBmilling;Preparationof(b)Cu6Sn5and(c)Cu3SnmicrobeamsattheSn99Cu1/Cuinterfaces.

Table2Mechanicalpropertiesofmaterialsinthefiniteelementanalysis.

Materials Young'sModulus Poisson'sratio

Cu6Sn5 115.5GPa[27] 0.31[ 30]

Cu3Sn 134.2GPa[ 31] 0.299[25]

Diamondindenter 1141GPa[ 32] 0.07[ 32]

Cu 129.8GPa[ 33] 0.339[ 34]

3Resultsanddiscussions3.1ObservationofIMCs,Cu6Sn5andCu3Snmicrobeams

Toacquireacomprehensiveunderstandingoftheinitialmicrostructure,anEBSDanalysisoftheSn99Cu1/CuinterfacesbeforemillingIMCmicrobeamswascarriedoutanditsimageisshowninFig.3.Fromthisimage,a

platinumlayerinthetoprightcornerwasdepositedforprotectionoftheadjacentmaterialsduringtheFIBmillingprocess(themillingdirectionwasfromrighttoleftinthisfigure).TheboundariesoftheSn99Cu1solder,Cu6Sn5layer,

Cu3SnlayerandCusubstratewithintheSn-richsolderjointsaremarkedbythewhitelines.ItcanbeobservedthatthesizesofCu6Sn5grainsrangefrom2to15μm,whilethesizesofCu3Sngrainsaresmallerthan1μmafterageing

upto1132.5h.

AccordingtoFig.3,theminimumthicknessoftheCu3Snlayerattheinterfacesislessthan4.5μmCu3SnmicrobeamswerepreparedfromthisthinCu3SnlayerbetweenCusubstrateandCu6Sn5layer.Moreover,inviewofthe

factthattheboundariesofeachphaseswerenotparallel,thewidthofCu3Snmicrobeamswerethereforesetas1.5μm,muchsmallerthanthewidth(3μm)ofCu6Sn5microbeams.

Fig.2Thefiniteelementmodelswithmeshesof(a)Cu6Sn5and(b)Cu3Snsamplesduringmicro-cantileverbending.

31

32

33 33

34 35

Fig.3AnEBSDimageoftheinterfacialmicrostructureattheSn99Cu1/Cuinterfaces.

FEG-SEMimagesoftypicalCu6Sn5andCu3SnmicrobeamsareshowninFig.4(a)and(b),respectively.Itisimportanttoremovethematerialsnearthemicrobeams,soastoleavesufficientspaceallowingthesubsequent

indentationwithoutanyblockagesprior to the fractureof IMCbeams.ThedimensionsofCu6Sn5andCu3Snmicrobeamswere3×3×10μmand1.5×1.5× 10 μm, respectively, as shown in Fig. 4(a) and (c). To confirm the

constitutionofthefabricatedbeams,theirchemicalcompositionwasexaminedbyEDXacrosstheseIMCbeamsasindicatedinFig.4(a)and(c).TheEDXresultswereillustratedinFig.4(b)and(d),confirmingthesetwotypesofIMC

beamsconsistofonlyCu6Sn5orCu3Snphase,respectively.

3.2FracturemechanismsofCu6Sn5andCu3SnIMCsThefracturesurfacesofCu6Sn5beamsafterthemicro-cantileverbendingtests arepresentedinFig.5.Themicrobeamsfracturedattheirbottomastheresultsoffailurelikelyduetothehighesttensilestressinducedin

thesebeamsunderthebendingload[26,35,36].InobservationonthefracturesurfacesamongCu6Sn5microbeams,twotypicalfracturesurfaces,whichindicatedifferentfracturemodesduetothebendingtest,werefoundandshown

inFig.5(a)and(c).

Fig.4SEMmicrographofaCu6Sn5microbeam(a)anditsEDXspectrum(b);SEMmicrographofaCu3Snmicrobeam(c)anditsEDXspectrum(d).

is

InFig.5(a), asmarkedwithdash lines, the initial area at thebottomsofCu6Sn5microbeamsbefore thebendingwere in rectangular shape.However, the actual fracture cross section (solid lines)was extendedwith a

significantincreaseintheareaafterthemicro-mechanicaltests.Thiswasattributedtotheinvolvementandfractureofadjacentmaterialsinconnectiontothebeambottomalongcertaincrystallineplaneswherefurtherextended

stressesinduced.Notably,theellipseregionhighlightedinFig.5(a)wasresultedfromthedamagebytherearoftheindenterafterthefracture.Generally,thefracturesurfaceofthisCu6Sn5microbeamisrelativelysmoothasthere

wasonlyasingleCu6Sn5graininvolvedinthefracturesection,whichindicatesacleavagefracturemode,asreportedelsewhere[26,27,37].

