Electronic Design

MEMS-Based Heat Exchanger Cools "Hot" CPUs

Employing silicon microchannels and a novel electrokinetic pump, it's the first practical and affordable system of its kind to hit the market.

The latest generation of gigahertz-clock-rate RISC and CISC CPUs is becoming more challenging to fit into designs. These chips are squeezing into tighter and tighter spaces with no place for heat to escape. Total power-dissipation levels now reside at about 110 W, and peak power densities are reaching 400 to 500 W/mm2 and are constantly climbing. As a result, higher performance and greater reliability levels are extremely tough to attain.

Until now, the solution was to use active heatsinks and passive fluid heat pipes. These types of structures are costly and bulky, and they take up too much real estate on the pc board. The heatsink can be some 3000 times larger than the CPU it's cooling. A standard 120-W Intel Pentium 4 microprocessor generally uses a heatsink that dwarfs the CPU hotspot that it's trying to cool.

Microelectromechanical-systems (MEMS) heat exchangers have found some success here, but the key missing element has been a practical means of removing the heat. The Pentium 4 currently consumes 82 W. Meanwhile, the Prescott is projected to consume 100 to 110 W when it's launched. And, the Itanium will consume about 130 W. These figures will climb with time.

Cooligy Inc. believes it has found the answer thanks to a deep knowledge of cooling microstructures and a novel electrokinetic pump that makes the solution practical and inexpensive (Fig. 1). The company is now ready to introduce to the market the first commercially available self-contained heat-exchange system based on a MEMS heat exchanger and an electrokinetic pump. Andy Keane, Cooligy's vice president of marketing, sees this device as an opportunity for his company to introduce a "disruptive" technology, much like MEMS itself was considered back in the 1980s when it was first commercialized.

This microfluidic system will initially target high-performance CPUs that are used in very restricted spaces in workstations, 1U servers, and small form-factor PCs. Later versions are being planned for other types of ICs, including graphics processors, FPGAs, DSPs, and other dense ICs.

As professors at Stanford University's Mechanical Engineering Department, Cooligy founders Ken Goodson, Tom Kenny, and Juan Santiago work extensively on MEMS heat exchangers and have become quite familiar with the problems of cooling hot structures and removing the heat. They figure that the best way to remove heat from a CPU is to minimize the distance heat must travel from a CPU's hotspot to the heat collector before it's carried away. In their new microchannel structure, that distance is a mere 1.5 mm, where heat travels to the microchannel that contains the pumped fluid (Fig. 2).

Made by reactive ion etching, the MEMS structure is so efficient that if it were unfolded and laid out flat, its cooling area would be 20 times larger than the actual structure and the die it cools. Its active cooling area is 10 to 20 times the area of the die, yet it fits directly over the die with very little extra space used. Other standout dimensions include a 3:1 aspect ratio (channel height to width) and channel widths of about 50 to 150 µm (about the width of a hair).

Simply having an efficient heat exchanger is only half the story, though. A practical means must carry that heat away. Goodson, Kenny, and Santiago hit upon a simple idea: an electrokinetic pump that can be made from conventional components—and thus at low cost in high volumes. Using a pump the trio developed at Stanford University as a model, they set about to optimize it for CPU heat removal and convection.

The electrokinetic pump works by having an electric field drive a fluid through a sintered glass disk. The electric field's strength controls the liquid's flow rate.

"It was a matter of scaling the volumes of liquid involved from what has already been demonstrated to make the pump practical," says Goodson, Cooligy's co-chief technology officer.

The pump consists of a small sintered glass filter disk as the active pumping structure, a catalytic gas recombiner, platinum electrodes, and Plexiglas machined parts (Fig. 3). The housing was made of Plexiglas in the prototype stage, but in production it will be made from glass, ceramic, or metal. An electric field drives the water that flows through the microchannel heat collector and then flows through the radiator. The radiator is a counter-flow type where the fluid and the air flow in different directions.

The pump has a high flow rate of more than 20 ml/minute with a 60-V/mm electric field. It's also silent. Most importantly, it's reliable since it has no moving parts. The flow rates can be scaled up to 200 to 300 ml/minute. The pumping structure has a diameter of 30 mm, is 2-mm thick, and has an effective pore diameter of 1 µm.

The pump requires only about 10% of the heat energy removed, which is about 10 W for 100 W of heat removed. Successful demonstrations of the pump have been performed on CPUs from Intel, Apple Computer, and Hewlett-Packard using buffered de-ionized water as the liquid. The water contains buffer chemicals to prevent growths in the water that could cause blockages.

Keane points out that the commodity parts for Cooligy's pump are similar to parts that go into filters widely used in industry. The sintered glass, the key element of the electrokinetic pump, is used as a filter in beer and food processing (for much larger liquid volumes) and as a pump component in the biomedical field in microfluidic arrays (for very small liquid volumes).

Cooligy notes that each heat-exchange system it designs is custom-made to the specific application involved. The MEMS heat exchanger and the pump can then be optimized differently for different applications, depending on the chip that's being cooled.

Nevertheless, cost for a closed-loop system can range around $30 during the early stages of manufacturing, which should come down with volume manufacturing. In contrast, a heat-pipe system might cost between $15 and $25, while a fan/heatsink combo might cost about $12 to $20. But these other systems can't deliver the benefits of Cooligy's MEMS-based system, which is about 20% smaller, not to mention the resulting higher reliability and CPU performance gains.

Andy Keane, (650) 417-0300, ext. 325

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