...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.
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