Retarding intermetallic compounds growth of Zn high-temperature solder and Cu substrate by trace element addition

Since the thickness of layers of intermetallic compounds (IMCs) at the joining interface has an apparent effect on the joint reliability, understanding the growth mechanisms of IMCs is important for die-attachment applications. In the present study, the interfacial reaction between elements (Ca, Mn, Cr, and Ti) added to Zn alloys and Cu substrate were investigated in detail, focusing on IMCs grown by isothermal aging at 150 °C. The reaction layers included two types of Cu–Zn IMCs i.e., γ-Cu₅Zn₈ and ε-CuZn₅ phases. The joining interface with trace elements containing Zn solder reduced the growth rate of the IMCs. In particular, the addition of 0.1 wt% Cr to pure Zn solder decreased the growth rate of the IMCs by approximately 50 %. The mechanism we propose assumes that a phase containing the small Cr atoms exists between the solder and ε-CuZn₅, which suppresses the diffusion of metal atoms. To summarize, the Cr additive showed beneficial effects in terms of suppressing the growth of IMCs during the solid-state isothermal aging.