Fluxless Tin Bonding Process With Suppressed Intermetallic Growth

Abstract

Soldering is a process that joins two metal parts with a low-melting-point metal or alloy, called solder. It is widely recognized that intermetallic compound (IMC) formation is essential for the solder to bond to metals. The IMC layer grows during soldering. It grows further during usage, in particular, at temperature near the solder melting temperature. Extensive IMC growth can result in reliability issues. A reason is that IMC is much less ductile than the solder and does not deform as much as the solder to manage the coefficient of thermal expansion mismatch between the parts joined. In this paper, we looked into and developed a soldering process to fabricate IMC-less solder joints. The fundamental concept is to choose a barrier metal that can bond to solder without forming IMC with the solder. With our experience and Cr-Sn phase diagram, Cr is selected as the barrier metal. Several bonding experiments have been performed and the preliminary results show that high quality Sn joints could be produced with a little IMC formation before high-temperature aging. After aging at homologous temperature of 0.9 of Sn for 100-200 h, Cu6Sn5 and Cu3Sn IMCs are observed. They are caused by the penetration of molten Sn through microcracks or pinholes on the Cr layer to react with the underlying Cu. In production environment, the density of microcracks and pinholes can be greatly reduced to inhibit IMC growth, making the realization of solder joints without IMC a possibility.