Fabrication of high density and high conplanarity lead-free solder bump by a novel process for advanced wafer level packaging

This paper aims to provide a fine-pitch Sn/0.7Cu lead-fee solder bumps fabrication process that is characterized by using a novel plating-friendly polishing mechanism to transform the plated-based Sn/0.7Cu lead-free solder bumps with huge height deviation into smooth and uniform ones. The final experimental results showed that the UIW (uniformity in wafer) of Sn/0.7Cu solder bumps at 50 mum pitch size could be sharply decreased from 9.19% after plating to 3.54% after polishing and even 1.9% after reflow throughout the entire 4 inch silicon wafer. Most importantly, after reflow uniformity could be controlled as accurately as within 1.25% in each dies (10 mm times 10 mm) respectively. This proposed polishing mechanism could assist the plating-based fine-pitch solder bumps in precisely getting better coplanarity so as to enhance packaging reliability and yield.