Limitations and accuracy of steady state technique for thermal characterization of thermal interface materials and substrates

The steady state method is a commonly used and in principle simple way to measure thermal resistance and conductivity of thermal interface materials (TIMs). The sample must be positioned between a hot and a cold plate with constant temperatures, whereby a heat flow through the sample and temperature gradient across the sample are generated. To determine the thermal resistance of the sample the heat flow and the temperature gradient have to be measured. This is also defined by the ASTM standard ASTM D5470 [4]. However, for the new generation of highly conductive and thin TIMs, die attach materials and substrate the resolution of the common steady state technique often reaches its limit. To increase the resolution of the steady state equipment beyond the state-of-the-art the test systems must be analyzed and parasitic effects be studied. Some options for increasing the resolution of the steady state method will be studied analytically and by FE simulation within this paper. Accuracy and resolution depend not only on the precision of the setup, but decisively on the selection and execution of the measuring method conformed to the specific measurement task. We will also present our test stand TIMA Tester for thermal characterization of TIMs, die attach materials and substrates based on the mentioned steady state method. It has been developed as a platform which allows the integration of various modules for characterization of different materials under different conditions, e.g. mated surface, finish, operation temperature, pressure, aging etc. Finally, selected studies of different materials will be presented in order to demonstrate the functionality and the accuracy of the test stand.