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Low Profile Heat Pipe Heat Sink and Green Performance Characterization for Next Generation CPU Module Thermal Designs
Keywords: heat pipe heat sink, green performance, CPU cooling
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 for two system designs with a phase change heat sink, the 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 passive phase change heat sink technologies are: multiple embedded heat pipes; and vapor chamber designs. 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. The CPU power dissipation continues to increase and the number of CPUs per server increases for next generation servers. This has resulted increased data room energy costs associated with supplying additional power to the server, and also cooling the server. Typically in the past if two heat sink technologies yielded the met thermal performance requirements, the least expensive technology would be utilized. In the future, heat sink thermal performance specifications will include the impact of energy cost savings attained through reduced air flow requirements if utilizing a supperior heat sink technology warrants a potential increase in heat sink cost. Data room energy cost versus heat sink technologhy cost trade-off characteristics will be presented utlizing the above heat sink technologies and associated range in the next generation system air flow and pressure loss characteristics. ... CFD analysis results indicated that an optimized all-metal heat sink would not meet the sink-to-air thermal resistance while also meeting the pressure loss and maximum mass requirements. The thermal performance achieved by the prototype passive phase change heat sinks met the thermal performance requirements. All other submitted competitive designs and prototypes which utilized similar cooling technologies were not able to meet the thermal specifications. Indication that the companies prototypes that met the thermal performance requirements have developed industry leading IP within the passive phase change heat sink technologies. The date room energy cost vs heat sink cost characterization showed that the cost optimization is very dependent on the system level air flow / pressure loss characteristics and the associated required fan input power, along with an acceptable time period required to recover the added expense of a higher performing heat sink technology.
Marlin Vogel, Thermal Engineering
Sun Microsystems
Santa Clara, CA

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