Effects Of Zn On The Interfacial Reaction And Electromigration Reliability Of Sn-Bi Lead-free Solder

Lead-free solder could eliminate the negative impact of Pb on human health andenvironment, which should be widely used in future. Eutectic Sn-Bi solder attractsmuch attention because of its low melting point （139oC）, good wetting ability andmechanical property. We investigated the effects of Zn on intermetallic compounds,its growth mechanism, activation energy, diffusion rate, and electromigrationreliability of eutectic Sn-Bi solder by interfacial reaction and electromigration.In interfacial reaction, Cu₅Zn₈would replace Cu6Sn5due to its lower activationenergy （35.341kJ/mol） in Zn-doped Sn-Bi solder. Time exponent of Cu₅Zn₈was0.5,which indicated the kinetic of its formation was diffusion-control. The diffusion rateof Zn increased with temperature while the increment became smaller.By electromigration, we concluded three main conclusions as follow.Firstly, in eutectic Sn-Bi solder, grain rotation was observed at the interface firstand then the grains in solder region began to rotate gradually with the increasing ofthe current stressing time. More grains rotated at cathode than anode because ofelectromigration. In Zn-doped Sn-Bi solder, the initial size of Sn and Bi grains werelarger than that of eutectic Sn-Bi solder. No grain rotation appeared while prominentsurface deformation was found by in-situ observation.Secondly, Cu6Sn5extrusion was found on Cu substrate in eutectic Sn-Bi whileCu₅Zn₈displayed bi-layer bulged structure in Zn-doped Sn-Bi after current stressing.In eutectic Sn-Bi solder, there was not enough space to hold Cu6Sn5as reactionproduct, because the reacting atoms were immigrant rather than native. In Zn-dopedSn-Bi solder, it was thought that the bulge was the volume expansion resulting fromthe formation of Cu₅Zn₈, because the molar volume of Cu₅Zn₈was larger than that ofCu and Zn.Thirdly, Cu₅Zn₈would replace Cu6Sn5due to its lower Gibbs free energy. Theeffect of interfacial reaction on Bi accumulation could be completely ruled out.Therefore, it was thoroughly proved that electromigration was the dominant reasonfor Bi accumulation.