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 degC 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 Cu₆Sn₅ 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 5times10³A/cm² at 35 degC , 55 degC and 75degC , were calculated.