A letter in a recent edition of The Lancet recalls how a patient had to be treated for minor burns on a very sensitive part of the body after using a laptop computer on his lap.
For the unlucky patient, and thousands like him, things could get worse because upcoming chips may produce 100 watts per square centimetre, which is the same as the heat generated by a light bulb.
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 appears to solve the problem.
The 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.
‘No internal redesign of laptops 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 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 vapour. The vapour 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, many laptops are cooled by a fan that blows the heat downward across a solid copper plate that acts as a heat sink. 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 as 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.’
Currently, microprocessors in desktop computers have to be situated adjacent to a heat sink several inches high and wide, with an attendant fan close by.
This design problem is said to create 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.
The wick in the Sandia heat pipe is made of finely etched lines about as deep as a fingerprint. 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 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.