Effects of Minor Ga Addition on Interfacial Reactions Between Sn-Ga Solders and Cu

This study investigated the effects of Ga doping (0.1–1 wt.% Ga) on Sn-Ga/Cu interfacial reactions. In liquid-state reactions at 250°C, the addition of 0.1 wt.% and 0.2 wt.% Ga induced the formation of scallop-shaped Cu6Sn5 and Cu3Sn phases, with ripening of Cu6Sn5 grains. However, Ga concentrations > 0.5 wt.% led to an unstable microstructure evolution. Initially, a thin layer of γ1-Cu9Ga4 formed but later detached and disintegrated in the solder, followed by Cu6Sn5 formation. Notably, Ga enhanced Cu6Sn5 nucleation, refining grains and promoting faster growth. The solid-state Sn-Ga/Cu reactions were also examined at 140°C to 200°C. Adding 0.2 wt.% Ga had no significant influence on the Cu6Sn5 and Cu3Sn formation, but the Cu3Sn/Cu6Sn5 thickness ratio increased with decreasing aging temperature, probably because of the higher Cu diffusion to Sn at lower temperatures. However, the interface with 0.5 wt.% Ga became unstable, particularly < 160°C, resulting in Cu6Sn5 dissolution, phase fracture, and Cu substrate dissolution. This instability was attributed to Cu outer-diffusion induced by Ga at lower temperatures. In contrast, the interface was relatively stable at 200°C. Similar interfacial results were observed with 1 wt.% Ga. Additionally, the phase equilibria of Sn-Ga-Cu system at 250°C were investigated to discuss the interfacial reactions.