By Dr. Phil Garrou, Contributing Editor

The March Issue of Electronic Cooling magazine contains a great summary article on the IBM effort in the DARPA ICECool Program [link]. I recommend reading the full article which I will summarize here.

When today’s standard cooling technology, air cooling with fans, does not meet the required needs, advanced water cooling approaches are examined. Traditional water cooling approaches replace the heat sink with a cold plate that provides more efficient heat transfer. But, because of its electrical conductivity, water cooling requires isolation measures to protect the chip, and requires large channels to cool large high-power die at reasonable pressure drops.

As part of the DARPA ICECool program, seeking to develop appropriate cooling technologies for 3D chip stacks, IBM developed a new chip-embedded cooling approach, utilizing a nonconductive fluid, doing away with the need for a barrier between the chip electrical signals and the fluid. This chip-embedded cooling technology pumps a heat-extracting dielectric fluid into ~100μm cooling channels, between the chips at any level of the stack. The coolant removes the heat from the chip by boiling from liquid-phase to vapor-phase. It then re-condenses, dumping the heat to the ambient environment. Since this system doesn’t need a compressor, it can operate at much lower power compared to typical refrigeration systems.

The dielectric coolant is fed in at the center of the die, moves through radially expanding channels, and exits at the edges of the die. This approach (shown below) provides better energy efficiency and maximum critical heat flux with the resulting reduced flow path.

To modify an IBM microprocessor module for embedded cooling the package lid was removed to expose the processor die, a deep reactive ion etch (DRIE) of the processor die was performed to generate the 120 µm deep cooling channels structures in the backside the processor and a glass die was bonded to the etched processor die to create the top wall of the micro-channels and a brass manifold lid, which provides for coolant supply and return, was bonded to the glass manifold die and the organic substrate using an adhesive. The coolant enters the module and passes through 24 inlet orifices to distribute the flow among the corresponding 24 radial expanding channels as shown below.

The figure below compares the performance of the standard air cooled module with the new embedded liquid cooled module. The cores temperature were measured with coolant inlet temperature in both cases at 25 ºC; a dielectric coolant mass flow rate of 9 kg/hr at a pressure drop of ~11 psi. The temp of the air-cooled processor levels off at around 70 ºC as the system fans speed up (~65%) to prevent overheating whereas the liquid cooled system is running at 40 – 45 ºC. At the highest power operation (4.3 GHz) the reduced operating temperature results in over a 10 watt decrease in the power consumed by the microprocessor along with a significant reduction in fan power (15+W) .

Apple to Replace Intel chips in Macintosh Computers

Bloomberg is reporting that Apple, which has used Intel processor chips in its computers since 1995, is planning to use its own chips in Mac computers beginning as early as 2020 (code-named Kalamata), replacing processors from Intel (link).

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