Development of Die Attachment Technology for Power IC Module by Introducing Indium into Sintered Nano-silver Joint

Wide band gap semiconductors have become very attractive for power electronics because of their excellent properties at high power and high junction temperatures above 300 degrees C. However, the maximum operation temperatures of conventional packaging materials, like tin-based solders or even tin-lead solders, are limited to around 220 degrees C. Thus, a new packaging material with a higher melting temperature must be developed. Sintered silver paste die attachment has been regarded as one of the most promising interconnection technologies for high temperature and high power applications because of its excellent electrical connection and thermal dissipation abilities and the low processing temperature and high operating temperatures up to 700 degrees C. Nevertheless, some critical problems still exist in this technology, for example high porosity, poor wettability at interfaces and severe oxidation problem when aging at temperatures above 200 degrees C in air. These serious issues cannot be neglected before this promising technology steps to practical application. Our research aims at solving these problems of sintered nano-silver die attachment by introducing indium into the sintered nano-silver joint. In our study, we put a slice of indium foil on nano-silver paste, which had already been stencil-printed with a certain thickness on substrate. Indium foil would begin to melt at the beginning of the bonding process and then permeated and filled into the pores between silver nanoparticles. Indium reacted quickly with silver and transformed the whole joint into a silver-indium intermetallic, which has a high melting temperature above 600 degrees C. Then, a much lower porosity joint was produced and the wettability between sintered joint and substrates also became better, which showed a better connection at interface. To compare the high temperature reliability between pure sintered nano-silver joint and silver-indium joint, we conducted high temperature storage test at 300 degrees C in air. The results showed that after thermal aging at 300 degrees C in air for 100h, the shear strength of pure silver joints dramatically reduced because of the oxidation of copper substrates. In contrast, no copper oxide could be observed inside the silver-indium joints, and even had twice higher shear strengths than the as-bonded ones. Therefore, we suggest that by introducing indium the high temperature reliability of the sintered silver joint can be significantly improved. Furthermore, the silver-indium joint also passed high temperature storage test at 300 degrees C in air for 1000h, and was proven to have anti-tarnishing property by other researchers. To sum up, we provide a simple method and an ideal packaging material that can successfully overcome some serious problems of the sintered silver paste die attachment technology and bring this technology much closer to practical use for high temperature and high power applications.