Without some basic understanding of the factors affecting heat sink performance the heat sink you select may be inadequate to meet the thermal requirements of the device being cooled. This leads to an increased number of designs and testing iterations ultimately delaying the completion of the product development. There are 3 common mistakes that are made when evaluating a heat sink to be used.
1. Using the manufacturer’s thermal resistance to predict performance
The thermal resistance of a heat sink is the most common way to assess the performance of a heat sink for a given application. By multiplying the heat sink thermal resistance, Rth by the power dissipation, Q of the device being cooled and adding the ambient temperature, Tamb to the results, the case temperature of the device, Tc can be determined, as shown in equation 1. The typical method by which heat sinks are selected is to first calculate the required heat sink thermal resistance using equation 1. A search of commercially available heat sinks with published thermal resistances less than or equal to the calculated value is then conducted.
The thermal resistances provided by the heat sink manufacturers is typically determined through testing of a heat sink with a square heat source, usually 25.4 mm x 25.4 mm (1″ x 1″) attached to the center of the base of the heat sink with a predetermined heat dissipation value. The temperature difference is measured and using equation 1 the thermal resistance is calculated.
Using this measured heat sink thermal resistance value can often times results in the selection of a heat sink that may not meet your thermal needs because the thermal resistance of the heat sink is not a constant. The thermal resistance of the same heat sink will change based on the size of the heat source relative to the base area of the heat sink. If the heat source you are using is significantly smaller than the heat source used by the manufacturer during testing of the actual heat sink the thermal resistance may be much higher than the manufacturer’s tested value. This is due to the difference in thermal spreading resistance which is caused by the flow of heat from the smaller area of the heat source to the larger area at the top surface of the heat sink. The smaller the heat source area relative to the base area of the heat sink the higher the thermal spreading resistance which then increases the overall thermal resistance of the heat sink. The effect of spreading resistance is most pronounced on heat sinks undergoing forced convection.
Figure 1. Sample heat sink