Influence of the aggregated Ag₃Sn on the improvement of electromigration phenomenon in the doped Sn58Bi solder joints

Electromigration (EM) phenomenon has become more troublesome in the Sn–Bi solder joints as the increase of current density. Doping method is reported as one of the solution to improve solder joints. However, the doped particles are easily aggregated and formed large sized intermetallic compounds (IMCs) in the solder matrix during reflow soldering. In this research, EM phenomenon was studied in the Sn57.6Bi0.4Ag, Sn58Bi with 0.4 wt% Ag doping (Sn58Bi + 0.4Ag) and Sn58Bi solder joints to fully understand the influence of the aggregated Ag₃Sn IMCs. The grain refinement effect of the Ag element was found to be the main reason of the improvement of EM phenomenon. Also, it was proved that Sn57.6Bi0.4Ag solder joints with uniformly distributed Ag₃Sn IMCs performed well in preventing the segregation of Bi phase. However, Sn58Bi + 0.4Ag solder joints with the aggregated Ag₃Sn IMCs did not perform well, forming thicker Bi rich layer at the anode. On the other hand, the addition of Ag element had minimal effect on the formation and consumption of Cu–Sn IMCs in the Sn–Bi solder joints. The thickness of Cu–Sn IMCs layer at the cathode in the Sn57.6Bi0.4Ag and Sn58Bi solder joints were found to be similar. However, the aggregated Ag₃Sn IMCs in the Sn58Bi + 0.4Ag solder joints could promote the migration of Cu–Sn IMCs to the cathode.