Mechanism for serrated cathode dissolution in Cu/Sn/Cu interconnect under electron current stressing

Dissolution of cathode metallization is one of the key degradation processes for solder interconnects under electron current stressing. Very recently, several studies have shown that this dissolution often results in a highly serrated interface, but little is known about how the serrated interface develops and what mechanism contributes to this intriguing microstructural feature. This paper presents detailed observations from a purposely designed experiment to uncover the responsible mechanism. These observations reveal that a pronounced and unstable grain boundary grooving between Cu6Sn5 grains occurs before the serrated cathode starts to develop. This grooving process is driven by preferential dissolution at the grain boundaries, a process similar to the formation of scallop-type Cu6Sn5 between molten solder and Cu. Furthermore, the diffusion and reaction are more significant at the valley of the grooves, which increases the local consumption rate of Cu. This non-uniform consumption of Cu eventually leads to a serrated cathode interface.