Theoretical and experimental investigations of microstructural changes in lead-free solders

Experiments show that the microstructure of solders changes over time. From a materials science point-of-view this phenomenon considerably affects the reliability and lifetime of microelectronic products. It is, therefore, important to quantitatively predict the amount of microstructural change. In the present paper we concentrate on a theoretical and experimental description of spinodal decomposition as well as subsequent phase growth in binary solder alloys. We present experimental results and a theoretical description, which is based on an extended diffusion equation of the phase field type, and which can be interpreted as an generalization of the Cahn-Hilliard equation. Moreover, it takes diffusion of the Fickian type, surface tensions along the phase boundaries as well as local thermo-mechanical stresses into account. In particular we turn our attention to the determination of the required material parameters, which can all be obtained consistently from atomistic models or adopted from literature. As an example the FCC-structured lead-free solder alloy Ag-Cu is considered and numerical results are presented.