Kinetic Analysis of Electromigration Enhanced Intermetallic Growth and Void Formation in Pb-Free Solders

The kinetics for electromigration (EM) enhanced intermetallic growth and void formation in Sn-based Pb- free solder joints with Cu under bump metallization (UBM) was analyzed. 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 EM parameters were obtained by solving the inverse problem of the EM- enhanced intermetallic growth and found to be consistent with the literature values. Finite difference method was used to solve the mass transport kinetics within the intermetallic phases and across each interface of interest. Simulation showed that with zero current and no EM effects, intermetallic growth follows a parabolic law, suggesting a diffusion controlled mechanism for thermal aging. However, under significant current stressing (4.12 x 10⁴ and 5.16 x 10⁴ Amp/cm²), the growth of the dominant intermetallic Cu₆Sn₅ clearly follows a linear law, suggesting a reaction controlled mechanism for electromigration. The kinetic results were consistent with the experimental observations. The vacancy transport under EM was also analyzed and the results showed substantial increase in vacancy concentration in the Cu₆Sn₅ phase near Cu₃Sn/Cu₆Sn₅ interface. The peaking of the vacancy concentration predicts EM-induced Kirkendall void formation at this region and its effect on solder reliability is discussed.