Reliability of materials for the thermal management of electronics

The main goal of this project was to research the properties and reliability of the thermal interface materials used in electronics. The selected materials for measurements and tests contained elastic pad sheets, phase change materials and greases. The thermal impedance at pressures of 0,1...0,8 MPa was measured from the 34 materials. From these 14 materials were selected for the reliability tests. A new measurement method for thermal impedance and a new type of test structure for material testing was developed. The new test structure for materials was designed for use in environmental tests. The test structure makes it possible to select test surface quality, test pressure and material thickness. The materials were tested in various environmental conditions (high/low temperature, temperature/humidity cycle, temperature cycling) to see how their properties change in these conditions and what is the life endurance in use. The main criteria for evaluation of materials was the measured thermal impedance (K·cm2/W) at power densities from 10 to 25 W/cm2. The high power density means that samples are heated during the measurements into the high operating temperature. Greases, phase change pads and graphite pads had the lowest impedance value. The measured thermal impedance showed that the pressure and the wettability of materials have in many cases a larger effect on the impedance than the thermal conductivity of the bulk material. Therefore the impedance should be measured with the low pressures (< 0,3 MPa), which are achievable in practical solutions. The tested materials (14 types) performed quite well in the reliability tests, although there were some samples, which had quite large changes of impedance values. The long term behaviour of the tested materials seems to be good. Quite important observation of the tests is that the variation from sample to sample in initial and final measurements is larger than the changes during the tests. Therefore the user should take care of the surface quality and contact pressure while calculating the total variation of thermal impedance for the whole life cycle of products.