IBM Researchers unveil details of chip cooling breakthrough

Unraveling the secrets of the “magic cross” to squeeze more performance out of chip cooling

(PresseBox) ( Stuttgart, )
Today, at the IEEE Semi-Therm conference, IBM researchers unveiled details on a new technique to significantly increase the ability to cool computer chips.

The technique, developed by a team of scientists at the IBM Zurich Research Laboratory in co-operation with Momentive Performance Materials, formerly GE Advanced Materials, overcomes a barrier in chip cooling by improving the application of the “glue” that binds chips to their cooling systems. The new technology could allow for faster computer chips to be cooled more efficiently.

In today’s computer chips, as the circuits on chips get smaller and smaller, the chip puts out more heat than ever before. To remove the heat from the chip, a cooling system is attached to the microprocessor using a special adhesive or glue. This glue is necessary to bind the two systems together, yet it poses a real barrier in heat transport.

To improve the glue’s heat-conducting properties, it is enriched with micrometer-sized metal or ceramic particles. These particles form clusters and build “heat-evacuation bridges” from the chip to the cooler to make up for the glue’s shortcomings. However, even highly particle-filled pastes are still inefficient, consuming up to 40 percent of the overall thermal budget, i.e. of the cooling capacity available to draw the heat away.

IBM researchers now unveiled the reason and presented a novel technique to solve this problem. By observing how the glue spreads when attaching a chip with its cooling element, the scientists noticed a cross forming in the paste, where large numbers of particles were pilling up, inhibiting the ability to thin out the layers of glue. The scientists were able to trace the cause of this back to the flow behavior of the paste, which simply follows the path of least resistance. Along the diagonals, the particles are pulled in opposite directions and as a result they do not move anywhere and pile up on each other as the squeezing process continues – forming the “magic cross”.

To overcome this problem, the team designed a special layout of micrometer-sized channels – or trenches – in a tree-like branched structure, consisting of larger and smaller channels, which functions like an irrigation system for the paste at exactly those spots where the particles would pile up. This allows the particles to spread more homogeneously, and reduces the thickness of the resulting paste gap.

The results obtained are impressive: The paste thickness could be reduced by a factor of three, and the pressure needed to squeeze the paste to the same bondline thickness could be reduced by a similar factor. These lower assembly pressures ensure that the delicate components and interconnects below the chip are not damaged as the chip package is created. The channels also allow pastes with higher fill factor and higher bulk thermal conductivity to be squeezed to thinner gaps, thereby reducing the thermal resistance of the paste interface considerably by more than a factor of three. The new technology allows air-cooling systems to remove more heat and helps to improve the overall energy efficiency of computers.

To further optimize the technology in real cooling systems and to demonstrate its feasibility, the IBM team co-operated with paste manufacturer Momentive Performance Materials, Wilton, CT, U.S.A.

Together with other industry-leading suppliers tools are developed to define the surface channels through the same copper stamping process currently used to fabricate high volume chip lids. This will define a full supply chain of low-cost parts to quickly integrate the new technique into products.

The work is being published in the paper "Hierarchical Nested Surface Channels for Reduced Particle Stacking and Low-Resistance Thermal Interfaces" by R. J. Linderman, T. Brunschwiler, U. Kloter, H. Toy, B. Michel, Proc. 23rd IEEE SEMI-THERM Symp., 2007
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