Conventionally, heat generated by a switching element in a semiconductor module is radiated through a heat sink in an electric power converter. In JP-A-20023-373970, a heat radiation sheet as a heat radiation member is formed between the semiconductor module and the heat sink (FIG. 3). A part of the heat radiation sheet is sandwiched between a metal electrode plate and the heat sink in the semiconductor module. An IGBT as the switching element is formed on the metal electrode plate. Accordingly, when the IGBT functions to turn on and off so that the IGBT generates heat repeatedly, the metal electrode plate expands and shrinks repeatedly.
When accuracy of a surface processing of the metal electrode plate and the heat sink is low, the surface of the metal electrode plate and the surface of the heat sink, which contact the heat radiation sheet, may include multiple small protrusions having a side surface slanting on one side of the expanding and shrinking direction. In this case, when the metal electrode plate repeatedly expands and shrinks, the heat radiation sheet sandwiched between the metal electrode plate and the heat sink moves to a direction opposite to the side surface of the protrusion. This effect is defined as “ratcheting phenomenon.” When the ratcheting phenomenon continuously occurs, the heat radiation sheet may protrude or removed from an interface between the semiconductor module and the heat sink.