Ag plating and its impact on void-free Ag/Sn bumping

We have already developed the eutectic Sn-Ag solder bumping process by alloying Ag/Sn electroplated metal stacks to overcome some problems concerning Sn-Ag alloy plating. As the dimensions of solder bumps shrink, the effect of voids in the solder bumps on electromigration resistance must be discussed. For the Sn-Ag alloy plated bumps, voids in the solder bumps as reflow processed are difficult to be avoided. The large amount of degassed species from the alloy-plated bumps due to strong chemical agents trapped in the plated films has a close relation to generation of voids in the bumps. In contrast, though the stack plating process shows less degassing, micro-voids would be left at the interface of the plated stack as embryos of residual voids in the solder bumps. In this study, surface roughness of the underlying Ag films and degassing behavior of the Ag/Sn stack-plated bumps has been investigated using different types of Ag plating solutions. Gas analyses and X-ray imaging inspections were performed for the Ag/Sn plated stacks. Surface roughness of the underlying Ag layer was also characterized by a laser scanning and scanning electron microscopy. From the experimental results, it has been confirmed that less degassing is the most important issue for the Ag/Sn plated stacks. In addition, improvement of surface roughness of the underlying Ag plated films must not be neglected.