The baseplate of a power semiconductor device module makes reliable and superior thermal contact with a heatsink when fixed to the heatsink. The baseplate includes a rectangular plate portion, a first downward-extending peripheral heel extension portion, and a second downward-extending peripheral heel extension portion. In one example, the plate portion has four mounting holes for receiving mounting bolts. There is one mounting hole located adjacent each of four corners of the rectangular plate portion. The central portion of the bottom surface of the plate portion has a slightly convex downward shape. The strip-shaped first heel extension portion extends along a first edge of the bottom surface. The strip-shaped second heel extension portion extends along a second edge of the bottom surface opposite the first edge. Each of the first and second heel extension portions extends downward from the bottom surface for a distance of between thirty and five hundred microns.