Kinetic analysis of Ni5Zn21 growth at the interface between Sn–Zn solders and Ni

Ni5Zn21 is the primary formed phase in the Sn-9wt%Zn/Ni interfacial reactions. In this study, the growth kinetics of the Ni5Zn21 phase was systematically investigated by solid-state aging between 150 °C and 170 °C, and liquid-state aging between 230 °C and 270 °C. The relevant kinetic parameters and the corresponding activation energies were determined. Particularly, the linear growth of the Ni5Zn21 layer was clearly found in the early stage of Sn-9wt%Zn/Ni solid-state reactions. When the Ni5Zn21 layer reached a critical thickness of about 6–10 μm, depending on the reaction temperature, the growth rate gradually decreased and followed a parabolic relationship. In addition, the reactions of Sn-5wt%Zn and Sn-3wt%Zn solders also demonstrated similar growth behaviors. Nevertheless, the Ni5Zn21 growth would significantly slow with decreasing Zn contents. Further, in the reactions with molten Sn–Zn solders, the Ni5Zn21 layer growth was also found to be linear prior to becoming about 3 μm thick, and then became parabolic. The appearance of the linear growth in both reflow soldering and isothermal aging suggested that the Sn–Zn/Ni interface was controlled by chemical transformation in the initial stage. The Ni5Zn21 growth mechanism is discussed in-depth in this paper.