Novel Thermal Ground Plane to Improve Cooling in Electronic Devices

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Ronggui Yang

Mechanical Engineering

Faculty expertise in micro- and nano-technologies has come together in a "perfect storm" at CU-Boulder, and the outcome promises to be anything but disastrous.

A new $3.95 million grant from the Defense Advanced Research Projects Agency (DARPA), managed by Thomas Kenny of the Microsystems Technology Office, is likely to result in significant improvement in thermal management in electronic devices, one of the critical constraints on today’s consumer and military electronic systems.

The four-year grant to researchers in mechanical and chemical engineering, with their long-time partner Lockheed Martin, will be used to demonstrate the feasibility of replacing the copper heat pipe that is common in electronics and space systems today with a thermal ground plane made of flexible polymer materials.

The concept developed by Assistant Professor Ronggui Yang at CU-Boulder has vastly superior ability to remove the heat generated in computers and cell phones, as well as in new generations of energy systems such as flexible solar cells and batteries where heat removal requirements have limited further improvements, according to Professor Y. C. Lee.

"Cooling is the number-one problem in electronics, and this concept represents a total paradigm shift," says Lee, who directs the DARPA Focus Center on Nanoscale Science and Technology for Integrated Micro/Nano-Electromechanical Transducers, known as iMINT. "Flexible thermal ground planes have 100 times better thermal conductivity than copper and will enable a new generation of high-performance, integrated microelectronic, photonic, or microwave systems operating at high power density without constraints resulting from complex thermal management solutions."

Researchers at iMINT plan to fabricate a thermal ground plane that is only one millimeter thick—comparable to a credit card—but with an area as large as a laptop computer. The thermal ground plane can be used as a stand-alone component or integrated as another layer in a printed circuit board connecting chips and other components. A smaller thermal ground plane could be fabricated in the same way for use in a device such as a cell phone. Or, since the polymer material is flexible, it could be folded back and forth in a stack configuration, although this would be a greater challenge, Yang says.

The polymer ground plane will encase a nano-scale wicking structure in which distilled water is alternately vaporized and condensed, as in a more conventional heat pipe, to remove heat from a laser diode, microprocessor, or transceiver. The polymer will be coated with alumina through atomic layer deposition to provide a vapor barrier, which will maintain the water for long-time operation.

Three faculty members in mechanical engineering who joined the university in 2006 established the core concept for the novel thermal ground plane: Yang, who brought expertise in nano-structured materials and heat transfer, Professor and Chancellor G. P. "Bud" Peterson, a world expert in heat pipes, and Research Assistant Professor Chen Li, who contributed to the heat transfer modeling and design.

Their concept matched well with the technologies already developed by CU-Boulder professors Steven George, who patented atomic layer deposition coating technology; Victor Bright, a micro- and nanotechnology expert with years of experience working with DARPA; and Lee, an expert in micro- and nano-scale manufacturing and packaging technologies.

"We have every piece of the puzzle here. It’s a ‘perfect storm’ of research interests coming together," Lee says.

Lockheed Martin has been a long-time partner in the research, having provided the first seed grant to improve atomic layer deposition for hydrophobic coating in 2002, a technology that is essential to the proposed thermal ground plane.

"Lockheed Martin considers it a unique opportunity to benefit from CU researchers and incorporate their innovative thermal management solutions into future aerospace systems," says Suraj Rawal, senior manager for research in advanced materials and structures at Lockheed Martin Space Systems.

The iMINT center, which was established in fall 2006 with another DARPA grant managed by Dennis Polla, attracted nearly $1.5 million in government and industrial funding in its first year of operation. With the new grant, the center’s research activities will be expanded to more than $2.5 million per year.

Additional industry sponsors are being sought to participate in the thermal ground plane development as well as other engineering research projects.

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