Effect of Thermal Processing on the Dissolution of Micro/Nano-Size Metal Particles in Lead-Free Composite Solders

The ever increasing miniaturization of electronic devices has pressed the need to find the alternate solders that can deliver the necessary strength and reliability of the solder joints. In this view the development of composite solders seems a promising option and has become the focus for many researchers in recent years. An assessment of terminal size of embedded metal particles in a composite solder after reflow process is critical to evaluate the quality and strength of the solder joint. During the actual reflow process the heating phase, holding time and cooling phase are parts of the necessary thermal cycle of the process. These thermal variations have critical effect on the terminal size of the particles and growth of intermetallic compound (IMC) around them during the process. This paper presents a mathematical model to simulate the dissolution behavior of metal particles in composite solders during reflow process. The mathematical model is based on basic mass diffusion process and involves the actual physical properties of various species (particle, IMC, solder etc) of the system. Thermal effects and related thermodynamic constraints together with the non-equilibrium interface kinetics of dissolving micro-size particles are also considered. Dissolution behavior of micro-size Cu particles in lead-free, Sn-Ag-Cu (SAC) alloy solder was studied as a representative example. The growth of IMC and terminal size of embedded particles for various reflow conditions were analyzed using the model. The end particle size and the thickness of the IMC layer around the particles are presented for various reflow conditions. Effects of heating rate, holding time and cooling rate on the dissolution of copper particles and growth of IMC are also investigated. Results from the model show the interesting behavior of micron-size particle dissolution in liquid solders. In addition to the holding time, cooling rate of the solder was also seen to play an important role in the dissolution of copper particles.