Electromigration enhanced intermetallic growth and void formation in Pb-free solder joints

A kinetic analysis was formulated for electromigration enhanced intermetallic evolution of a Cu-Sn diffusion couple in the Sn-based Pb-free solder joints with Cu under bump metallurgy. The simulated diffusion couple comprised the two terminal phases, Cu and Sn, as well as the two intermetallic phases, Cu3Sn and Cu6Sn5, formed between them. The diffusion and electromigration parameters were obtained by solving the inverse problem of the electromigration enhanced intermetallic growth, and they were compatible with the literature values. Finite difference method was applied to the whole simulated domain to solve for the mass transport kinetics within the intermetallic phases and across each interface of interest. Simulation showed that, when electromigration effect was absent (zero current), intermetallic growth followed a parabolic law, suggesting a diffusion controlled mechanism for thermal aging. However, under significant current stressing (4x10(4) A/cm(2)), the growth of the dominant intermetallic Cu6Sn5 clearly followed a linear law, suggesting a reaction controlled mechanism for electromigration. Simulation results were consistent with the experimental observations. The analysis of vacancy transport was also incorporated with the model, and the results showed substantial increase in vacancy concentration at the Cu6Sn5 phase near the Cu3Sn/Cu6Sn5 interface. The peaking of the vacancy concentration explains the substantial Kirkendall void formation under electromigration at this region. (c) 2006 American Institute of Physics.