Patent ID: 8405995

Claim:
A vehicle mount semiconductor device comprising: a semiconductor element; a forced-cooling type cooler to which heat generated in the semiconductor element is conducted; and a heat mass that is joined onto a first side of the semiconductor element, so as to be thermally coupled to the semiconductor element, wherein the forced-cooling type cooler is provided on a second side of the semiconductor element, the heat mass is formed such that the thermal resistance of a part of the heat mass that corresponds to a high temperature part of the semiconductor element in a heat generating state is lower than the thermal resistance of a part of the heat mass that corresponds to a low temperature part of the semiconductor element, the heat mass has a plurality of projections, each projection has a distal end that is joined to the semiconductor element with solder, and the length of projections that are located in a peripheral portion of the heat mass is greater than the length of the other projections.