Roger Howorth

The difficulty of keeping computer chips cool is one of the most immediate challenges for the IT industry. Researchers at IBM’s Zurich lab are using nano-scale technologies to make self-contained water-cooling systems that are much smaller and can handle much higher power densities than the air-cooled copper heatsinks in use today.

Nanoscale devices are made of components that measure less than 100nm. A nanometre is equivalent to one billionth of a metre.

Dr Bruno Michel, manager of advanced thermal packaging research at the Zurich lab, said the paste between the chip and the heatsink, called the thermal interface material (TIM), currently accounts for 50 percent of the thermal resistance of chip-cooling systems. The TIM is needed because the silicon chip and the copper heatsink have different thermal expansion coefficients, so they can’t be directly joined together.

IBM is working on TIMs made with nanoscale particles to conduct heat. Using such small particles enables the two surfaces to get closer together and to bridge the gap with a material that is an excellent thermal conductor.

But another problem with TIMs also needs to be solved to improve the reliability or lifetime of the heatsink. Currently, as time passes small voids appear in the TIM as air is drawn in and out of the TIM through repeated heating and cooling cycles, thus reducing its effectiveness as a thermal conductor. Michel said IBM found that adding microchannels into one of the surfaces allows the paste to flow back and therefore prevents the voids from forming.

However, other technologies will also be needed to cope with future chips that run hotter than today’s devices. In particular, as chip capabilities increase, the number of connectors needed to link the chip to the motherboard increases. For example, Pentium III chips used 370 pins, early Pentium 4 chips used 423 pins, and modern single-core Xeon processors use 604 pins. The problem is that the more pins you have, the less space is left that can be used for cooling.

One of the more interesting solutions is to make microscopic water-cooling systems. Water has a much higher heat capacity – you need about 4,000 times less volume of water than air for equivalent cooling. IBM is developing water-cooled heatsinks that are smaller than current air-cooled ones. The new heatsinks use microscopic ducts to direct the flow of water in a sealed chamber inside the heatsink. The ducts are between 30 to 50 micrometres wide. An enhancement of this design uses jets to direct the flow of fluid onto the backside of the silicon chip, so such systems do not require a TIM.

Such systems should be able to cope with the increased heat produced by forthcoming multicore processors. For example, passive air-cooling systems can handle around 35 watts per cm2, and those with powerful fans can handle about 90 watts per cm2. Microchannel-based cooling systems can handle about 300 watts per cm2 while jet impingement systems go up to about 500 watt per cm2.

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