The heatsink material is normally aluminum or copper, with internal passages to allow the coolant, such as water, to flow through. Semiconductors are then fastened to the flat surface of the heatsink. Heat flows from the semiconductor towards the heatsink by virtue of the metal to metal mating surfaces of the semiconductor and heatsink. There are three areas of thermal resistance in the traditional heatsink. First, the interface of water to the metal heatsink has a thermal resistance.
Heat is transferred from the heatsink material to the cooling water.
The thermal resistance of this connection is proportional to the surface area of the water to heatsink interface. Second, the metal in the heatsink has a thermal resistance, and the thermal resistance is proportional to the thickness of the heatsink. Heat sink materials are chosen for their high thermal conductivity. Third, the connection of the semiconductor to the heatsink relies upon the heatsink being manufactured with a very flat surface. By optimizing the flatness of the heatsink and flatness of the semiconductor, the thermal resistance between the two devices is minimized.