Heatsink Design and Selection
Design factors which influence the thermal performance of a heat sink
For semiconductor devices used in a variety of consumer and industrial electronics, the idea of thermal resistance simplifies the selection of heat sinks. The heat flow between the semiconductor die and ambient air is modeled as a series of resistances to heat flow; there is a resistance from the die to the device case, from the case to the heat sink, and from the heat sink to the ambient. The sum of these resistances is the total thermal resistance from the die to the ambient. Thermal resistance is defined as temperature rise per unit of power, analogous to electrical resistance, and is expressed in units of degrees Celsius per watt (C/W). If the device dissipation in watts is known, and the total thermal resistance is calculated, the temperature rise of the die over ambient can be calculated.
The idea of thermal resistance of a semiconductor heat sink is an approximation. It does not take into account non-uniform distribution of heat over a device or heat sink. It only models a system in thermal equilibrium, and does not take into account the change in temperatures with time. Nor does it reflect the non-linearity of radiation and convection with respect to temperature rise. However, manufacturers tabulate typical values of thermal resistance for heat sinks and semiconductor devices, which allows selection of commercially manufactured heat sinks to be simplified.
Commercial extruded aluminum heat sinks have a thermal resistance (heat sink to ambient air) ranging from 0.4 C/W for a large sink meant for TO3 devices, up to as high as 85 C/W for a clip-on heat sink for a TO92 small plastic case. The famous, popular, historic and notable 2N3055 power transistor in a TO3 case has an internal thermal resistance from junction to case of 1.52 C/W. The contact between the device case and heat sink may have a thermal resistance of between 0.5 up to 1.7 C/W, depending on the case size, and use of grease or insulating mica washer.