General requirementsof Pb-free in mobile phone applicationsExperimentalVoiding vs. Au thicknessPhase identificationAu contents in Sn-Ag-Cu-Sb joint by EDXEffect of package sizeAcknowledgements1 W. Peng Pb-free, 2002Microstructural Aspects & Performance Implications of Sn-Ag-Cu-Sb Solder in the Presence of GoldWeiqun Peng, Steven O. DunfordPuligandla Viswanadham and Stephen QuanderNokia, Dallas, TX75039, U.S.A.2 W. Peng Pb-free, 2002General requirementsof Pb-free in mobile phone applications• WEEE requires mobile phones be lead-free in 01, 2006• Solder paste must be lead-free• PWB board finish must be lead-free• Component termination finishes & balls must be Pb-free3 W. Peng Pb-free, 2002Purposes• To investigate reliability of totally lead-free solder joints• To study effects of gold coating thickness on Sn-Ag-Cu-Sb solder joint performance• To discern failure mechanisms & microstructural evolution differences in Sn-Ag-Cu-Sb and Sn-Pb-Ag solder joints after thermal cycling4 W. Peng Pb-free, 2002Experimental• Materials/Hardware• 14 leadless component packages, Ni/Au termination finish• FR4 high density micro-via test boards, Ni/Au surface finish• Sn-Ag-Cu-Sb and Sn-Pb-Ag solder pastes• Test• After reflow, after 200 cycles, after 500 cycles between −40 and 85°C• Analysis• SEM, EDX for microstructural analysis5 W. Peng Pb-free, 2002Sample MatrixFlash Au0.02-0.05umOrganicsubstrateIntermediate Au0.15-0.5umCeramicsubstrateThick Au0.5-1.2umCeramicsubstrate14 component packagesAfter reflow200 cycles (-40 ~ 85°C)500 cycles (-40 ~ 85°C)6 W. Peng Pb-free, 2002Aspects of Analysis• Voiding• Phase identification• Microstructure & interface• Effect of gold content • Effect of substrate• Effect of package size• Crack generation and development• What is the maximum Au content that can be tolerated ????7 W. Peng Pb-free, 2002ResultsVoiding vs. Au thickness Flash gold, round voidsSn-Ag-Cu-Sb solderThick gold, irregular voidsSn-Ag-Cu-Sb solder8 W. Peng Pb-free, 2002Voiding vs. solder pasteVoidsVoidsVoidsVoidsSn-Ag-Cu-Sb, irregular shape.Thick goldSn-Pb-Ag, round shape.Thick gold9 W. Peng Pb-free, 2002Effect of gold content on voiding Solubility of gold in Sn-Ag-Cu-Sb is greater than in Sn-Pb due to high Sn and high reflow temperature Fraction of AuSn4in the molten Sn-Ag-Cu-Sb joint is greater than in the Sn-Pb-Ag joint AuSn4crystals in molten solder will increase solder viscosity and degrade its spreadability Increased viscosity restricts the release of gasses, resulting in higher void content  Fast cooling rate and large fraction of AuSn4results in irregular voids in thick gold joint10 W. Peng Pb-free, 2002Phase identificationSn-Ag-Cu-Sb solder joint with thick gold terminal finish11 W. Peng Pb-free, 2002AgAuSn4Ag3SnSnAuSn12 W. Peng Pb-free, 2002Au contents in Sn-Ag-Cu-Sb joint by EDXFlash gold, ~1.5wt.%Medium gold ~ 3 wt. %Special case, 12~17wt.%Thick gold, 5~7wt.