Failure mechanisms of solder interconnects under current stressing in advanced electronic packages: An update on the effect of alternating current (AC) stressing

The relentless pursuit of miniaturization and integration in electronic industry has put challenges on the reliability of electronic products in aspects of the ever more severe environmental loadings, for example, thermal, electrical, and mechanical stresses. The failure mechanism of solder interconnects under direct current (DC) stressing has been studied and summarized as electromigration (EM), thermomigration (TM), Joule heating, stress-related mechanical degradation. However, the current flow through some paths for example clock buses and signal lines is bidirectional, i.e. alternating current (AC). The damage made in the positive half will be partly healed in the negative half, leading to less EM issues than that in DC stressing. Except for the TM damage, the thermal cycling will also induce thermal-mechanical fatigue issues under AC stressing. Considering these different failure mechanisms between DC and AC stressing, a better understanding of AC stressing will assist in finding out the root cause of failures in practical use. In this review, we discuss the research activities on the effect of AC stressing and their focus on the microstructure evolution of solder or metal interconnects. We firstly explained the discrepancies of reported temperature distribution and analyzed the possible reasons. We then discuss about microstructure evolution of interfacial intermetallic compounds under both DC and AC stressing, as well as the effect of various frequency. Finally, we discuss the failure mechanisms of solder interconnects under AC stressing, especially the unique thermal fatigue. The outlook of AC stressing study has also been discussed.