Metastable CoSn₄ formation induced by minor Ga addition and effective suppression effect on the IMC growth in solid-state Sn–Ga/Co reactions

Solid-state interfacial reactions between Co and Sn-based solders doped with different levels of Ga (0.02 to 0.5 wt%) were investigated. With minor Ga addition of less than 0.05 wt%, the CoSn₃ layer was uniformly formed at the interface. Compared to the pure Sn/Co reaction, the CoSn₃ growth was greatly suppressed by 95 % with only 0.02 wt%Ga addition at 160 °C. The growth kinetics of CoSn₃ were systematically studied at temperatures of 160 to 200 °C. The CoSn₃ phase clearly exhibited a linear growth with aging time and the suppression effect was more significant with decreasing the aging temperature. It could be attributed to the fact that Ga atoms doped in the CoSn₃ phase retarded the nucleation and growth of CoSn₃. In particular, with 0.1 wt%Ga addition, the reaction layer changed to the metastable CoSn₄ phase, rather than CoSn₃. Most importantly, the CoSn₄ growth was also strongly suppressed. It was only ~3-μm thick at 180 °C even after aging for 480 h. Similarly, a thin layer of CoSn₄ was formed in the reactions with 0.2 to 0.4 wt%Ga. When the Ga content was increased to 0.5 wt%, the dominant reaction phase changed to the CoGa, which was very thin and stably present at the interface. The CoSn₄ and α-CoSn₃ phases with similar orthorhombic crystal structures and the reasons for the phase change were further discussed.