Abstract:The flip chip processes of UBM fabrication and solder reflow were studied. By varying the thicknesses of Ni barrier layer and Cu wetting layer, the variation of the mechanical strength of the solder joints at the interface between SnAgCu solders and Al/Ni/Cu thin films was investigated. The results prove that the mechanical strength of the solder joints between SnAgCu solders and Al/Ni/Cu thin films is influenced mostly by the thickness of Ni barrier layer. And the mechanical strength can also be enhanced as the thickness of Cu wetting layer increases. After ball shear tests, the morphologies of the solder joints, observed by SEM and analyzed by EDS, were concluded as four modes of failure, that is, pad lift, brittle break, ball lift and ductile break. Each mode represents different reflow quality, mainly determined by the thickness and composition of UBM. The research results provide a theoretical guidance for the flip chip process optimization.
