Direct-Soldered Mounting Inverter Modules for Traction Motor Drives ^†*

Mounting power module by directly soldering them onto the heatsink can significantly reduce the thermal impedance 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 solder paste selection, heating procedure, and temperature profile. 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 interface impedance, a constant current source injection into power diode along with curve tracer measurement was adopted. Experimental results indicate that the reduction of void rate also significantly reduces the thermal interface.