Systematic Study of Liquid-State Interfacial Reactions Between Co and In-Sn Solders with Varying Sn Contents

This study investigated the interfacial reactions between Co and In-Sn solders, with various compositions, up to 90 at% Sn, at 350°C, with the aim of evaluating their potential for use in the solid–liquid interdiffusion (SLID) process. The results demonstrated that the reaction phases formed at the interfaces exhibited significant variations depending on the Sn content present in the In-Sn solders. For Sn content below 2 at%, the reaction phase was CoIn₃. Notably, the CoIn₃ in the In-2 at% Sn/Co reaction exhibited a linear growth at a rate of ~ 15 μm/h, which was significantly higher compared to the In/Co reaction. The accelerated growth rate could be attributed to the minor addition of Sn, which facilitated both the nucleation and growth of CoIn₃. With an increase in Sn content to 2.5–3.5 at%, the dominant reaction phase shifted to Co(In,Sn)₂, but its growth was significantly hindered. With a further increase in Sn content within the range of 4–35 at%, the irregular Co(Sn,In) phase became dominant. However, as the Sn content exceeded 36 at% and extended up to 90 at%, the Co(Sn,In)₂ phase remained stable at the interface, and its growth decreased significantly with increasing Sn content. The observed shift in the reaction phases is closely related to the local phase equilibrium. The suggested phase diagram of Co-In-Sn system was proposed to further understand the relationship between interfacial reaction and phase equilibrium. The Sn content of In-Sn solders not only influenced the formed reaction phase but also the growth rates and microstructures. Careful control of Sn content is crucial for the SLID process of In-Sn/Co system.