ALBUQUERQUE, N.M. — Laptops make laps hot, as users of mobile lightweight computers sometimes learn dramatically. 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.)
Evacuating heat is one of the great problems facing engineers as they design faster laptops by downsizing circuit sizes and stacking chips one above the other. The heat from more circuits and chips increase the likelihood of circuit failures as well as overly heated laps. “Space, military, and consumer applications, are all bumping up against a thermal barrier,” says Sandia researcher Mike Rightley, whose newly patented “smart” heat pipe seems to solve the problem. The simple, self-powered mechanism transfers heat to the side edge of the computer, where air fins or a tiny fan can dissipate the unwanted energy into air. The technology is being licensed to a start-up company “that has a very interested large customer in the [civilian] laptop market,” says Rightley. “No internal redesign of laptops — a bugaboo for computer makers — is needed. The new design exactly duplicates in external form the heat transfer mechanism already in place in laptops,” says Rightley. “Industry won’t even see the difference.” The technique also interests the military, which seeks wearable computers with the smallest possible cooling fans. (Powerful fans are electronically noisy and give away the location of the user.) In colder climates, the heat could be dumped into hand warmers rather than undesirably into fabric and the flesh beneath. A paper describing the work has been accepted for publication by Microelectronics Journal. The method replaces the typical laptop heat sink — a chunk of metal that absorbs heat from circuits and then gives it up to air blown by a cooling fan — with tiny liquid-filled pipes that shuttles heat to pre-chosen locations for dispersal. 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 out below the computer rather than moved to a particular location. Such air-cooled spreading, says Rightley, will work however uncomfortably — till the hundred-degree range is exceeded. Then liquid cooling is essential. Outputs greater than 100 watts/cm² can melt circuits. “Formerly, thermal management solutions have been backend issues,” says Rightley. “It’s clear now that the smaller we go, the more that cooling engineers need to be involved early in product design.” 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 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. “We thought one application would be for a wearable computer for the military,” says Rightley. A box 6” by 1.5” by 4” could contain microprocessors, wireless web cards, information from planes, AWACS information, and weather, 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 pipes to put the heat out to fins so the computer cools naturally.” 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 anisotrophic capability to cool all sources of heat directionally,” says Rightley. “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 program is part of the DARPA HERETIC program (Heat Removal by Thermal Integrated Circuits), a joint project of Sandia’s with the Georgia Institute of Technology.