Thermal Conformance Between Metal-Metal Contacts Under Transient Conditions – A Study (The Effect of Tims, Metal Thermophysical Properties, and the Interfacial Pressure)

Abstract

A study of interfacial heat transfer between metal-metal contacts is essential in various engineering applications for designing thermal systems with increased efficiency. Interfacial heat transfer between metal-metal contacts under transient conditions is discussed in the present investigation. A novel technique based on solution to inverse heat conduction problem was adopted to estimate the interfacial heat flux transients between metal contacts. Thermal conformance parameters and a thermal conformance assessment parameter (TCAP) were proposed to assess conformance of metal-metal contacts. The metal-metal contacts with different combinations, such as Cu-Cu, Al-Al, Brass-Brass, Cu Al, and Al-Cu, were subjected to varying degrees of interfacial pressure, and the heat flux transients were estimated using lumped heat capacitance and inverse heat conduction approaches. The effects of thermal interface materials (TIMs), thermophysical properties and surface roughness on the thermal conformance were also investigated. The study used polymer-based TIMs with thermal conductivity ranging from 0.193 W/m K to 7 W/m K. The interfacial pressure had a significant impact on heat transfer across the interface under bare conditions (without TIMs). However, the effect of interfacial pressure on contact heat transfer is reduced with the application of TIMs of high thermal conductivity. The study also revealed that the effect of pressure was negligible on heat transfer beyond a limiting value of interfacial pressure. A smooth surface texture of the contacting materials resulted in enhanced heat transfer across the interface. The addition of multi-walled carbon nanotubes (MWCNT) in low conductive polymer-based TIMs on heat transfer was investigated. At low weight percentages, the heat flux transients increased marginally by about 6%. However, increasing the weight percentages of MWCNTs resulted in deterioration in heat flux owing to the agglomeration of MWCNTs. The study also found that the thermophysical properties of metals in contact affected the interfacial heat transfer with thermal conformance parameters increasing linearly with TCAP for all the combinations of metal-metal contacts except for the Cu-Cu combination, where interfacial pressure becomes dominant. Further, 1-Methyl-2-pyrrolidone (NMP), a cleaning solvent generally used in the semiconductor industry, was applied on the source and sink surfaces to assess its effect on heat transfer at the interface. The use of NMP resulted in a significant increase in the thermal conformance at the interface. The increase in the thermal conformance on the application of NMP was attributed to the (i) removal of oxide films on the source and sink surfaces and (ii) surface smoothening effect increasing the microscopic contact points and effectively reducing the mean gap width. The surface smoothening effect was validated by carrying out surface roughness measurements.