Before discussing the heat sink selection process, it is necessary to define common terms and establish the concept of a thermal circuit. The objective is to provide basic fundamentals of heat transfer for those readers who are not familiar with the subject. For the purpose of selecting a heat sink, the maximum operating power dissipation is used. Since the case temperature of a device depends on the location of measurement, it usually represent the maximum local temperature of the case. Again, this represents the maximum temperature of a heat sink at the location closest to the device.
T is the temperature difference between the two locations.
Thermal resistance circuit Consider a simple case where a heat sink is mounted on a device package as shown in Fig 1. Using the concept of thermal resistance, a simplified thermal circuit of this system can be drawn, as also shown in the figure. In this simplified model, heat flows serially from the junction to the case then across the interface into the heat sink and is finally dissipated from the heat sink to the air stream. This resistance is specified by the device manufacturer. Although the R jc value of a give device depends on how and where the cooling mechanism is employed over the package, it is usually given as a constant value.