Investigation of Intermetallic Compound Growth Enhanced by Electromigration in Pb-Free Solder Joints

An efficient numerical method based on simulated annealing was developed to extract the diffusion and electromigration (EM) parameters for multi-phase intermetallic compounds (IMCs) formed between under-bump-metallurgy (UBM) and solder bumps. This method was applied to the growth of Cu-Sn IMCs during thermal aging and electromigration in Pb-free solder joints with Cu UBM. Diffusion coefficients and effective charge numbers of Cu and Sn in IMCs were derived in this study. The simulated annealing approach provided superior efficiency and accuracy over the conventional grid heuristics, and was proven to be particularly useful for analyzing multiple parameters in multiphase systems, such as solder joints. The effect of EM-enhanced IMC growth on stress fields in solder joints was also studied using finite element analysis. The negative volumetric strain appeared to be an important factor contributing to the degradation of EM reliability of solder joints.