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Performance of Passive 2-Phase Immersion Cooling of Server Hardware
Keywords: Data Center, Immersion, open bath
Recently published work has suggested the utility of a new cooling strategy termed "open bath immersion cooling" for datacom equipment. In this simple concept, servers are immersed side-by-side in a nonflammable dielectric liquid that boils where it is in direct contact with the heat generating devices (Figure 1). The entire apparatus is enclosed within modular "semi-open" baths, so-called because they are closed when access is not needed. These baths operate at atmospheric pressure and have no specialized hermetic connections for electrical inputs and outputs. This technique provides numerous advantages compared with more traditional liquid cooling strategies. Among these advantages is power density. Power densities as high as 4kW/L have been demonstrated with modules that simulate CPU packages (Figure 2). The data suggest that junction-to-fluid resistances <0.05°C/W may be possible with a thinned die soldered to a conventional integrated heat spreader (IHS) modified with a porous metallic boiling enhancement coating or BEC (Figure 3). Another advantage is that the waste heat removed from the electronics can be recovered and used remotely for district heating or other applications. This is accomplished by transferring the heat from the dielectric fluid in a closed-water heat exchanger operating at approximately 75°C This presentation will include performance data from immersion cooling experiments conducted with real computing hardware. Off-the-shelf CPU packages were modified by soldering a BEC atop the IHS. Experiments were also performed to determine the suitability of the fluid for transmitting high-speed signals on boards and connectors immersed in the fluid. Initial experiments measured the dielectric constant and loss tangent of the fluid up to 20 GHz. Next the electrical performance of previously characterized boards and connectors was measured up to 20 GHz while immersed in fluid. The results from these experiments will be presented.
Phillip Tuma,
3M Company
St. Paul, MN

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