Observation of solder fillers coalescence in resin for development of self-organization assembly process

A novel assembly method for electronic devices replacing conventional methods is demanded. Self-organization assembly method using resin containing solder fillers has the possibilities to replace them. The method enables us to form reliable interconnects equal to that of solder bump method with underfilling, through low cost process like that using Anisotropic Conductive Adhesives (ACAs) with flexibility against design change. The process of this method is based on movement, coalescence, and wetting phenomena of the solder fillers. And the coalescence phenomena were focused on. Activator is contained in the resin to eliminate the oxide films around the solder fillers. The particle-size distribution after heating process ensured the effects of the oxides and the activator on the coalescence, which is essential for forming of conductive paths. In-situ observations and temporal change of coalescence frequency revealed that 110°C pre-heating for 4 minutes make the activator eliminate the oxide film enough before melting of the fillers, enables fillers to coalesce well.