%13 W. Peng Pb-free, 2002Microstructure change of Sn-Ag-Cu-Sb jointFlash gold, reflowed Flash gold, 500cyThick gold, reflowed Thick gold, 500cyNo noticeable change in microstructure after thermal cycling14 W. Peng Pb-free, 2002Microstructure change of Sn-Pb-Ag jointFlash gold, reflowed Flash gold, 500cyPb-rich phase becomes irregularThick gold, reflowedThick gold, 500cyCoarsened Au-needles & Pb-rich phase15 W. Peng Pb-free, 2002Interface of Sn-Ag-Cu-Sb joint after 500 cyclesCuNiAuSn4(Cu,Ni)6Sn5Flash gold jointAuSn4Ni-CoThick gold joint• (Cu,Ni)6Sn5 between Ni &solder• Au, Ag, Cu & Ni mixed IMCs• Micro-cracks at interface16 W. Peng Pb-free, 2002Interface of Sn-Pb-Ag joint after 500 cyclesPb phaseAuSn4Flash gold joint Thick gold joint• Ni3Sn4at interface• No Pb-rich phase at interface• Pb-rich layer at interface• AuSn4layer at interface • AuSn4needles throughout joint17 W. Peng Pb-free, 2002Good joints: Flash gold finish & organic substrateorganicSn-Ag-Cu-Sb500cyorganicSn-Ag-Cu-Sbreflowedorganic500cySn-Pb-AgorganicSn-Pb-AgreflowedPackage size: 5.6x5.6x1.6mm18 W. Peng Pb-free, 2002Bad joints: Thick gold finish & ceramic substrate200cyceramicSn-Ag-Cu-SbceramicSn-Ag-Cu-Sb500cyreflowedPackage size, 3.1x3.1x1.2mmMedium Au joints are similar to thick Au jointsceramicceramicSn-Pb-Ag Sn-Pb-Ag500cy19 W. Peng Pb-free, 2002Effect of package size9.5x7.5x2mm, 200 cyclesSn-Ag-Cu-Sbceramic ceramic3.1x3.1x1.2mm, 200 cyclesSn-Ag-Cu-Sb• The joint stress is higher in larger pacakges. • Sn-Pb-Ag presents the same situation as Sn-Ag-Cu-Sb.20 W. Peng Pb-free, 2002Void in solder gapCrack generationComponent cornerRough fillet surface Joint tip21 W. Peng Pb-free, 2002Crack developmentreflowed200cy500cy• Sn-Ag-Cu-Sb solder joints• Package size: 9.5x7.5x2mm22 W. Peng Pb-free, 2002• Sn-Ag-Cu-Sb solder joint• Cracks originate at void in gap area• Easier crack propagation along IMC boundaryCrack development23 W. Peng Pb-free, 2002Crack start from tipPWBCrack developmentPbSnAuSn4• Sn-Pb-Ag solder joint• Cracks start from tip• Propagated along Pb/Sn interface or in Pb-rich phase• Cracked joint smoother than Sn-Ag-Cu-Sb joint• Role of Pb-rich phase24 W. Peng Pb-free, 2002High Au content may be toleratedSn-Ag-Cu-Sb solder joint after 500 cycles20Sn80AuAuSn2AuSn420Sn80AuAuSn4SMALL PACKAGE3.0x3.0x1.2mm12~17wt.% Au25 W. Peng Pb-free, 2002High Au content may be toleratedNo solder joint cracksIIIAuAu-SnAuAu-Sn20Sn80AuAuSn4Au-Sn IMCSMALL PACKAGE3.0x3.0x1.2mm12~17wt.% AuSn-Pb-Ag solder joint after 500 cycles26 W. Peng Pb-free, 2002PrimaryAuSn4EutecticAu-Sn-Pb microstructure27 W. Peng Pb-free, 2002High Au can be prevented20Sn80AuConfined wellIIIIIINo big AuSn4in Sn-Pb-Ag jointSn-Au solder is confined in component joint as indicated by the white arrow. Sn-Pb-Ag solder joint after 500 cycles28 W. Peng Pb-free, 2002Conclusions• Microstructure of Sn-Ag-Cu-Sb solder joint is more stable than that Sn-Pb-Ag joint in the temperature range studied. (p13-16)• Presence of Au affects voiding because of Au-Sn IMC formation.9 Thick gold finish results in irregular voids (p7)9 Flash gold finish results in spherical voids (p7)9 Sn-Ag-Cu solder joint has more, bigger & irregular voids (p8)• High Au content may be tolerated both in Sn-Ag-Cu-Sb and Sn-Pb-Ag solder joints in specific instances, but further study is