Kinetics of the Au-Ni-Sn Ternary Intermetallic Layer Redeposition at the Solder/under-Bump Metallurgy Interface during Aging Treatment

The Au-Ni-Sn ternary intermetallic layer redeposition at the interface between the solder and the Au/Ni/Cu under-bump metallurgy (UBM) was measured experimentally and analyzed theoretically. The predicted thicknesses of the layer, assuming Au diffusion in the Sn-3.5Ag solder matrix, were thicker than those measured experimentally. The thicknesses predicted in the Sn-37Pb solder matrix, on the other hand, were thinner than those measured experimentally. The discrepancies between the predicted and measured thicknesses were discussed based on findings, such as formation of the ternary phase and presence of AuSn₄ depletion zones in the solder matrices, and the stoichiometry of the ternary phase.