Thermal interfacing techniques for electronic equipment - A perspective

This paper reviews the existing knowledge base about thermal contact resistance in cooling electronic equipment, and also highlights some novel issues that are emerging with the advent of compact electronic equipment. Where a high contact pressure is tolerable, such as in cooling power electronic devices, the experimental data and the theoretical models that have been developed to this day provide useful guides for the management of contact resistance. In such applications the compression load and a technique to enhance interface heat transfer need be examined, weighing their relative importance in the entire heat transfer system. Using the Yovanovich correlation for contact resistance and assuming water-cooled or air-cooled heat sinks, the contact pressure ranges of practical importance are identified. The case studies revealed that contact pressures around and less than 1-4 MPa are often sufficient to make the contact conductance comparable to the convective conductance in the water-cooled channel. The threshold pressure is much lower for the air-cooled case, around 0.2-0.6 MPa. However heat transfer data in such intermediate pressure ranges are relatively few. For compact electronic equipment, such as laptop computers, the contact conductance to a thin heat spreader plate is becoming an issue of prime importance. In a constrained space the heat flow across the interface is affected by the heat conduction paths beyond the interface. This is illustrated using an example where warped heat sources are in contact with a heat spreader. It is shown that, with decreasing heat spreader thickness, the warping of the heat source has an increasing influence on the contact resistance.