However,another typical fracturesurfaceofCu6Sn5microbeamwasalsoobserved,aspresented inFig.5(c),where thebeam fracturedanddirectly alignedat thebottomof thebeamwithout any further extension.By

examiningthedetailswithinthefracturedzoneasdefinedbythesolidlinesinFig.5(c),itisapparenttoseetherearemultiplegrainsinvolvedinthefractureatthatspecificcrosssectionwheretheboundariesofCu6Sn5grainsare

markedbythedashlinesinFig.5(c).Astheresult,atypicalintergranularfracturehasoccurred,whichshouldbeconsideredasanotherfracturemodeofCu6Sn5microbeamsunderthemicro-mechanicaltests.

Therefore,differentfracturemodesofCu6Sn5microbeamsshouldbeconsideredsubjecttothecrystallitestructureofthebeamnearorclosetothebottomlinesofthebeam,forinstance,thenumberofCu6Sn5grainsinvolved

atthepointoffracture.ThishasbeenclearlyobservedfromtheEBSDimagepresentedinFig.3,wherethevariationofthegrainsizesrangingfromapproximately15μm–2μmcanhaveconsiderableeffectsonthefracturemechanism

ofCu6Sn5microbeamsatthesolder/Cuinterfaces.Accordingly,subjecttotheselectedlocationsoftheIMCbeamsforFIBmilling,therewasasingleCu6Sn5graininvolvedatthebottomofaCu6Sn5microbeamwhenthecrackwas

initiated,thusacleavagefractureoccurred.However,asitisclearlyseenamixtureofmultipleCu6Sn5grainswasinvolvedinthefractureatthebottomofanotherbeams,whichhasledtoatypicalintergranularfracture.Basedonthe

EBSDresults,theschematicsgiveninFig.5(b)and(d)indicatethelocationswithintheCu6Sn5IMCslayersatthesolder/CuinterfacewhereCu6Sn5microbeamswaslikelyselectedandtested,whichhaveresultedinthecleavageand

intergranularfractureshowninFig.5(a)and5(c),respectively.

IncomparisontoCu6Sn5microbeams,onlyonefracturemodewasfoundinCu3Snmicrobeams.Fig.6(a)showsthetypicalfracturesurfaceofCu3Snmicrobeamafterbendingtests.Accordingly,ashighlightedwiththebroken

lines,theoriginalcross-sectionalareaatthebottomofthebeamfracturedunderthebendingload,whichhasresultedinthefracturesurfaceofCu3SnbeamashighlightedbysolidlinesinFig.6(a).Asithasbeenclearlyobservedin

theEBSDimagesinFig.3,thisCu3Snlayeratthesolder/copperinterfaceconsistsnumerousrefinedgrainspresentingapolycrystallinestructure,therebytherearealwaysmultiplegrainstopartakethefracturenearthebottomofthe

beam(Fig.6(b)),whichcanbeassmallassub-micronsizes(approximately0.4μm)duetotheageing.ThegrainsizeofCu3Snseeninthisstudyarewithinthesimilarrangeasreportedinapreviousstudy[38].Thebottomofthe

beamswasthestressconcentrationsitewherethemaximumstresslocates.Onceacrackinitiatesatthislocation,itwillpreferentiallypropagatealongthegrainboundarieswheretheadhesionstrengthisweaker.Hence,thegrain

boundariesofCu3Sncanfacilitatethecrackpropagationnotablyoncethecracksinitiated,leadingtointergranularfractureinaCu3Snmicrobeam.Andthisfracturemode hasalsobeenobservedbytheotherresearchers[39].

