Silver-indium joints produced at low temperature for high temperature devices

A two-step fluxless bonding process adopted to produce high temperature silver-indium joints (80 wt% silver and 20 wt% indium) at relatively low process temperature of 206°C has been developed. After annealing the joint continuously for 26 h at 145°C, its melting temperature increases to 765-780°C, as confirmed by a de-bonding test. The technique thus developed provides a viable alternative to packaging many high temperature devices running at 350°C and above. The bonding quality of the Ag-In joints produced was examined using scanning acoustic microscopy. The joint cross-section was also studied using a scanning electron microscope equipped with an energy dispersive X-ray (EDX) spectroscope to find the local microstructure and composition. The results have shown that the joint is nearly void-free and uniform in thickness ranging from 7.2 to 7.8 μm. The annealed sample joint, as determined by EDX, is mainly composed of AgIn₂, Ag₂In, and AuIn₂ grains embedded in an Ag-rich Ag-In alloy matrix. During joint formation, the intermetallic compound AgIn₂, in particular, prevents the indium layer from oxidation, and therefore, no flux is needed. In addition, low process temperatures help to reduce the thermal stresses developed in the bonded structure due to thermal expansion mismatch. Finally, reliability tests were conducted on three sets of annealed joints using a high temperature oven running continuously at 500°C for 10, 100, and 1000 h each. Scanning acoustic microscopy (SAM) images on these samples confirmed that the joints have an excellent survivability in a high temperature environment.