Early stage soldering reaction and interfacial microstructure formed between molten Sn–Zn–Ag solder and Cu substrate

The soldering reaction and interfacial microstructure formed between liquid Sn–Zn–Ag solder and Cu at the early stage of soldering at 250 °C for 15 s were studied primarily with the aid of transmission electron microscope (TEM). To achieve the early stage reaction information, the soldered specimens, 5 mm × 5 mm × 500 μm solder on 10 mm × 10 mm × 20 μm Cu, were rapidly quenched in liquid nitrogen after soldering. The results of TEM interfacial analysis show that a Cu–Zn reaction zone, consisting of β′–CuZn and γ–Cu₅Zn₈, formed near Cu while a Ag–Zn zone, consisting of γ–Ag₅Zn₈ and ϵ–AgZn₃, formed near solder. The innermost layer adjacent to the Cu substrate is an amorphous Cu-Zn diffusion region that contains dispersed β′–CuZn nanocrystalline cells. The β′–CuZn also precipitates in the γ–Ag₅Zn₈ and ϵ–AgZn₃ layer due to the supersaturation of Cu.