Low Temperature Solders for Electronics Packaging Applications

Most optoelectronic devices, sensors, and materials either have a low heat tolerance or low melting temperature. Thus, the bonding process without high-temperature treatment is essential to prevent bond failures and increase yield simultaneously. Among various low-temperature bonding methods, solid-liquid interdiffusion (SLID) bonding is the most promising technique. Typically, the structure of SLID joints comprises two kinds of layered alloys, of which the one with a higher melting point is capped with a low melting point solder. At a reflow temperature over the melting point of the solder, these two metals react with each other and transform into one or more intermetallic compounds, which have higher melting points than the original solder. In this investigation, we studied the interfacial reaction and the mechanism of Cu with various In alloys at a relatively low temperature for soldering times from tens of seconds to hundreds of minutes. A novel milling technique was applied to address the problems caused by mechanical polishing. The microstructure evolution and the mechanism of the interfacial reaction between Cu and In alloys were studied and are discussed. Additionally, the dissolution of intermetallics and the growth kinetics of intermetallics are clarified. Metallurgical systems containing substantial amounts of metallic indium have always been very challenging to investigate due to the difficulties involved with sample characterization caused by the substantial softness of indium. Such difficulties are overcome in this study, and artifact-free microstructures are successfully obtained for the first time by using cryogenic broad Ar beam ion polishing. In this study it will be shown that the CuIn₂ is in fact a stable phase, and the Cu-In binary phase diagram is revised. The new phase stabilities for the CALPHAD database will be presented. The reaction kinetic data between Cu and several In alloys will also be presented.