3.3TensilefracturestrengthofCu6Sn5andCu3SnmicrobeamsFig.7(a)and(b)showtheselectedinitialregionwheretheincreaseoftheloadwiththedisplacementwasrecordedtillthefractureofthemicrobeamsforbothCu6Sn5andCu3SnIMCs,respectively.Thecompleteprofilesof

load-displacementdataofCu6Sn5andCu3SnmicrobeamswerealsoplottedasembeddedintheFig.7(a)and(b)toprovideanoverviewofthebendingtests.Accordingtotheseembeddedcurves,theentirebendingtestcanbedivided

intothreestages,includingtheinitialloadingstage(stage1),middlestage(stage2)andunloadingstage(stage3).Inthesecondstage,theindenterwassubjecttoaswiftmotionduetothefractureofthebeamtillitreachedthe

maximumdepth,fromwhichpointtheunloadingcommenced.TheswiftmotionoftheindentermayleadtothedamageaccidentallyasshowninFig.5(a)inthisstage.Itisfoundthatthefractureshadtakenplacewithintheinitial

loadingstage(stage1).Therefore,theinitialloadingregionisofparticularinterests,primarilypossessingthemechanicalresponseofIMCmicrobeams,andenlargedasshowninFig.7(a)and(b)tofurtherelaboratethemechanical

propertiesoftheseCu

Fig.5FracturesurfacesandtheirschematiclocationsofthetestedCu6Sn5microbeams:(a)SEMmophologyofcleavagefracturesurface;(b)schematiclocationofCu6Sn5beamsinCu6Sn5layer;(c)SEMmophologyofintergranularfracturesurface;(d)

schematiclocationofCu6Sn5beamsinCu6Sn5layer.

have

Fig.6TheintergranularfracturesurfaceofCu3Snmicrobeamsduetothecantileverbending:a)SEMimageonthefracturesurafecsurface,b)schematicillustrationofthebeamlocationintheCu3SnIMClayerattheinterface.

SnIMCs.ThispresentstheelasticdeformationofbothCu

SnIMCmicrobeamsasindicatedbythelinearrelationshipofload-displacementcurvespriortothefracture.

As reported inaprevious study [40], ifmaterials exhibit elastic-plasticbehaviourunder cantileverbending, the load-displacement curvesmustdeviate from the linear relationshipuntil reaching the yield strengthof the

materials.Inthiswork,bothCu6Sn5andCu3Snmicrobeamsonlyshowedelastic(linear)behaviourbeforefailures.FromthedatainFig.7(a)and(b),thefractureforceofCu6Sn5andCu3Snmicrobeamscantherebybeestimated,

whichareintherangeof2.11mN–2.27mNand0.6mN-0.64mNforCu6Sn5andCu3Snmicrobeamswiththecorrespondingdeflectionof0.45μm–0.47μmand0.54μm-0.63μm,respectively.

Accordingtotheloadingnatureandthegeometryofthemicrobeams,thefractureoftheIMCmicrobeamscouldbepossiblyresultedfromthecombinationofbothtensileandshearstressinducedwithinthebottomofthe

microbeamsduetothecantileverbending.It isthereforenotpossibletosimplyconductacalculationbasedonthemechanicsofbendingtoobtainthefracturestrengthoftheIMCmicrobeamssincethefracturemechanismsof

Cu6Sn5andCu3Snmicrobeamswereunclear.InordertodeterminethefracturestrengthoftheIMCmicrobeams,finiteelementsimulationsofCu6Sn5andCu3Snmicrobeamsunderthebendingwerecarriedout,whichcanalsoassist

to furtherelaborate the fracturemodes (The "fracturemechanism"was replacedwith "fracturemode" followed the reviewer's suggestion. )byvisualising thestressdistributionacross the IMCmicrobeamsunder the

bending.

Withtheparametersandboundaryconditionspresented inSection2.3, thesimulationswereperformedto the initial stageof thebendingup to thepointof fracture,andvalidatedbasedon the load-displacementcurves

obtainedthroughexperimentalbendingtests.Theresultsofload-displacementcurvesgeneratedbythemodellingareplottedwiththeexperimentallyderivedcurvesforCu6Sn5andCu3SnmicrobeamsinFig.8(a)and(b),respectively.

ThesimulationresultsprovidedaverycloseapproximationandanexcellentconsistencytotheexperimentalresultsforbothCu6Sn5andCu3Snmicrobeams.ThiscanprovideanapproachtodeterminethefracturestrengthofCu6Sn5andCu3SnmicrobeamsbyextractingthetensileorshearstressesthatwereinducedatCu6Sn5andCu3Snsamplesbasedonthemodellingresultsasthiswasfounddifficulttoderivepurelythroughtheexperimentaldata.

