Cooling Off Hot Spots

As processors run faster and add more functions, keeping them cool is becoming more difficult. A Stanford University spinoff has devised a solution that combines both liquid cooling and a solid state pump to move heat away from a microprocessor's hot spots.

The 2-year-old company, Cooligy Inc. (www.cooligy.com) of Mt. View, CA, is currently ramping production of a design that can be customized for any system, from portables to much larger workstations and embedded systems. The technology is designed for coming generations of chips with high performance and high power. "Chips are now starting to climb above 110W. Fans and heat pipes run out of gas around 110 to 120F because of hot spots," says Andy Keane, Cooligy's marketing vice president.

Those hot spots occur in parts of the chip that have most of the logic circuitry, which is generally in one section, while cooler running memories and other parts are in other sections. The temperature of the logic section, which can generate 80% of a chip's heat, is critical not only for the lifetime of the device. "Hot spots can really determine system performance. If the chip gets too hot, it slows down," Keane says.

Cooling is a growing concern, according to the latest Thermal Road Map from the National Equipment Manufacturing Initiative (NEMI, www.nemi.org) of Herndon, VA. Smaller chassis size prevents using larger heat sinks, while fans and blowers are limited by acoustic and EMI limits. However, NEMI says the development of cost effective, compact, reliable water-cooling techniques offers the potential to provide the necessary cooling solutions.

Focusing specifically on the Cooligy-Stanford technology that was funded in part by DARPA, NEMI notes, "Two-phase convection heat transfer devices yield the highest cooling rates per unit volume in electronic systems. Far greater rates of cooling can be achieved using high-pressure pumping of the liquid phase."

Cool Running: To deal with rising power levels, Cooligy's new active cooling system is designed to move 200W or more across a chassis.

The technology uses the basic concepts of a traditional finned heat sink, but instead makes the heat sink using silicon. That permits micro-machining very thin walls and channels, each measuring only 50-200 microns wide, which greatly increases the available surface area. The distance to these fins is also short, typically less than 2 mm, so heat is moved from the chip quickly.

However, the small size means heat isn't moved very far away. Water is used as a complement, flowing through the micro-channels, then carrying heat far from the chip. "Water isn't that good at carrying heat, but it moves," Keane says.

The water's movement is facilitated by an electro-kinetic pump that provides pressure needed to keep the water moving in one direction, completing its circuit even though part of the path may be uphill. The pump is a solid-state device driven by electric fields. The tubing has a slightly negative field when it's charged, while the water has a slightly positive charge.

Two-Phase Cooling: Cooligy's system pulls heat from the chip using silicon microchannels. It's then pumped through pipes to the outside where the fans can remove it.

These tubes, which measure 4-5 mm in diameter, can move pumped water as far as 24 inches. That is usually enough to reach a chassis or outside wall where it can be vented out, often with a conventional fan, Keane says.

The extra cooling capability comes with a price, though one Keane feels is not prohibitive. In this range, a fan heat sink costs from $7-$10, while a heat pipe ranges from $15-$25. "We sit on top of heat pipes, at $25-$30," Keane says.

He adds that reliability will be as good as with heat pipes, since the system is completely sealed.