Measuring the Surface Flatness of Heaters and Cold Plates and Estimating Its Effect on Heat Transfer

Thermal contact resistance between microprocessors and their cold plates plays a critical role in the processor junction temperature. Surface flatness is one of the important design parameters defining the interface's quality. It is directly related to contact resistance between surfaces and lack of it negatively impacts a cold plate's thermal performance. A lack of flatness results in imperfect contact which greatly reduces the contact area and creates air gaps between them. In this paper, the authors developed a confident flatness measurement method comparing three flatness technologies and investigated the effect of surface flatness on the thermal performance of the cold plate. A Thermal Test Vehicle (TTV) with uniform heat flux heaters is used to simulate the thermal characteristics of processors. A server- level cooling loop with cold plates is attached and tested on the TTV. The non-uniformity of thermal resistances between cold plates motivated authors to investigate the contact quality further. The flatness of the cold plates and TTV heaters was measured using a Height Gauge, Micrometer laser Microscope, and TAKAOKA HVI-8000. The contact gap between the TTV heater and its cold plate is calculated using the flatness results. A comparison study is conducted between the results of three methods. The value with a lower discrepancy with others is considered as the gap value. The results of this study reveal that the gap value under 100 μm does not influence the junction temperature.