The study investigates the electromigration effects upon the Sn/Cu interfacial reaction. A Sn/Cu/Sn/Cu/Sn couple was prepared by the casting method. The couple was reacted at temperatures ranging from 150 to 190^oC with 5000A/cm² current density. No massive Cu₆Sn₅ phase formation is found in the couples reacted at 160^oC for 120 hours. However, after current stressing at 180^oC for 120 hours, at the interfaces with electrons flowing from Cu to Sn, large irregular Cu₆Sn₅ phase was formed and penetrated into the Sn matrix. However, at the interfaces with the opposite electron flow direction, two continuous reaction layers, Cu₆Sn₅ and Cu₃Sn, were formed. At the Cu/Sn interfaces, atoms are driven into the Sn phase by the electron current flow. Since grain boundaries are faster paths for Cu diffusion, large amounts of Cu atoms cram into the Sn region with the electron current flow through grain boundaries, reacted with Sn and large Cu₆Sn₅ compounds were formed. Sn grains grow with reaction time as well. When the sizes of Sn grains are large, only limited grain boundary paths are available. Instead of the grain boundary diffusion, the Cu atoms diffuse along the free surfaces, and the reaction Cu₆Sn₅ compounds are found at the surfaces of the couples. It is found that Cu dissolution in the molten Sn is significant when the casting method is used for sample preparation, and fine Cu₆Sn₅ phases precipitate after quenching. The Cu₆Sn₅ precipitates are an important source for the Cu supply at the interfaces with electrons flowing from Sn to Cu. The Cu atoms of the precipitates move toward the interfaces under current stressing, and the growth of the reaction layers are thus faster in the initial stages at the interfaces where electrons are from the Sn side to Cu side.

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