The effect of non-reactive alloying elements on the growth kinetics of the intermetallic compound between liquid Sn-based eutectic solders and Ni substrates

The effect of non-reactive alloying elements on the growth kinetics of the intermetallic compound formed between liquid Sn-based eutectic solders and Ni substrates at 250–280 °C was studied, with Ag, Pb and Bi as the alloying elements. Experimental results showed that only the intermetallic compound (IMC) Ni3Sn4 was present as the reaction product. Based on the diffusion controlled reaction mechanism, a parabolic kinetic equation in which both Sn concentration and alloying element effects were quantified, was derived and verified by comparing the kinetic data obtained by using four different solders with different Sn concentrations and alloying elements. The apparent activation energies for pure Sn and eutectic Sn–Ag, Sn–Pb and Bi–Sn solders reacting with Ni, the substrate, are in the range of 28.7–32.7 kJ mol−1, which indicates slight variations in activation energy. The similarity between the activation energies of these four solders suggests that the diffusion of Sn atoms through the IMC is the rate controlling step. The effect of non-reactive alloying elements exerted on the pre-exponential factor of the equilibrium constant which multiplied by the bulk concentration of Sn in solder gives the surface concentration of Sn in Ni3Sn4.