Electron Attachment: A New Approach to ${\rm H} _{2}$ Fluxless Solder Reflow for Wafer Bumping

Wafer bumping, via fluxless solder reflow, can be performed using electron attachment (EA) with nonflammable mixtures of hydrogen (= 4 vol%) in nitrogen. The EA process facilitates dissociation of molecular hydrogen at ambient pressure and a temperature significantly lower than that of thermal dissociation. Our studies suggest that the EA process promotes the formation of atomic hydrogen anions, which reduce solder oxides and facilitate fluxless solder reflows at temperatures close to the solder melting point.