Interfacial segregation of Bi during current stressing of Sn-Bi/Cu solder interconnect

Effect of electromigration on the interfacial structure of solder interconnects was examined in a Sn-Bi/Cu interconnect system. At current densities above 104 A/cm2, Bi migrated along the direction of the electron flow in the solder alloy. A continuous Bi layer was found at the solder interface on the anode side, while a Sn-rich region formed at the cathode side of the electrical connection. The presence of the Bi layer inhibited further growth of Cu-Sn intermetallic phase at the interface by acting as a diffusion barrier to the reacting species.