TheresultsoftensilestressdistributionderivedfromthesimulationofCu6Sn5andCu3SnmicrobeamsareillustratedinFig.9(a)and(b),respectively.Thetensilestressesofcross-sectionsatthebottomofbeamsarealsogiven

intheembeddedimagesinFig.9(a)and(b)formoredetailsofthetensilestressdistribution.Apparently,besideslightconcentrationsofstressesatthelocationsincontactwiththeindenter(region1inFig.9(a)and(b)),inbothcase,

asignificantstressconcentrations is locatednear thebottomsof thebeams (region2 inFig.9(a)and (b)).This isbecauseof themaximumbendingstresses in this regionas the resultof cantileverbending.Therefore, it isnot

surprisingthatthefractureoccurrednearthebottomofthebeamswhenthestressesreachedthefracturestrengthwiththeincreaseofthebendingforceappliedbyindenter.However,it isstilldifficulttodeterminethefracture

Fig.7Theload-displacementcurvesfromthemicro-cantileverbendingtestson(a)Cu6Sn5microbeamsand(b)Cu3Snmicrobeams.

fracturemechanism

Fig.8Theload-deflectioncurvesobtainedfromexperimentandmodellingof(a)Cu6Sn5,and(b)Cu3Snmicrobeamsunderthesameconditionsofcantileverbending.

strengthasithastobecorrelatedtothelocalstresses(tensileandshearstress)ofconcentrationandthepotentialfailuremodes.

Toclarifytheeffectofshearstress,theshearstressdistributioninCu6Sn5andCu3SnmicrobeamswasalsoobtainedandpresentedinFig.10(a)and(b),respectively.Furthermore,thecross-sectionalviewsatthebottomsof

beamsarealsoprovidedtorevealthedetailsofthedistributionofshearstressasembeddedinsidetheFig.10(a)and(b).Itindicatesthepotentiallocationsoffracturewhentheshearstresscausesthefailure.FromFig.10(a)and(b),

thehighestshearstressislocatedatthepointofcontactonthebeamwithindenter(region1inFig.10(a)and(b))otherthanthebottomofIMCmicrobeams(region2inFig.10(a)and(b)).Clearly,themaximumshearstressatthe

pointcontactingwithindenterdidnotreachtheshearstrength,thereby,nofailurescanbeobservedatthesecontactingpoints.ThiswasconfirmedbytheexperimentalresultsthathavebeenpresentedinFigs.5and6,whichdisplays

thefinalfailureofCu6Sn5andCu3Snmicrobeamsnearthebottomofbeamsotherthanthecontactpointwithindenter.Thissuggeststhatthefractureofthesemicrobeamswereprimarilyattributedtothemaximumtensilestresses

thathadbeenresultednearthebottomsofthemicrobeams,i.e.region2showninFig.9(a)and(b).Therefore,thefracture modesofCu6Sn5andCu3Sncantileverbeamswerebothdeterminedbythelimitofmaximum

tensilestressthatthemicrobeamscansustainnearthebottomofthebeams,leadingtothetensilefracture,whichisinaccordancewiththeworkpreviouslyreported[26].

Basedontheabovefailure modes,thetensilefracturestrengthwasestimatedasthemeanvalueofsimulationsconductedusingthreesetsofexperimentaldata,whichisapproximately1.13±0.04GPaforCu6Sn5microbeams.Similarly,thetensilefracturestrengthofCu3Snmicrobeamscanalsobeacquiredintherangeof2.15±0.19GPa.ThisvaluealmostdoubledthevalueoftensilefracturestrengthofCu6Sn5microbeams.FromtheEBSD

imagespresentedinFig.3,itisobviousthattheCu3SnIMClayerconsistsofmorerefinedpolycrystallinemicrostructureinvolvingsignificantamountofgrainboundaries,whichisoneofthereasonsthathasmadethistypeofIMCs

muchstronger.BasedonHooke'slawwiththeobtainedtensilestrengthestimatedaboveandYoung'smodulusoftheCu

SnIMCslistedinTable2,thetensilestrainsofCu6Sn5andCu3Snatthepointoffractureinthisstudycanbededucedas0.01and0.016,respectively.

