Microstructure evolution during electromigration between Sn–9Zn solder and Cu

This study investigated the electromigration behavior between Cu and Sn–9Zn solder under a current density of 1.0 × 10³ A/cm² for up to 230 h. The experimental results indicated that Cu₅Zn₈ was formed at the interface between Cu and the cathode side of the Sn–9Zn solder as well as in the bulk near the anode. Consumption of Cu was also observed for the Cu plating on the cathode side and anodic side, but with less compound formation and Cu consumption at the anode. The intermetallic compound layer on the cathode side was always thicker than that on the anode side after the same current-stressing time. The effect of chemical potential overwhelms electromigration in inducing Zn diffusion when a counterflow of electrons and chemical potential gradient exists. Voids formed at the Cu₅Zn₈–solder interface inside the solder regardless of the direction of current flow.