Interfacial reactions in Au/Sn/Cu sandwich specimens

The Au/Sn/Cu structure is frequently encountered in flexible electronics. Interfacial reactions in Au/Sn/Cu sandwich specimens with limited Sn supply were examined. The thickness of the Sn layer varied from 5, 8, 14, 17, 27–32 μm and the reaction temperature was at 210 °C. The reaction intermetallic phases were determined and found to evolve with reaction time. Initially, the reaction path in the Au/Sn/Cu specimen is Au/AuSn/AuSn2/AuSn4/Sn/Cu6Sn5/Cu3Sn/Cu. With longer reaction time, the (Cu,Au)6Sn5 phase is formed on the Au side as well. The (Cu,Au)6Sn5 phase is the Cu6Sn5 phase with Au solubility. The (Cu,Au)6Sn5 phases at the Cu and Au sides merge together after the Sn layer is entirely consumed. The path evolution is Au/AuSn/AuSn2/AuSn4/(Cu,Au)6Sn5/Sn/(Cu,Au)6Sn5/Cu3Sn/Cu, Au/AuSn/AuSn2/AuSn4/(Cu,Au)6Sn5/Cu3Sn/Cu, Au/Au5Sn/AuSn/AuSn2/(Cu,Au)6Sn5/Cu3Sn/Cu, and Au/Au5Sn/AuSn/(Cu,Au)6Sn5/Cu3Sn/Cu. The evolution will continue until the reactants are completely reacted, and the reaction system reaches phase equilibrium. In addition, in the AuSn4 matrix, a ternary Au25Sn50Cu25 phase was found, which merits further characterization.