Thermal interface void reduction techniques for direct-soldered power modules

Mounting power modules by directly soldering them onto the heatsink can significantly reduce the thermal resistance between power module and heatsink as compared to the conventional bolted method that requires thermal grease or other interface materials in between. However, the problem of this mounting technique is the voids existing in the thermal interface. In this paper, void reduction techniques are studied through heating procedure, temperature profile using NC-559^TM solder paste, which is commercially used for surface mount devices (SMD) mounting. In order to study the effect of void rate, a 15 MHz scanning acoustic microscope (SAM) is used as an inspection tool. To measure the thermal resistance of the interface, a constant current source injection into power diode along with thermal coupling test was adopted. Experimental results indicate that the reduction of void rate in the interface also significantly reduces the thermal resistance. Moreover, experimental results of the thermal resistances using the direct-soldered method and Thermstrate^TM are compared. The thermal performance and temperature distribution are also studied by the finite element analysis using Flotherm^TM.