Effect of liquid-solid electromigration on interfacial reaction in Cu/Sn-9Zn/Cu solder joint

The diffusion behavior of Zn atoms and the interfacial reactions in line-type Cu/Sn-9Zn/Cu solder joints undergoing liquid-solid electromigration (L-S EM) were investigated under a current density of 5.0×10³ A/cm² at 230 °C. Liquid-solid (L-S) reaction without current at 230°C was also conducted for comparison. It was clearly observed that there existed a reverse polarity effect induced by the Zn atoms migration towards the cathode undergoing L-S EM, i.e., the interfacial intermetallic compounds (IMCs) at the cathode grew continuously and were significantly thicker than those at the anode. At the anode, dissolution and massive spalling of the interfacial Cu-Zn IMCs occurred and the depletion of Zn resulted in the formation of Cu₆Sn₅ IMC. At the cathode, the interfacial IMCs remained as Cu₅Zn₈. It was deduced that the effective charge number (Z*) of Zn atoms in both liquid Sn-Zn solder and interfacial Cu-Zn IMCs turned into positive, resulting in the abnormal diffusion behavior of Zn atoms undergoing L-S EM.