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

A two-step fluxless bonding process adopted to produce high temperature silver-indium joint (80 weight % silver and 20 weight % indium) at relatively low process temperature of 206 °C has been developed. After annealing the joint continuously for 26 h at 145 °C, the melting temperature of the joint increases to 765-780 °C, as confirmed by the 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 afterward using the scanning acoustic microscopy. The joint cross-section was also studied using scanning electron microscope equipped with energy dispersive X-ray (EDX) spectroscope to find the local microstructure and composition. The characterization 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 in atmosphere, and therefore, no flux is needed. In addition, low process temperature also helps to reduce the thermal stresses developed in the bonded structure due to the thermal expansion mismatch. Finally, the 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. The scanning acoustic microscopy (SAM) images on these samples confirmed that the joints indeed do have an excellent survivability at high temperature environment. This technique opens up a new window of opportunity to packaging various high temperature devices involved in numerous applications in a very effective fashion.