The effects of Ag, Cu compositions and Zn doping on the electromigration performance of Pb-free solders

A systematic study of the effect of Ag, Cu and Zn alloying elements on the EM performance of Sn-rich Pb-free solder is reported. EM performance was found to strongly depend on the types of UBMs, alloying elements, compositions, and microstructures in solder. While leaded solders were found to be more robust than Pb-free solders, SnAg solders, in general, has better EM performance than SnCu solders. Greater than 1% Ag in SnAg solder is needed to stabilize the Sn grain microstructure and suppress the rapid diffusion process that drives early EM failures. Cu on top of Ni UBM improves the EM reliability. A minor amount of Zn doping in SnAg solder stabilizes both Ag₃Sn and Cu₆Sn₅ IMC networks in the solder microstructure and slows down the EM process.