Non-equilibrium dissolution kinetics of micro-size metal particles in lead-free solders

Soldering is the default joining process in the electronic packaging industry. Solder joints are obtained by interaction of the substrate with the molten solder. Soldering reaction and the resulting dissolution of substrate material is a complicated process. Reliability and strength of solder joints during service have always been the critical issues in electronic packaging industry. Mechanical strength of solder joints can significantly be increased by employing composite solders. These composite solders are sometimes obtained by incorporation of micro- and nano-size metal particles in the solder paste before the soldering process. Better understanding of substrate-solder interaction is important for the proper selection of the reinforcing particle size and composition. In the present research the relative importance of interface reaction and diffusion has been studied. Dissolution kinetics of a planar substrate and spherical particles has been investigated. Our results show that the dissolution is governed both by interface kinetics and long-range diffusion. Non-equilibrium behavior has been observed in the early stage of the process. It has been observed that at the early stage the dissolution process is governed by interface kinetics, while diffusion became the rate controlling mechanism at the later phase. A mathematically rigorous model has been proposed for simulating the dissolution of the substrate in the liquid lead-free solders. The study is extended to investigate the dissolution of spherical particles in molten solders. The results show that the initial particle dimension plays a critical role in the end particle size after the reflow process.