Experimental Evaluation of Immersion-Cooled Strategies for High-Powered Server Modules

Abstract

Cooling of server-level high-performance computing equipment is an ongoing challenge. Early approaches, based almost entirely on forced air cooling, resulted in unsustainably high energy usage in the air-conditioning units. Subsequently, several other schemes including a hybrid air-cooling/liquid-cooling concept, natural convection cooling for data centers located in cold climates, and provisioned liquid cooling have been attempted. This study explores the direct liquid immersion cooled option first implemented in the Cray supercomputing platforms of the 1980s. Heat removal via pool boiling in FC-72 was tested. Tests were conducted on a multi-chip module using 1.8 cm × 1.8 cm test die with multiple thermal test cells with temperature sensing capability. Measurements with the bare silicon die in direct contact with the fluid are reported. Additionally testing includes the test die directly indium-attached to copper heat spreaders with surface enhancements. A screen-printed sintered boiling-enhanced surface (4 cm × 4 cm) was evaluated. This was followed by testing on surfaces that feature grooved (shallow-finned) surface enhancements. Tests have been conducted on an array of five die. Parameters tested include heat flux levels, dielectric liquid pool conditions (saturated or subcooled), and effect of neighboring die. Information was gathered on surface temperatures for a range of heat flux values up to 12 W/cm2. The highest heat dissipated from a circuit board with five bare die was 195 W (39 W per die). Addition of the heat spreader allows heat dissipation iii of up to 160 W per die. High-speed imaging was also acquired to help examine detailed information on the boiling process.