Galvanic contact formation of a Cu-Sn alloy layer was investigated using an ionic liquid, trimethyl-n-hexylammonium bis[(trifluoromethyl)sulfonyl] amide, as a solvent for an electrolytic bath. The use of the ionic liquid rendered it possible to raise the processing temperature beyond 100 degrees C so as to form the alloy layers faster than in the case of an aqueous media. The layer obtained was silver-gray speculum metal, which is composed of Cu6Sn5, Cu3Sn, and Cu10Sn3 intermetallic phases. The rate-determining step of the alloy formation was found to be the interdiffusion of Cu (i.e., substrate) and deposited Sn atoms with the activation energy of 58 kJ mol(-1). (c) 2006 The Electrochemical Society.