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A new method for determining effective thermal conductivity and Young’s modulus in thermal interface materials is demonstrated. The method denoted as the Bulk Resistance Method (BRM) uses empircal thermal resistance data and analytical modeling to accurately predict thermophysical properties that account for insitu changes in material thickness due to external loading and thermal expansion. The BRM is demonstrated using commercially available sheets of Grafoil GTA. Tests were performed on thermal joints consisting of two Al 2024 machined surfaces with layers of Grafoil GTA in the interface. Test conditions included a vacuum environment, 0.2–6.5 MPa contact pressure, a nominal 50°C mean interface temperature and a continuous loading and unloading cycle. Test results indicated that the BRM consistently predicted thermal conductivity independent of the number of layers tested and that the predicted results were significantly lower than values reported using conventional ASTM test procedures.

The conductivity of thermal interface materials are typically determined using procedures detailed in ASTM D 5470. The disadvantages of using these existing procedures for compliant materials are discussed along with a proposed new procedure for determining thermal conductivity and Young’s modulus. The new procedure, denoted as the Bulk Resistance Method, is based on experimentally determined thermal resistance data and an analytical model for thermal resistance in joints incorporating thermal interface materials. Two versions of the model are presented, the Simple Bulk Resistance Model, based on the interface material thickness prior to loading and a more precise version denoted as the General Bulk Resistance Model, that includes additional parameters such as surface characteristics and thermophysical properties of the contacting solids. Both methods can be used to predict material in situ thickness as a function of load.

Thermal joint conductance and resistance models are presented for grease-lled joints formed by conforming rough surfaces under light contact pressures. One model includes the thermal effect of contacting asperities, whereas the second, simpler model is based on conduction across the gaps only. The models are compared against recently published grease and phase-change material (PCM) data obtained at one contact pressure, copper surfaces having three levels of surface roughness, four values of grease thermal conductivity, and two values of PCM conductivity. The models and the data are found to be in agreement over a wide range of a joint parameter de ned as the ratio of the effective joint roughness and the thermal conductivity of the gap substance. The models can be used to predict an upper bound on the joint conductance and a lower bound on the speci c joint resistance for surfaces that are turned and milled.