Strong effects of minor Ga addition on liquid-state Sn–Ga/Co interfacial reactions

Abstract

Liquid-state interfacial reactions between Co and Sn–Ga solders at 250 °C were examined with addition of various Ga contents (0.1wt.%–1wt.%). Minor Ga addition in Sn-based solders strongly affected thegrowth of intermetallic compounds (IMCs) at the interface and even the IMC species. With addition of less than 0.3wt.%Ga, one continuous CoSn3 layer was formed. Most importantly, the CoSn3 growth was greatly decreased by 95% with only 0.2wt.%Ga addition. Multi-stage interfacial reactions with different solders demonstrated that the strong inhibition of IMC growth was likely due to minor Ga doping in the CoSn3 phase and a possible mechanism was proposed. When the Ga content was increased to 0.4wt.%–0.6wt.%, only a thin uniform CoGa layer, being ∼1 μm thick, was stably present at the interface. Even after aging for 12 h, it had no significant growth. At higher Ga contents (0.7wt.%–1wt.%), the dominant phase changed to the CoGa3 phase in the initial reaction. Subsequently, the CoGa3 gradually transformed to the CoGa. In addition, the partial isothermal section (less than 40at.%Co) of the Sn–Ga–Co phase diagram at 250 °C was experimentally determined. Two three-phase regions, L–CoSn3–CoGa and L–CoGa–CoGa3 tie-triangles, were identified. The phase equilibria relationship can reasonably interpret the transitions of interfacial products with varying Ga contents.