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, Cu
₃
Sn and Cu
₆
Sn
₅
, 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 10
⁴
and 5.16 x 10
⁴
Amp/cm
²
), the growth of the dominant intermetallic Cu
₆
Sn
₅
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 Cu
₆
Sn
₅
phase near Cu
₃
Sn/Cu
₆
Sn
₅
interface. The peaking of the vacancy concentration predicts EM-induced Kirkendall void formation at this region and its effect on solder reliability is discussed.