Effects Of Mn Doping On Interfacial Reaction And Mechanical Properties Of Sn-0.3Ag-0.7Cu Lead-free Solder

With the trend towards high performance and miniaturization of electronic products, thesize of microelectronic components and thus of solder joints has been continuously scaleddown. Solder joints are exposed to very high load, resulting in requirement of higherreliability. The low-Ag lead-free solders of Sn-Ag-Cu family become more popular for itsrelatively low cost and acceptable performance. However, the excess interfacial IMC(intermetallic compounds) growth and inferior mechanical properties of low-Ag lead-freesolder will influence long-term reliability of solder joint and restrict its high-end application.The effects of Mn doping on interfacial reactions and mechanical properties inSn-0.3Ag-0.7Cu-xMn(x=0,0.02,0.05,0.1,0.15wt.%) lead free solder joints in as reflowedand aged condition were investigated in this work. Mechanism of Mn influence the interfacialreactions, mechanical properties and aging resistance have been explored by analyzing themorphology of interfacial microstructure, the growth of interfacial IMC layer, the tensilestrength and shear strength of solder joint, and the morphology and chemical composition offracture surface. Results show that thinner interfacial IMC layer with scallop-shaped Cu6Sn5grains is formed as a result of Mn addition. Mn doping could significantly restrain the growthof IMC layer for both Cu6Sn5and Cu3Sn. The effects of Mn restrain the growth of theinterfacial IMC layer are enhanced with Mn content increasing from0to0.15%. Inhibition ofthe interfacial IMC growth could attribute to the heterogeneous nucleation mechanism and thepinning effect. Localized Mn enrichment near the interfacial IMC layer provides moreheterogeneous nucleation sites for IMC particles, resulting in more refined IMC particlesprecipitate in the valleys between grains or nucleate on the surface of the interfacial IMClayer. These IMC particles would restrain the interdiffusion of Cu and Sn flux, and retard thedeterioration of solder joints during thermal aging treatment.Results also reveal that adding Mn into the Sn-0.3Ag-0.7Cu solder could improve thetensile strength and suppress the deterioration during thermal aging treatment. The tensilestrength of the Sn-0.3Ag-0.7Cu solder joint reach the highest value when0.05wt.%Mn is added. The improved tensile properties could attribute to the thinner interfacial IMC layer,increased contact area and decreased local stress concentration. The modified interfacial IMClayer increases the contact area between solder and interfacial IMC layer. More refined IMCparticles precipitate on the surface of the interfacial IMC layer which act as obstacles for themovement of dislocations, thus the local stress concentration is decreased. Shear strength andthermal aging resistance of solder joint are enhanced by Mn addition. A maximum shearstrength is obtained by adding0.05wt.%Mn into Sn-0.3Ag-0.7Cu solder. The strengtheningeffect attribute to refined microstructure and fine IMC particles dispersed in solder matrixwhich would hinder the movement of dislocations.