SOS wafer Cu pillar bumping process development for flip chip package application

Cu pillar bumping process has been developed on SOS (Silicon on Sapphire) wafer for flip chip package application while in the initial packaging experiment open failure has been found. Voiding in bump pad metal has been found in package failure analysis. The cracks initiated from the voiding have also been captured. Further investigation has confirmed that the surface roughness of bump pad metal is closely related to this failure. The voiding was formed for that the bump pad metal was etched away by plating solvent in bumping process. Solutions on process and bump metal structure design have been discussed and presented with evaluation experiment results in this paper.