Experimental investigation of mounting thermal resistance of flatpacks on circuit boards

Thermal tests were performed on radar digital module circuit boards to measure the thermal effect of various size gaps between the circuit board and the flatpack integrated circuit (IC) case and also the thermal resistance of the circuit board. These IC's are cooled by conduction through the circuit board to an air-cooled heat exchanger. The thermal effect of the gap filled with air, and also filled with an adhesive, was measured. The temperature differences between pairs of locations on the flatpack IC case, particularly between the top and the bottom center, also were measured. These tests are described; the test results are presented, discussed, and compared with analytical predictions; and conclusions are given. The key conclusions are that the filler is quite effective in lowering the IC junction temperatures and, in fact, has a larger thermal effect than the gap size. For example, filling a 5-mil 1.27 x 10 to the -4th m gap reduces the junction temperature by 12 C, whereas reducing the gap from 5 mils 1.27 x 10 to the -4th m to 0 reduces it by only 3 C. The combined thermal resistance of the gap and the circuit board is a linear function of the gap.