TINY showerheads could be used to spray computer chips with water and keep them from overheating. It might sound foolhardy, but if chips are to go on getting smaller and more powerful, such cooling attachments will be vital.
Chips built today consume around 10 times as much power per square centimetre as they did 10 years ago, and the increase looks set to steepen. Within three years, power consumption could leap from 70 to 120 watts per square centimetre, says Chandrakant Patel, a researcher at Hewlett Packard Labs in Palo Alto, California. 鈥淎nd you can be sure of one thing,鈥� says Patel. 鈥淣early all of that is going to come off as heat.鈥�
The problem of preventing ever more powerful chips from overheating is getting computer engineers in a sweat. They have already fitted fans to computers, and chips now bristle with ultra-thin metal fins that radiate their heat to the surrounding air. The latest trick is to add layers of tiny water-filled copper tubes that efficiently conduct heat away from the chip. But at the rate chips are hotting up, even this won鈥檛 do the trick for long.
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Patel鈥檚 team has had a different idea: if you want to cool really hot chips, why not hose them down? The effect is like flicking water onto a hot oven hob: a quick fizz and it turns into vapour. 鈥淏y using this kind of effect, we think we can remove about 200 watts of heat from a square centimetre of chip,鈥� he says.
Patel and his team tested the idea using an printer inkjet cartridge that had an array of 512 separate nozzles. They replaced the ink with water and fixed the cartridge just above a hot metal plate. Spraying the plate with water cooled it, as they expected, but more importantly they found they could cool different parts of the plate by different amounts, simply by controlling which nozzles were firing.
That control is essential, says Patel. 鈥淵ou don鈥檛 need to cool the memory as much as the central processing unit,鈥� he says, and if you spray the same amount of water all over the surface the chip, you鈥檒l either run dry on the hot areas or swamp the cooler parts. Chips already have built-in heat sensors, so these could be used to work out precisely how much water is needed where, Patel adds.
To ensure the water doesn鈥檛 go everywhere and damage components, Patel has designed a waterproof coating for the chip and the case that clips tightly over it. As the water evaporates, the vapour rises up and condenses where it can be fed back into the reservoir (see Graphic). And since surface tension keeps pulling the tiny droplets through the nozzles, the system doesn鈥檛 need a pump.
Patel says the system could also use fluorocarbons instead of water. These liquids are electrically insulating, so they could be sprayed directly onto an unprotected silicon chip without shorting it out. However, fluorocarbons have several disadvantages compared with water. Patel found that water was 60 per cent more efficient at cooling chips. What鈥檚 more, certain fluorocarbons damage the ozone layer.
To see if spray cooling will work with computers used in more extreme environments, Patel has now started testing computers at different angles. 鈥淲e鈥檙e confident it works with the computer flat and on its side 鈥� it鈥檚 all about the distance between the chip and the nozzles,鈥� he says.
鈥淭his is a provocative idea and has some merit. It also needs a lot of work,鈥� says Ken Goodson, whose group at Stanford University in California specialises in chip cooling. 鈥淭he pros are that it is a proven pumping technology from another application.鈥�
The biggest challenge is making sure all the fluid can be recovered, he adds.