Comparativestudyofthedissolutionkinetics ofelectrolyticNiandelectrolessNi–Pbythe moltenSn3.5Ag0.5Cusolderalloy M.N.Islam,Y.C.Chan * ,A.Sharif,M.O.Alam Department of Electronic Engineering, City University of Hong Kong, 83 Tat Chee Avenue, Kowloon Tong, Hong Kong Received28January2003;receivedinrevisedform25May2003 Abstract Lead-free solders have high Sn content and high melting temperature, which often cause excessive interfacial reactionsattheinterface.Sn3.5Ag0.5Culead-freesolderalloyhasbeenusedtoidentifyitsinterfacialreactionswith two-metal layer flexible substrates. In this paper we investigate the dissolution kinetics of Sn3.5Ag0.5Cu solder on electrolyticNi/electrolessNiPlayer.Itisfoundthatduring1minofreflowelectrolessNiPlayerdissolvesslightlylower thantheelectrolyticNiduetothebarrierlayerformationbetweentheintermetalliccompounds(IMCs)andelectroless NiP layer. Faster nucleation of IMCs on the electrolytic Ni layer is proposed as the main reason for higher initial dissolution.TheappearanceofP-richNilayeractsasadiffusionbarrierlayerbetweenthesolderandelectrolessNiP layer,whichdecreasesthedissolutionrateandIMCsgrowthratethanthatoftheelectrolyticNilayer,butweakenthe interfaceandreducestheballshearstrengthandreliability.AfteracquiringcertainthicknessP-richNilayerbreaksand increasesthediffusionrateofSnandasaconsequenceboththeIMCsgrowthrateanddissolutionratealsoincreases.It isfoundthat3 lmthickelectrolessNiPlayercannotprotecttheCulayerformorethan120minat250 °C.Inelec- trolyticNishearstrengthdoesnotchangesignificantlyandlowerdissolutionrateandmoreprotectiveforCulayer duringlongtimemoltenreaction. Ó 2003ElsevierLtd.Allrightsreserved. 1. Introduction Solderingisthemostimportantmethodforjoiningof mechanical components in electronics. The traditional tin–leadsolderingmethodhasbeenusedformanyyears. However, Pb containing solder is become a conscious issueofmanufacturingduetoenvironmentalandhealth concerns.Becauseofthelackofinformationregarding reliabilityinpractice,alotofreliabilitytestareneeded before lead-free solder can replace traditional tin–lead solder. Oneofthemostinfluentialfactorsinthesolderjoint qualityofaBGAcomponentisthemetalsurfacefinish on the pads. The pads, which are Cu, easily oxidized, need to have a protective surface finish to ensure sol- derability during assembly. The most common surface on BGA is electrolytic Ni/Au or electroless NiP/Au plated over the copper pad. Electrolytic Ni/electroless NiPlayeractsasadiffusionbarrierlayerandAulayer acts as an oxidation barrier layer between the Cu and thesoldermaterials[1,2].Thethicknessofintermetallic, composition,microstructure,mechanicalpropertiesand reliability of solder joints are strongly dependent on surfacefinishlayers. Interaction and interdiffusion behavior between sol- derandCuhasbeenstudiedelsewhere.Itisfoundthat attheSn-containingsolder/Cuinterface,tinreactsrap- idly with Cu to form Cu–Sn intermetallic compounds (IMCs).Thestrengthofthesolderjointdecreaseswith increasingthicknessofIMCformedattheinterface[3– 5].Manyworkshavebeenreportedthatthegrowthrate * Corresponding author. Tel.: +852-2788-7130; fax: +852- 2788-7579. E-mail address: eeycchan@cityu.edu.hk (Y.C.Chan). 0026-2714/$-seefrontmatter Ó 2003ElsevierLtd.Allrightsreserved. doi:10.1016/S0026-2714(03)00190-2 MicroelectronicsReliability43(2003)2031–2037 www.elsevier.com/locate/microrel