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

Abstract

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.