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Can you get too close to the fire? Cooling Efficiency From Air-Cooled Heat Sinks to Bare-Die Direct Liquid Impingement
Keywords: liquid cooling, energy efficiency, high power
In the evolution of data center thermal management, engineers have found that cooling system efficiencies increase as the distance between the CPU heat sink and conditioned liquid/refrigerant in the data center. Hence the progression from random rack placement to hot/cold aisles (shortens the path for air to and from racks and minimizes mixing), to in-row cooling, to above-rack or rear-door heat exchangers, etc. This progression seems to be entering the next phase in earnest with the introduction of a number of different commercial direct-to-chip cooling technologies, including single- and two-phase cold plates, oil immersion, dielectric fluid immersion, loop heat pipes, etc. In this study we ask the question, Can you get too close to the fire with direct liquid cooling? In other words, do the efficiency gains continue if you go directly to the die instead of stopping at the package? Is it optimal to use a heat spreader on the die or impinge the die directly? While the answers to these questions may seem obvious, published experience in laboratories and commercial applications suggest that, in fact, the conclusions are not so clear. This presentation will present and discuss data from a range of cooling solutions applied to commercial bare-die GPUs (nVidia GTX 980). The results indicate that significant gains can be made in cooling effectiveness and energy efficiency with the introduction of liquid cooling. Direct impingement of liquid on the die gave somewhat non-intuitive results, but indicate that further efficiencies may be achieved with this most aggressive implementation.
Timothy A Shedd, President
Ebullient, LLC
Madison, WI

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