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Microchannels—fine channels approximately the width of a human hair, etched into a silicon wafer—are built with a very high aspect ratio to increase their total surface area. As fluid flows through the microchannels, their large surface area enables them to cool hot spots as with temperatures as high as 1000 watts per square centimeter.
The results, shown above right, are from a simulation of two microchannel collectors with different channel widths but identical length and height. The experiment used identical fluid flow and input power. Under identical conditions of flow and power, narrower channels result in lower wall temperature and correspondingly lower chip temperature.
The Microchannel Advantage
Microchannels as a means of cooling integrated circuits have been theorized since 1981, when Stanford professors Dr. David Tuckerman and Dr. Fabian Pease published research proving that microchannels etched into silicon could remove heat densities as high as 1000 watts per square centimeter.
Cooligy’s Microstructure Heat Collector puts that theory into practical use. The Cooligy system contains high-aspect-ratio channels that are very narrow, about the width of a human hair, and very tall relative to their width. Fluid is pumped through the channels to carry away heat conducted from the chip.
There are two reasons for the efficiency of the Microstructure Heat Collector. First, the heat generated by the chip travels a relatively small distance from the transistors on the chip, where the heat is generated, to the walls of the microchannels. Second, the heat from the walls of the microchannels conducts a very small distance into the fluid before the heat energy is carried away to the radiator. In other words, “h”, the heat transfer coefficient, varies inversely with the width of the channel. As the microchannels get narrower, the walls of the channels stay cooler.
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What about turbulence?
Large cold plates with macro-scale channels need turbulence to increase cooling efficiency. If flow in the large channels is laminar, heat will conduct slowly to the center of the channel. Because the channels in standard cold plates are so wide, fluid in the middle of the channel stays relatively cool. With turbulent flow, cold fluid in the middle of the channel mixes with hot fluid next to the wall of the channel. This improves the performance, but less so than reducing the channel width to the dimensions of a microstructure heat collector.
Microstructures neither require nor create turbulent flow. Instead, they take advantage of the fact that the heat transfer coefficient, “h,” is inversely proportional to the width of the channel. As width decreases, “h” increases. A very narrow channel completely heats a very thin layer of fluid as it travels through the collector.
Cooligy microstructures are efficient for another reason. The length scales of the cold plate design (channel height, width and thickness as well as the footprint) have been optimized to enable heat transfer with a minimum loss due to spreading. By maintaining close proximity of the microchannel to the hot spots spreading resistance has been minimized.
