Improved methodology for calculating interfacial thermal resistance and uncertainty for steady-state TIM testers with embedded probes

Abstract

Abstract-Efforts to miniaturize electronic components within the semiconductor industry continue to intensify stresses on the primary thermal pathways that are used for heat dissipation in electronics packaging equipment. This is particularly true for heat flow pathways that traverse interfaces. Consequently, an increasing priority for thermal engineers is to design materi als that are capable of reducing the impedance to heat flow acrosss device junctions. However, the equipment most often used to measure the resistance to heat flow across interfaces (ASTM D5470) is becoming increasingly insufficient for the characterization of next-generation thermal interface materials (TIMs) , as evidenced by the wide variability in the reported results for current state-of-the-art TIMs. Through the use of statistical analyses, we show that one possible reason for these discrepancies is the method by which the temperature difference across the interface is calculated. Additionally, we find that there exists a lack of consideration for many potential sources of positional uncertainty that exist within the measurement system, including: 1) the thermal conductivity mismatch between the thermal probes, the heat meter bars and any interstitial filler material used to increase contact conductance between them, 2) drill drift during manufacturing, 3) the temperature and positional uncertainties of each probe along the length of the heat meter bars, 4) the tolerance associated with the location of each thermal probe's junction and 5) the number of thermal probes that are used to determine the temperature difference across the interface. We find that these factors result in an unavoidably large uncertainty in the position of the thermal probes, which produces a significant measurement uncertainty when RT is on the order of 1.10-5 m 2 ·KJW or lower, regardless of the temperature measurement accuracy that can be achieved with the probes. Using numerical simulations, we conduct a parametric study to determine the magnitude of these effects on positional uncertainty. Results suggest that the lowest positional uncertainty is achieved when the thermal probe is significantly less thermally conductive than its surrounding filler, and that drill drift and the uncertainty associated with the location of the actual thermal probe junction account for a significant increase in the overall uncertainty of the measurement. It is expected that these results will allow for the development of steady-state TIM characterization instruments with improved measurement resolutions and for greater consistency between the results of different groups that use thermal probe-based TIM testers.