HP Applies Inkjet Technology To Cooling Chips

Theres nothing more refreshing on a hot summers day than a cool stream of water—and that goes for chips, too.

On a balmy day at the companys headquarters in Palo Alto, Hewlett-Packard research engineers disclosed new methods of keeping both semiconductors and datacenters cool. Whats novel is the way they are doing it: adapting an inkjet printer head to spray a fine mist of liquid on an overheating semiconductor, and using a mobile robot to look for "hot spots" in a data center.

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"We believe we have to take a holistic approach to cooling," said Chandrakant Patel, principal scientist and principal researcher on HPs thermomechanical research team at HP Laboratories. "We can not say we are going to cool the chip and not the data center."

Patel is expected to make a presentation on the importance of data center cooling at Intels Developer Forum the week of Sept. 9, labs researchers said.

Mist for microprocessors

HP, a company that makes billions on its inkjet printers, is exploring adapting an inkjet-like cartridge to deliver a fine mist of fluid onto the chip. Engineers are exploring whether it makes the most sense to spray the mist onto the chip package, or the surface of the chip die itself.

"What were proposing to do is take the cartridge right over the silicon chip," said Cullen Bash, a member of the technical staff at HPs System Technology Department. Fluid will be sprayed from the cartridge to the chip…then there will be a pump which will pump this fluid back into the cartridge."

HP believes that in the near future, chips could dissipate 200 watts of thermal energy – heat – in a space an eighth of an inch on a side. Thats because CPUs dont dissipate heat uniformly. In a microprocessor, most of the available space is actually cache memory, where instructions are stored. The computations take place in the arithmetic logical units (ALUs), where heat is concentrated.

"And those heat sources will move around," Patel said. "The difficulty is targeted cooling."

At the chip level, the problem isnt the overall heat produced by a chip, but the heat density. Although Intel CTO Pat Gelsinger first noted publicly that upcoming chips could produce more heat per square millimeter than a nuclear reactor, HP engineers said they first began thinking about the problem in the mid-1990s.

Like a car, when chips overheat, they malfunction. Commercial-grade chips can generally tolerate temperatures up to 100 degrees centigrade. But heat can be a problem both in tiny spots on a chip as well as the ambient air swirling around banks of processors.

Today, computer and silicon makers use several different ways to cool microprocessors. Most common is a heat sink or heat spreader, a metal attachment to a microprocessor package that passively dissipates heat across a wide surface area. "Finned" heat sinks use vertical plates to add even more surface area, and fans can be clipped onto these as well. In notebook PCs where heat is even more critical, "heat pipes"—thin tubes containing liquid—passively ferry heat away from chips through evaporation and recondensation. Some firms also sell water pumps that expedite this process, cooling the chip further.

Bash said that HP found that a finned heat sink (also called a "thin fin") dissipates between 30 to 35 watts of heat. Evaporative cooling can dissipate up to 100 watts, or heat densities of 200 watts per square centimeter. HP eliminated heat pipes as an option because the water can boil inside of them, destroying the convection process.

"The only reason we would consider this is the limit of the heat pipe itself," Patel said. "The (thermal) interface is the problem. We would try to change that interface."