Abstract: A through slot is provided in the central apex portion of a U-shaped heat sink clip and results in an advantageous assembly and method of manufacture. A heat sink and power transistor are clamped together by arm portions of the clip. The slot is used either to permit the passage of an insertion tool for holding the clip open during attachment of the clip to the heat sink and power transistor device or to permit passage of external leads of the power transistor. In either case, the resultant heat sink assembly does not require opening of the clip by providing access to ends or outer projections of the clip arm portions by mounting tools or by providing additional exterior clip projections. Thus a much more compact and readily manufacturable heat sink assembly is provided.