Experimental Investigation of Direct Attach Microprocessors in a Liquid-Cooled Chiller-less Data Center

As part of a US Department of Energy cost shared grant, a liquid cooled chiller-less data center test facility was designed and constructed with the goal of reducing total cooling energy use to less than 5% of the total IT and facilities energy usage by utilizing warm water cooling of the electronic rack. A server compatible Liquid Metal Thermal Interface (LMTI) [1] was developed and integrated to improve the thermal conduction path of the hot server components to the ambient cooling of the data center. This LMTI has a thermal resistance an order of magnitude better than that achieved with most commercially utilized thermal interface materials (TIMs). When integrated directly between a bare die and a water cooled heat sink, this technology achieved a significant improvement in thermal conduction and enabled the computer devices to operate in a much higher ambient temperature environment. Initial studies on single modules showed substantial improvement in operating temperature when utilizing LMTI. Based upon this result, a detailed study was completed using two liquid cooled System X 3550 servers, comparing the thermal performance between the commercial thermal solution of a standard lidded module interfaced with a thermal grease to a cold plate, and the solution where the lid was removed and LMTI was used between the bare die and the same cold plate. The servers were first characterized using bench top investigation and then in a Data Center Liquid Cooled System with the standard lidded module and subsequently reassembled with a direct die attach LMTI. The servers CPU core temperatures showed a 5 to 6 degrees C advantage in CPU core temperature for the direct attach LMTI compared to the standard lidded module with thermal grease.