A novel bumping process for fine pitch Sn–Cu lead-free plating-based flip chip solder bumps

Electroplating is the best process for the manufacture of fine pitch flip chip solder bumps. However, certain unstable electroplating parameters usually cause poorer coplanarity, which affects packaging reliability and yield. This paper attempts to utilize a CMP-like polisher to reduce the nonuniform height deviation after electroplating. The optimization of three major polishing parameters—pad hardness, loading pressure, and polishing speed—enables the polisher to have a higher material removal rate (MRR) and an easier manipulation as compared with chemical mechanical polishing (CMP). After polishing at a pitch size of 100 μm, the overall coplanarity could be decreased sharply from 33±2.5 μm (coplanarity=7.5%) to 28±1 μm (coplanarity=3%) and it even reached 26±0.5 μm (coplanarity=1%) after reflow.