Researchers unveil details of chip cooling breakthrough

March 22, 2007

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Schematic of the interface technology and cooling system. The image shows a cross-sectional schematic of the cooling architecture using the branched channel design. A highly viscous paste is brought between the chip cap and the hot chip in order to reduce the thermal resistance. Thanks to the tree-like branched "trenches" in the copper cap, the paste spreads very homogeneously and attains a thickness of less than 10 micrometers. With this technique, only half the pressure is needed to apply the paste and a twofold increase in cooling performance can be achieved. Credit: IBM

At the IEEE Semi-Therm Conference 2007, IBM researchers unveiled details of a new technique to significantly increase capabilities to cool computer chips.

The technique, developed by a team of scientists at the IBM Zurich Research Laboratory in cooperation 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 faster computer chips to be cooled more efficiently.

The right-hand image shows the paste after being applied using the new technology. The pattern arises from the hierarchical channel design of the interface that controls and optimizes the spread of the paste. The schematic on the left shows the hierarchical microchannel design for a 14-mm chip. Solid lines represent the first-level hierarchy, which is 220 micrometers wide, long-dashed lines = 150 micrometers width, short-dashed lines = 150 micrometers width. An example of a third-level channel design is shown in the upper left-most cell. Credit: IBM

In today's computer chips, as the circuits on chips become increasingly smaller, chips generate 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 that build "heat-evacuation bridges" from the chip to the cooler to compensate 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. the cooling capacity available to draw heat away from the chip.

IBM researchers now unveiled the reason and presented a novel technique to solve this problem. By observing how the glue spreads when a chip is attached to its cooling element, scientists noticed that a cross formed in the paste as large numbers of particles were piling up, inhibiting the layers of glue from spreading out. 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 remain where they are. As the squeezing process continues, however, they begin to pile up, forming what scientists call 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. This structure 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 was reduced by a factor of 3, and the pressure needed to squeeze the paste to the same bondline thickness was reduced to a similar extent. 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 factors and higher bulk thermal conductivity to be squeezed into thinner gaps, thereby reducing the thermal resistance of the paste interface considerably by more than a factor of 3. 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 cooperated with paste manufacturer Momentive Performance Materials, Wilton, CT, USA.

Together with other industry-leading suppliers, tools are being 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 entitled "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 will be published in Proc. 23rd IEEE Semi-Therm Symp. 2007.

Source: IBM