In the present paper, a new method is presented for evaluating heat-transfer materials. When electronic devices are mounted on a metallic heat sink, a heat-transfer material is required in order to handle the thermal flow from the devices while maintaining electrical insulation. However, the thermal characteristics of heat-transfer materials are strongly affected by their contact with the heat sink. The concept proposed in the present paper is to measure the thermal characteristics of a heat-transfer material placed on an aluminum plate that is machine milled to form precise grooves, which represent the surface roughness of the heat sink. When pressure is applied to the heat-transfer material, the material is pressed into these grooves, reducing the gaps between the two materials and consequently increasing the thermal conductance. By varying the shape of the grooves and the amount of pressure applied, the contact conditions can be precisely controlled. Thus, complex thermal parameters can be measured with good repeatability. In order to verify the effectiveness of the proposed method, an evaluation system was constructed, in which the temperature was controlled using flowing water. Tests were then carried out on two types of heat-transfer materials, with hardnesses of C10 and C75, as determined using an Asker C type hardness meter. Four aluminum plates with different groove shapes were used. For each combination of heat-transfer material and aluminum plate, an increase in the thermal conductance was clearly observed as the pressure increased, until saturation finally occurred at a particular pressure. This result was expected because, as the pressure increases, the gaps between the heat-transfer material and the aluminum plate are expected to decrease and finally disappear. A trade-off relationship between the thermal conductance of the heat-transfer material and its softness, the latter of which affects the pressure at which the gaps become filled, was also found to exist. The results obtained in the present study confirm the reliability and effectiveness of the proposed method. Based on an analysis of the coupled electrical and thermal phenomena in contact, we also propose the electrical method including the contact condition factor.

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