Effect of contact metallization on electromigration reliability of Pb-free solder joints

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...