Microstructural evolution by electromigration in line-type Cu/SnBi/Cu solder joint

In this study, the microstructural evolution of eutectic SnBi solder in the solid and molten states were clarified using line-type Cu/SnBi/Cu solder joints. When the eutectic SnBi solder was in the solid state during the electromigration (EM) test, a Bi-rich layer formed at the anode side while a Sn-rich band formed at the cathode side, and the intermetallic compound (IMC) at the cathode side was thicker than that at the anode side. The growth of the Bi-rich layer followed a linear dependence on the time of electronic current stressing. While the actual temperature of the solder joint was above 140°C and the solder was in molten state or partially molten state, two separate Bi-rich layers formed at the anode side and a great many Cu₆Sn ₅ IMC precipitates formed between the two Bi-rich layers. Also, the IMCs at the cathode side were thinner than those at the anode side. In this case with a current crowding-reduced structure, the products of diffusivity and effective charge number of Bi in the eutectic Cu/SnBi/Cu solder joints stressed with 5×10³A/cm² at 35°C, 55°C and 75°C, were calculated. ©2008 IEEE.