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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, Cu 3 Sn and Cu 6 Sn 5 , 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 current͒, intermetallic growth followed a parabolic law, suggesting a diffusion controlled mechanism for thermal aging. However, under significant current stressing ͑4 ϫ 10 4 A/cm 2 ͒, the growth of the dominant intermetallic Cu 6 Sn 5 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 Cu 6 Sn 5 phase near the Cu 3 Sn/ Cu 6 Sn 5 interface. The peaking of the vacancy concentration explains the substantial Kirkendall void formation under electromigration at this region.

The effect of underbump metallization (UBM) on electromigration (EM) lifetime and failure mechanism has been investigated for Pb-free solder bumps of 97Sn3Ag composition in the temperature range of 110–155°C. The EM lifetime of the SnAg Pb-free solders with either Cu or Ni UBM was found to be better than the eutectic SnPb (63Sn37Pb) solders but worse than high-Pb (95Pb5Sn) solders. In the test temperature range, the EM lifetimes were found to be comparable for Cu and Ni UBMs but with different activation energies: 0.64–0.72eV for Cu UBM and 1.03–1.11eV for Ni UBM. EM failure was observed only in solder bumps with electron current flow from UBM to the substrate. Failure analysis revealed that EM damage was initiated by the formation of intermetallic compounds (IMC) at the UBM∕solder interface which was found to be significantly enhanced by mass transport driven by the electron current. Under EM, the continued growth of IMC with the dissolution of the UBM and the accumulation of Kirkendall voids resulted in the formation of interfacial cracks and eventual EM failure of the solder bump. For Ni UBM, the IMC formation was dominated by the Ni3Sn4 phase while for Cu UBM, a bilayer of Cu3Sn∕Cu6Sn5 was found. Void formation at the Cu6Sn5∕solder interface was found to be important in controlling the EM lifetime of the Cu UBM solder.

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