4ConclusionsBothexperimentalandmodellingtechniqueswereemployedtoinvestigatethefracturebehavioursofCu6Sn5andCu3Sncantileverbeamsatamicro-scaleunderbendingtests.TheCu6Sn5andCu3Snmicro-

cantileverbeamswerepreparedbyFIBalongtheinterfacesoftheSn99Cu1/Cusolderinterconnectsafterageingfor1132.5hat175°C.ThecorrelationbetweenthemicrostructureoftheinterfacialIMClayersand

thefracturecharacteristicswasrevealedbasedontheexaminationsofthefracturesurfacesastheresultsofbeambendingthroughSEMandEBSDanalysis;assuchthefracture modesforbothCu6Sn5and

Cu3SnmicrobeamscanbeproposedandtensilestrengthofCu6Sn5andCu3Snmicrobeamsweresubsequentlydeterminedwiththeassistanceofnumericalsimulations.

1. Duringthemicro-cantileverbendingtestsusingthenanoindentationtester,bothCu6Sn5andCu3Snmicrobeamsdeformedelasticallywiththeincreaseofappliedloads,leadingtothefinalfractureofthesemicrobeamsneartheirbottom.

2. SEMexaminationshasindicatedthatbothcleavageandintergranularfracturecanoccuratthebottomofCu6Sn5microbeamssubjecttothenumberofCu6Sn5grainsinvolvedatthebottomofCu6Sn5microbeamsduetothenon-uniform

polycrystallinestructureattheinterfaceofsolderjoints.However,themuchfineranduniformcrystallinestructureofCu3SngrainshasresultedinonlyintergranularfractureasobservedfromthefracturesurfaceoftheCu3Snmicrobeams.

Fig.9Thedistributionoftensilestressundertheappliedloadpriortothefracture:(a)aCu6Sn5microbeam.(b)aCu3Snmicrobeam.

mechanisms

Fig.10Thedistributionofshearstressundertheappliedloadpriortothefracture:(a)aCu6Sn5microbeam.(b)aCu3Snmicrobeam.

mechanisms

mechanism

3. Giventheloadingconditionsunderthecantileverbendingtests,throughnumericalsimulationitwasrevealedthatthefracture modesofIMCbeamswereprimarilyattributedtothemaximumtensilestressthathadexceededthe

tensilestrengthofthemicrobeamssurroundingthebottomofthebeams.Thishasbeenconfirmedbytheexperimentalresults.Themaximumshearstress,whichwasfoundtobelocatedatthecontactpointbetweenindenterandmicrobeams,

wererelativelylower,thusunabletocauseanyfracturesasitalsohasbeenobservedexperimentally.

4. Basedonthemodellingresults,itispossibletodeterminethetensilefracturestrengthandstrainoftheCu6Sn5andCu3Snmicrobeamsunderthebendingtests;theyare1.13±0.04GPa(strength)and0.01(strain),2.15±0.19GPa(strength)

and0.016(strain),respectively.

Uncitedreference[30]..

AcknowledgementsThis researchwas supportedbyaMarieCurie InternationalResearchStaffExchangeSchemeProjectwithin the7thEuropeanCommunityFrameworkProgramme,No.PIRSES-GA-2010-269113,entitled

“Micro-Multi-MaterialManufacturetoEnableMultifunctionalMiniaturisedDevices(M6)”,aswellasaNationalNaturalScienceFoundationofChina(Number:60976076).

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Graphicalabstract

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Highlights

• Cu6Sn5andCu3Snmicro-beamswerepreparedbyFIBforcantileverbendingtestusingananoindentationsystem.

• ThecorrelationbetweenIMCscrystallinestructuresandthefracturecharacteristicsofCu6Sn5andCu3Snwas proposed.

• Finiteelementanalysiswasconducted

toclarifythefracturemodesofCu6Sn5andCu3Snundermicro-cantileverbending.

• ThefracturestrengthofCu6Sn5andCu3Snwereestimatedbasedonsimulationandmicro-

mechanicaltest.

also

FiniteelementanalysiswascarriedouttoclarifythefracturemechanismofCu6Sn5andCu3Snmicro-beamsbyelaboratingthestressdistributionacrossthebeamsunderthecantileverbending.

ThefracturestrengthofCu6Sn5andCu3Snwereestimatedbasedonthenumericalsimulationandmicro-cantileverbendingexperiments.