Growth behavior and kinetics of interfacial IMC for ZnSnCuNiAl/Cu solder joints subjected to isothermal aging

Zn–30Sn–2Cu–0.5Ni–0.2Al (ZnSnCuNiAl) solder joints were isothermally aged in the temperature range of 100–170 °C for up to 360 h, and the growth behavior and kinetics of the intermetallic compound (IMC) formed between the ZnSnCuNiAl solder and Cu substrate during the aging process were investigated. The results showed that the interfacial reaction between the liquid solder and the solid Cu substrate caused the formation of IMC layers, which were composed of a thin irregular scallop-shaped ε-CuZn₅ layer and a thick γ-Cu₅Zn₈ layer. With the increase in aging temperature and time, the serrated ε-CuZn₅ layer gradually peeled off from the IMC interface and entered the solder due to thermal mismatch induced cracks between ε-CuZn₅ and γ-Cu₅Zn₈. The growth of the IMC layer at the solder joint interface was controlled by the bulk diffusion, and its thickness had a linear relationship with the square root of the aging time. Diffusion coefficients and activation energy for the interfacial Cu₅Zn₈ IMC layer were plotted as a function of aging temperature, and the values were calculated by the Arrhenius equation. The calculated activation energy of the Cu₅Zn₈ IMC layer was 77.78 kJ/mol, which was higher than that of the similar IMC layer in other lead-free solder joints, indicating a slower growth rate of the IMC layer between the solder and substrate during aging.