A Composite Solder Alloy Preform for High Temperature Pb-Free Soldering Applications

High temperature Pb-free solder materials are needed in die attach, power semiconductor and optical device packaging, flip-chip packaging, and heat-sink joining, etc. In this work, a special laminate composite preform foil has been developed for high temperature Pb-free soldering applications, where a melting temperature of 280 °C or higher is required. The special laminate composite preform foil is composed of a high-melting, ductile metal or alloy core layer and a low-melting solder coating layer at either side of the core layer. During soldering, the core metal and the liquid solder layer together with the substrate metals to be joined react to form high-melting intermetallic compound phases (IMCs) to consume the low-melting solder phase rapidly. The resultant solder joint is composed of a ductile core layer sandwiched by the IMCs layers at substrate sides, with a much higher remelt temperature than the melting temperature of the initial solder alloy coating. The laminate composite structure in the solder joint increases strengths and fracture toughness of the joint. The microstructure and mechanical properties of the solder joints formed by use of the SnAg/Ag/SnAg composite preform foil are investigated for the die-attach applications. Effects of the process variables, such as the reflow peak temperature, time above the liquidus of solder (TAL), applied pressure, and flux vs. fluxless, on the microstructure and mechanical properties and reliability of the solder joints will be discussed in the paper.