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 degrees 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 epsilon-CuZn5 layer and a thick gamma-Cu5Zn8 layer. With the increase in aging temperature and time, the serrated epsilon-CuZn5 layer gradually peeled off from the IMC interface and entered the solder due to thermal mismatch induced cracks between epsilon-CuZn5 and gamma-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.