On the initial stages of solid state reactions in Ni/Sn-Ag solder system at 150–210 °C

Abstract

For the first time, the initial stage(s) of solid state reaction (150–210 °C) between Sn-2wt%Ag and Ni have been investigated. A smooth round, scalloped shape Ni3Sn4 intermetallic (IMC) layer is observed for shorter interaction times (1min-1hr) while for longer times (1hr-4hr) the scallop-like morphology is gradually transformed to a facetted growth morphology. The growth kinetics of the reaction layer follows a power law regime with a growth exponent close to 1/3. The apparent activation energy of this regime was found to be 53 kJ mol−1. To determine the mechanisms of Ni3Sn4 layer growth, grain growth of the IMC layer was also analyzed. Together with this analysis and kinetic growth of the layer, a simple model is proposed, which gives the insight for possible rate limiting diffusion process for solid state reaction. With this model, we can conclude that the rate limiting diffusion process is grain boundary diffusion. Finally, a theoretical thermodynamic and kinetic analysis for the initial stage of growth of Ni3Sn4 layer was done and grain boundary diffusion coefficient for Ni3Sn4 phase was determined.