Interfacial reaction mechanism and kinetics between Au–20Sn and Sn

Sn is a promising solderable layer to join Au–20Sn solder alloy with common substrates at low temperature. The interfacial reactions between Au–20Sn and Sn during liquid/solid state diffusion and solid state diffusion were investigated. AuSn2 and AuSn4 layers formed at the interface of Au–20Sn/Sn. During liquid/solid state diffusion reaction (LSDR), Au atoms diffuse continuously into liquid Sn. After solidification, lath-like AuSn4 grains gradually precipitated adjacent to the interface in the Sn side. The solidified Sn matrix gradually transformed from eutectic microstructure to hypereutectic microstructure, and finally to peritectic microstructure. Potential mechanisms of LSDR at different temperatures were explained. Growth kinetic analysis of solid state diffusion reaction indicated that the growth rate of AuSn2 is lower than that of AuSn4. The time exponent, n ≈ 0.5, indicates that volume diffusion is the dominant growth mechanism of AuSn2 layer, while grain boundary diffusion is the rate-controlling mechanism for the thickening of AuSn4 with n close to 0.33. The activation energies for the growth of AuSn2 and AuSn4 layers are determined to be 10.97 kJ/mol and 7.79 kJ/mol, respectively.