A Study on Reliability of High-Temperature Joint in Packaging Structure

In recent years, the research and development of hybrid cars and electrical vehicles become one of the top targets. High electric power is necessary for those cars to run in motor drive, and power devices such as inverter are employed for the control. Since operative temperature of Si power device is from −40 to 150°C level, a big cooling system is needed to control the temperature lower than 150°C. Recently the SiC chip had been developed to downsize the cooling system and reduce the loss of energy, because the operative temperature of SiC chip can be increased to over 300°C. However mounting method that can be used at high temperature environment is not established yet. The author’s group has proposed a new mounting method using Ag-Nano material to mount the SiC chip on a metal board. In existing mounting method, stress was relieved by transformation of comparatively soft solder region. Thus, in the new method, the junction Ag-Nano is too hard to relax the stress. So stress relaxation facility is given to the pure aluminum substrate side. In addition, jointing Ag-Nano on aluminum board is not possible, enabled by Ag and Ni plating on substrate. And these films prevent aluminum board from oxidizing. This mounting method achieves low temperature mounting and high reliability in thermal cycle. In this study, reliability of Ni plating was investigated because it was brought out that Ni plating becomes as a new weak point during high temperature cycle test ranged from −40 to 300°C. Mechanical properties of Ni plating were investigated first. Test specimens with plating and without plating for four point bending test were prepared to compare the difference. And stress-strain relation of plating was evaluated. In addition, fatigue strength was investigated by cyclic bending test. With these material properties, fatigue life of Ni plating in packaging structure was evaluated by finite-element-analysis. And optimum dimension of the structure was studied.