Au-Sn alloy preforms are produced by a dual electron-beam co-evaporation process, giving a deposit free of impurities with precise composition, a stable melting point and a long liquid-phase retention time. They provide reliable hermetic sealing and low thermal resistance between chips and substrates. Preforms of different sizes and thicknesses can be patterned to customer requirements and are widely used in the packaging of laser heat-sink chips, optical submounts and other optoelectronic devices.