Smart heat pipe makes for way-cool laptops
Laptops make laps hot, as users of mobile lightweight computers quickly learn, and things could get worse: upcoming chips may produce 100 watts per square centimeter — the heat generated by a light bulb — creating the effect of an unpleasantly localized dry sauna.
Current chip emanations are in the 50 watts/cm2 range.
More technically, increased heat generation is one of the great problems facing engineers trying to downsize circuit size or stack chips one above the other to increase mobile computing intelligence. Heat greater than 100 watts/cm2 can melt circuits.
"NASA researchers, as well as those working in military and consumer applications, are all bumping up against a thermal barrier," says Sandia researcher Mike Rightley (1745), who thinks he knows how to bypass it.
His group’s newly patented version of a passively "smart" heat transfer mechanism uses small amounts of vaporized liquid sealed in tiny flat pipes to move heat to the side edge of the computer, where air fins or a tiny fan can dissipate the unwanted energy into air or even, in colder climates, into hand warmers, rather than undesirably into fabric and the flesh beneath.
"Because the new flat heat pipe design exactly duplicates in external form the less ‘intelligent’ heat transfer mechanism already in place, no internal redesign — a bugaboo for computer makers — is needed," says Mike. "Industry won’t even see the difference. We’ll just replace the heat sink with a heat pipe."
The method is being licensed to a start-up company "that has a very interested large customer in the laptop market," says Mike. A paper describing the work has been accepted for publication by Microelectronics Journal.
"We thought one application would be for a wearable computer for the military," says Mike. A box 6 x 1.5 x 4 inches could contain microprocessors, wireless Web cards, information from planes, AWACS information, and weather information on a hard disk with graphics capability and peripherals. "But using a fan to cool a field device will never work because of mud and muck and water. It’s a perfect opportunity for heat pipe, to put the heat out to fins so the computer cools naturally."
In the heatpipe loop, heat from the chip changes liquid — in this case, methanol — to vapor. The vapor yields up its heat at a pre-selected site, changes back to liquid, and wicks back to its starting point to collect more heat.
Currently, typical laptops are cooled by a fan that merely blows the heat downward across a solid copper (formerly aluminum, when chips were cooler) plate that acts as a heat sink; thus, hot laps. The heat is spread rather than moved to a particular location. Such air-cooled spreading, says Mike, will work — however uncomfortably — until the hundred-degree range is exceeded. Then liquid cooling is essential.
"Formerly, thermal management solutions have been back-end issues," says Mike.
"It’s clear now that the smaller we go, the more that cooling engineers need to be involved early in product design."
Powerful fans are electronically noisy
More circuits installed per unit area improve capability but reduce reliability, since increased heat increases the possibility of circuit failure; the problems are multiplied when chips are stacked one atop the next.
Currently, microprocessors in desktop computers have to be situated adjacent to a heat sink several inches high and wide, with attendant fan close by. This design problem creates enormous difficulties for designers interested in stacking chips for greater computational capacity yet reducing overall computer size. A further difficulty for the military is that powerful fans are electronically noisy and give away the location of the user.
A heat pipe can move heat from point A to point B without any direct geometrical relation between the points. This means that heat can be displaced to any desirable location, and a much smaller, quieter fan or even silent cooling fins can be used to dissipate heat.
The wick in the Sandia heat pipe is made of finely etched lines about as deep as fingerprints. These guide methanol between several locations and an arbitrary end point. The structure, which works by capillary action like a kerosene wick, consists of a ring of copper used to separate two plates of copper. Sixty-micron-tall curving, porous copper lines (slightly less thick than the diameter of a human hair), made with photolithographic techniques, allow material wicking directionally along the surface to defy gravity.
"An isotropic method [that sends out heat in all directions] doesn’t work because it only cools the first heat source; you need anisotropic capability to cool all sources of heat directionally," says Mike. "We use laws of fluid mechanics to derive the optimum wick path to each heat source." The curvilinear guides can be patterned to go around holes drilled through the plate necessary to package it within the computer.
The computer program was developed by Chris Tigges (1742) and the device was modeled by Rick Givler (9114).
Paul Smith (1321), Chris, and Mike formulated the idea at a meeting two years earlier. Charlie Robino (1833) "figured out how to perform the microscale hermetic welding of the device," says Mike, who also expressed appreciation for technologists JJ Mulhall (1745), Mark Reece (1833), and Cathy Nowlen (1745).
The program is part of the Defense Advanced Research Project Agency’s HERETIC program (Heat Removal by Thermal Integrated Circuits), a joint project of Sandia’s with the Georgia Institute of Technology.