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John O | August 2017

IBM team breaks thermal barrier in 3-D chip stacks with liquid cooling

researchers at ibm in new york and zurich have announced that they have reached a breakthrough in cooling three-dimensional chip stacks, breaking the thermal barrier with a nonconductive fluid that removes the need for a barrier between the chips and the liquid that is dissipating heat.


icecool uses a nonconductive fluid to take the next step of bringing the
fluid into the chip. (ibm)


according to a report from ibm research, icecool was a project begun in 2103 by the defense advanced research projects (darpa). after four years, the researchers have created a way to flow liquid between the layers of the stack and directly to the chips.


“it not only delivers a lower device junction temperature (tj), but also reduces system size, weight, and power consumption (swap),” the article explained. “our tests on ibm power 7-plus chips demonstrated junction temperature reduction by 25?c and chip power usage reduction by 7 percent compared to traditional air cooling.”


heat sinks and cold plates limited the creation of chip stacks because neither solution could reach the chips in the middle or the bottom of the stack. the article added, “ibm’s icecool technology circumvents that problem by pumping , a heat-extracting dielectric fluid right into microscopic gaps, some no thicker than a single strand of hair, between the chips at any level of the stack.”


the fluid can come into direct contact with electrical connections, so there is no limit to the number of chips in the stack and could also lead to enhancements such as adding memory directly on the stack.


icecool removes heat through boiling, with thermal energy being removed in the transition from liquid to vapor and pushed into the ambient. the vapor re-condenses and is pushed back into the system. because ambient temperatures are cooler than the chips, icecool, does not require a compressor, which could lower energy costs for data centers.


in fact, icecool has the potential to be a solution not only for chip stacks but for data center cooling as well, according to the researchers.


“icecool has the potential to eliminate the chiller, plus the crac (computer room air conditioning) unit, and most of the fans because it can be in direct contact with any and all electronic components. based on our tests with ibm power systems, icecool technology could reduce the cooling energy for a traditional air-cooled data center by more than 90 percent.”


recent reports have indicated that the market for data center cooling solutions is poised for big growth in coming years, so icecool has the potential to be a game-changer for that industry.


the full research was published in ieee transactions on components, packaging and manufacturing technology. the abstract stated:


“experimental investigation of data center cooling and computational energy efficiency improvement through advanced thermal management was performed. a chiller-less data center liquid cooling system was developed that transfers the heat generated from computer systems to the outdoor ambient environment while eliminating the need for energy-intensive vapor-compression refrigeration. this liquid cooling system utilizes a direct-attach cold-plate approach that enables the use of warm water at temperature a few degrees above outdoor ambient to achieve lower chip junction temperatures than refrigerated air.


“using this approach, we demonstrated a cooling energy reduction by over 90% and computational energy reduction of up to 14% compared to traditional refrigerated air-cooled data centers. to enable future computational efficiency improvements through high-density 3-d-chip stacking, we developed a 3-d compatible chip-embedded two-phase liquid cooling technology where a dielectric coolant is pumped through microscale cavities to provide thermal management of chips within the stack.


“in two-phase cooling, liquid is converted to vapor, which increases the capacity to remove heat, while the dielectric fluid enables integration with chip electrical interconnects. a test vehicle simulating an eight-core microprocessor was fabricated with embedded cooling channels.


“results demonstrate that this volumetrically efficient cooling solution compatible with 3-d chip stacks can manage three times the core power density of today’s high-power processor while maintaining the device temperature well within limits.”

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