High temperature joints manufactured at low temperature

A fluxless bonding process using either indium-silver or tin-copper multilayer composite to produce high temperature joints at relatively low temperature has been developed. The process temperatures for indium-silver and tin-copper system are 210°C and 280°C, respectively. Joints, which are almost void-free, with melting temperatures of 210°C and 415°C, respectively, are made. After further annealing at 150°C for In-Ag and 280°C for Sn-Cu, the re-melting temperature of the joints increases to above 700°C. The technique, thus, provides a quantum jump to the post-processing temperature of component fabrication. In either process, the joints are examined using a scanning acoustic microscope to confirm the bonding quality. The joint cross-sections are studied using SEM and EDX to find the microstructure and composition. Upon deposition, In interacts with Ag to become In-AgIn₂ composite, and Sn interacts with Cu to become Sn-Cu₆Sns composite. The intermetallic compounds AgIn ₂ and Cu₆SnS prevent the In and Sn layers from oxidation in atmosphere. Thus no flux is needed. Besides the fluxless feature, the low process temperature would significantly reduce the stresses developed due to thermal expansion mismatch comparing to the otherwise high temperature processes with temperature exceeding the melting temperature. The multilayer bonding method also facilitates precise control of the alloy composition and the joint thickness. The new technique should find applications in the emergent high temperature electronic devices