Thermal resistance is the key factor in determining the power handling capability of any electronic package. Ambient Temperature Ambient temperature, denoted TA, is the temperature of the undisturbed ambient air surrounding the package. Ambient temperature is usually measured at a specified distance away form the package. In the laboratory test environment, ambient temperature is measured 12 inches upstream from the package under investigation. In a system environment, ambient temperature is the temperature of the upstream to the package and in its close vicinity.
Special care is required when measuring the case temperature to ensure an accurate measurement.
Thermocouples are often used to measure case temperature. Before any temperature measurement, the thermocouples have to be calibrated. When measuring the temperature of a surface which is at a different temperature from the surrounding ambient air, errors should be introduced in the measurements. The errors could be due to having poor thermal contact between the thermocouple junction and the surface, heat loss by radiation or by conduction through thermocouple leads. The laboratory testing was done using a thermocouple made by Omega.