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Passive Phase Change Tower Heat Sink & Pumped Coolant Technologies for Next Generation CPU Module Thermal Designs
Keywords: phase change heat sink, pumped coolant , heat exchanger
Increasing thermal demands of high-end server CPUs require increased performance of air-cooling systems to meet industry needs. Improving the air-cooled heat sink thermal performance is one of the critical areas for increasing the overall air-cooling limit. One of the challenging aspects for improving the heat sink performance is the effective utilization of relatively large air-cooled fin surface areas when heat is being transferred from a relatively small heat source (CPU) with high heat flux. In order to meet the next generation CPU thermal requirements with a phase change heat sink, two heat sink technologies and their associated prototypes will be described. Each of the heat sink technologies use internal liquid-to-vapor phase change to efficiently spread the local CPU power to the air-cooled fin structure. The two passive phase change heat sink technologies are: multiple embedded tower heat pipes; and a hybrid vapor chamber / muliple tower heat pipe design. Increased electrical performance for the computer industry has created thermal design challenges due to increased power dissipation from the CPU and due to spatial envelope limitations. Local hot spot heat fluxes within the CPU are exceeding 100 W/cm2, while the maximum junction temperature requirement is 105 C, or less. Next generation CPU thermal designs will incurr additional increases in overall power, or increases in local power density, or reduction in junction temperature requirements, or a combinaton of the above. Maintaining the same CPU module spatial envelope and air flow requirements for follow-on CPU designs in air-cooled servers will require on board, self contained, pumped coolant solutions that incorporate micro-channel cold plates with relatively low coolant pressure loss due to the current practical performance limitations of the passive phase change heat sink evaporator and condenser. Three pumped coolant technologies and their associated prototypes that met the below thermal performance requirements will be described. The passive phase change heat sink and pumped coolant requirements for this study are, sink-to-air thermal resistance: 0.065 C/W (passive phase change heat sink) 0.045 C/W (pumped coolant) heat source size: 25mm x 25mm (passive phase change heat sink) 22mm x 22mm (pumped coolant) module air pressure loss: 140 Pa module air flow rate: 120 cfm multi-flow direction: front-to-back (perpendicular to gravity), bottom to top (parallel / same direction as gravity), top to bottom (parallel / opposite direction to gravity) module spatial envelope: 200mm height x 100mm width x 220mm flow length mass: 1800 grams altitude: sea level multi-orientation: bottom heating, gravity assisted fluid return, side heating, non-gravity assisted fluid return A hybrid, all-metal heat sink was optimized to yield the minimum sink-to-air thermal resistance while not exceeding the pressure loss and mass requirements. A commercial CFD software tool was used to optimize the heat sink design. The optimized design had a 15mm thick copper base, with 44 6063 Aluminum fins. The fin thickness is 0.5mm. It was found that for the above requirements, the sink-to-air thermal resistance of the all-metal heat sink design is 70% greater than the measured thermal resistance of the passive phase change prototype heat sinks. Additionally, the thermal performance achieved by the prototype passive phase change heat sinks and the pumped coolant system designs exceeded the thermal performance levels for all other submitted competitive designs and prototypes which utilized similar cooling technologies. Indication that the companies that met the thermal performance requirements have developed industry leading IP within the passive phase change and pumped coolant technologies.
Marlin Vogel, Sr. Staff Engineer
Sun Microsystems, Inc.
Santa Clara, CA

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