Fluxless Sn-Ag solder ball formation for flip-chip application

A technique for the formation of self-aligning Sn-Ag solder balls is presented. By controlling the solder reflow conditions through the optimization of the solder adhesion to the surface, a very thin layer of solder can be used. This makes possible the use of conventional photolithography and metal evaporation for the definition of the solder volume, thereby assuring its precision. Both wettable and nonwettable materials and their effects on the balling- up process are presented. The solder ball formation process and optimization are discussed along with the uniformity and scalability of the results.