Thermal Resistance of IC Packages

Over the past few years, integrated circuit production has been dominated by the manufacture of digital circuits which typically operate at rather moderate power levels. For example, 5400 and 7400 series TTL circuits will dissipate approximately 150 mW. Recently, the use of linear circuits in consumer applications has been on the increase, and it is not unusual for such circuits to operate at power levels an order of magnitude higher than their digital counterparts. As an example, the Sprague Electric Company now offers the ULX-2277 dual audio amplifier which delivers 2 watts per channel of continuous power. Heretofore, power levels such as this were encountered only in power transistors packaged in metal cans. It was a relatively simple matter to measure thermal resistances of such packages by attaching a thermocouple to the base of the can, or to a mounting stud or heat sink, and using this reading as case temperature. The measurement would then be carried out according to some procedure, such as that outlined in MIL STD 883.