Modeling microstructure effects on electromigration in tin-based solder bumps

Abstract

This work aims at gaining a better understanding of the effect of microstructure on the driving forces and resulting mass diffusion process associated with electromigration and the resulting mechanical response in tin-based solder bumps.The electromigration and stress driven diffusion equation is solved coupled to the mechanical equilibrium and constitutive equations by introducing a diffusional inelastic strain. A crystal plastic constitutive model is developed to capture the elastic and plastic anisotropy in tin-based solder alloys. Both the steady state and transient models are developed and a finite element based simulation approach is used to predict the effect of the microstructure. In the steady state model, the current-driven diffusion is balanced by the resulting stress gradient, resulting in infinite life. The effect of the crystal orientation on the limiting current density is investigated. In the transient model, the evolution of the stress and atomic flux divergence is simulated, and the effect of the crystal orientation on the electromigration-induced degradation is predicted. Electron Backscatter Diffraction Analysis (EBSD), is used to characterize microstructure of tin-based solder bumps in wafer level chip scale packages (WLCSP). EBSD, in combination with the accelerated electromigration tests, provide valuable information on the effect of microstructure on electromigration. And finite element analysis is performed to correlate with the experimental data.