A second type of heat sink fin arrangement is the straight fin. These run the entire length of the heat sink. The idea of thermal resistance of a semiconductor heat sink is an approximation. Another parameter that concerns the thermal conductivity of the heat sink material is spreading resistance. The most common heat sink materials are aluminium alloys. Its use is primarily as a temporary heat sink. Ericsson's incorporation of a 'regenerator' heat sink for his engine made it tremendously fuel-efficient. Thermally conductive tape is one of the most cost-effective heat sink attachment materials. A pin is one of the more common heat sink types available on the market. The 11.8 axle cover doubles as a heat sink. It contains an auto controlled fan and a heat sink. To understand the principle of a heat sink, consider Fourier's law of heat conduction. To determine the thermal performance of a heat sink, a theoretical model can be made. A straight fin heat sink is cut at regular intervals. A variation on the straight fin heat sink is a cross cut heat sink.