Phase segregation under reversed current stressing in eutectic Sn-based solder joints

Alternating current will lead to more complex phenomenon in electromigration (EM) compared to one directional current, in terms of the different controlling mechanisms. Therefore, Sn-based joints will exhibit unique characteristics of electromigration under reversed current stressing. Eutectic SnBi solder was used as the main research object in the current study. The ambient temperature was set to 50°C, and the DC current input was 5A, leading to a corresponding current density of 10⁴A/cm² during the EM test. Bi was observed to be segregated along the anode side, which was considered to be the dominant diffusing element during the whole reversed current testing process. The polarity effect of interfacial intermetallic compounds (IMC) also existed in whole reversed current testing process. During the one directional EM process, phase segregation can be observed with obvious phase coarsening effect, where Bi and Sn continuous phases were formed at the anode and cathode, respectively. During the reversed EM process, the initial continuous phase in the eutectic SnBi solder joints migrated towards the opposite direction and finally disappeared. The thickness of continuous phase was inversely proportional to the rate of migration, but the average sizes of phase were reduced.