Self-linking mechanism of micro solder particles dispersion in liquidus polymer

In the self-forming micro bump bonding by the solder paste mixed with a low-viscous epoxy thermosetting resin, voids generated during reflow is a drawback since it affects micro bump formation. We investigated the effect of vacuum degassing and pre-treatment of substrate surface with low-pressure plasma on the micro bump formation. Self-forming micro bumps were collectively assembled on the array copper lands of FR-4 substrate with the paste consisting of Sn-57Bi-1Ag solder particles (10, 20 micrometers in diameters) and the resin. Effect of the surface pretreatment conditions on the void generation and bump shape was found to be small. It was suggested that voids originated from the chemical reaction at the interface between the resin and solder powder surface.