Thermal Interface Material Enablement of Off-Board Two-Phase Cooling

Two-phase, refrigerant-based cooling offers significant promise for increasing heat densities and energy efficiencies of electronic components. One barrier to wide spread adoption of two-phase cooling solutions is preserving product features deemed essential for product functionality. An important feature present in a wide range of product applications, particularly within the telecommunications area, is allowing a circuit pack card to be removed and re-inserted within an operating equipment shelf without interruption of the shelf function and without undue requirements on end-user skill or time. This feature is often referred to as “hot-swappability” or “plug-and-play capability”, and is readily accommodated with air-cooling approaches, where the circuit pack card is effectively immersed in the air cooling medium. Two-phase cooling solutions to be used for these applications must therefore possess this essential “hot swappability” feature for widespread commercial adoption. This paper presents the details of a hybrid (air- and two-phase) cooling solution which incorporates a two-phase, refrigerant-cooled cold plate (evaporator) located at the rear of an equipment shelf and placed between the electrical backplane and circuit pack cards that slide into the shelf. The heat from the circuit pack components is transferred to the cold plate via a highly-conductive heat transfer element comprising, for example, either a heat pipe or a vapor chamber. Thermal interface materials facilitate efficient heat transfer at the interfaces between the heat-generating component and the highly-conductive heat transfer element, and the highly-conductive heat transfer element and the cold plate, respectively. Moreover, to ensure “hot swappability”, a mechanically-compliant (compressible) thermal interface material that maintains low thermal resistance under low applied force after multiple mate and de-mate cycles, is required. Details of the experimental apparatuses adopted in this study for evaluating the performance of the thermal interface materials as a function of applied pressure and the performance of the highly-conductive heat transfer elements as a function of imposed heat load and operating temperature are presented. Test results are reported for a range of suitable candidate thermal interface materials and heat pipe and vapor chamber assemblies. Finally, estimates are also provided for the overall heat removal capability of this approach given typical maximum component case temperature specifications, measured thermal interface material properties and contact areas, as well as highly-conductive heat transfer element properties and cold plate surface temperatures.