Factors affecting semiconductor device thermal resistance measurements

The usual industry practice is to assume that thermal resistance of a semiconductor device is constant, and thus independent of power dissipation level or ambient environment temperature. In fact, thermal resistance is not a constant and varies with both power and temperature. The degree of variation is dependent on several factors: the range of power and temperature variation, the type of device, chip layout, mounting methods, etc. Rather than attempting to cover all aspects of the topic in overview fashion, only power dissipation and environmental temperature considerations are considered. Power dissipation considerations for a diode, a transistor, and an integrated circuit are described, to show that there are significant power-dissipation-induced variations of thermal resistance. Ambient environment temperature considerations are discussed to demonstrate that thermal resistance is also dependent on the environmental temperature to which a device is subjected during thermal testing