Microchannels Take Heatsinks to the Next Level

Nov 1, 2006 12:00 PM
By Stephen A. Solovitz, Mechanical Engineer; Ljubisa D. Stevanovic, Advanced Technology Leader, Ener

Step Three: Experimental Validation

Finally, the thermal performance of the fabricated microchannel heatsink is measured to validate the simulations. For this example, the four diodes were operated in the forward-voltage drop mode, producing constant heat fluxes on the order of 100 W/cm2. The diode top-surface temperatures can be measured using infrared thermography, and these values can be converted directly to thermal-resistivity values. The heatsink should be connected to a chiller that controls the flow rate and temperatures of a water-cooled loop. The pressure losses and coolant temperatures are then measured across the device. Based on the instrument capabilities, the uncertainties in thermal resistivity and pressure loss measurements were ±5% and ±3%, respectively, for GE's integrated microchannel heatsink.

The thermal performance for the GE heatsink was studied for a range of diode power dissipations (0 W to 200 W) and coolant flow rates (0 LPM to 2 LPM). Fig. 6 displays an infrared image of the diode temperature contours for a dissipation of 199 W (104 W/cm2) when cooled by 20°C water at 0.24 LPM. Note that the higher temperatures on the right side are due to power dissipation in the wirebonds and not due to diode variation.

The measured thermal results are compared to several existing commercial heatsinks in Fig. 7. This comparison assumes an identical electrical stack to the GE microchannel heatsink, using silicon diodes attached to an AMB substrate. Because these other heatsinks require a thermal grease or epoxy layer for attachment, a thin layer (75 mm) with relatively high thermal conductivity (9 W/m/K) was assumed. Note that the GE heatsink did not include this layer, as there was no such interface required due to the heatsink design. Because of this benefit, the performance is superior to any of these existing heatsinks. The experimental results demonstrated about 15% lower thermal resistivity than the predicted values, validating the simulations. This discrepancy is partly explained by power-dissipation losses in the experimental connector and wirebonds, as well as by the uncertainty of the Icepak simulation absolute temperatures. Overall, the comparison is quite accurate, clearly demonstrating that the heatsink is very effective. In fact, the overall thermal resistivity of a power module equipped with this heatsink would be less than 0.15 (K)(cm2)/W, resulting in less than 75°C junction-to-coolant temperature rise for the heat flux of 500 W/cm2. This thermal performance is better than any existing heatsink using a comparable material stack.

References

1. Moore, G.E., “Cramming More Components onto Integrated Circuits,” Electronics, Vol. 38, No. 8, April 19, 1965.

2. Tuckerman, D.B. and Pease, R.F.W., “High-Performance Heatsinking for VLSI,” IEEE Electronic Device Letters, Vol. EDL-2, 1981, pp. 126-129.

3. Wang, B.X. and Peng, X.F., “Experimental Investigation on Liquid Forced Convection Heat Transfer Through Microchannels,” International Journal of Heat and Mass Transfer, Vol. 37, 1994, pp.73-82.

4. Adams, T. M.; Abdel-Khalik, S.I.; Jeter, S.M.; and Qureshi, Z.H., “An Experimental Investigation of Single-Phase Forced Convection in Microchannels,” International Journal of Heat and Mass Transfer, Vol. 41, 1998, pp. 851-857.

5. Hetsroni, G.; Mosyak, A.; Pogrebnyak, E.; and Yarin, L.P., “Fluid Flow in Microchannels,” International Journal of Heat and Mass Transfer, Vol. 48, 2005, pp. 1982-1998.

6. Knight, R.W.; Hall, D.J.; Goodling, J.S.; and Jaeger, R.C., “Heatsink Optimization with Application to Microchannels,” IEEE Transactions on Components, Hybrids and Manufacturing Technology, Vol. 15, No. 5, October 1992, pp. 832-842.

7. Shah, R.K. and London, A.L., “Laminar Flow Forced Convection in Ducts,” Advances in Heat Transfer, Supplement 1, Academic Press, 1978.

8. Gnielinski, V., “New Equations for Heat and Mass Transfer in Turbulent Pipe and Channel Flow,” International Chemical Engineering, Vol. 16, 1976, pp. 359-368.

9. Kakaç, S., Shah, R. K., and Aung, W., Handbook of Single-phase Convective Heat Transfer, New York: John Wiley & Sons, 1987, pp. 68-70.

The Circular Path of the Microchannel Cooling

Twenty-five years ago, Tuckerman and Pease introduced the concept of microchannels[2] to the electronics cooling industry. Their idea was fairly simple. Because heat-transfer coefficients generally increase with decreasing size, the passage size should be made as small as possible. This results in a dense package with higher heat transfer and a larger surface area-to-volume ratio than a conventional cooling device.

However, the benefits are tempered by increased pressure losses with minute passages, in addition to manufacturing challenges. Using traditional fluids analysis, Tuckerman and Pease determined that there was an optimum passage size for realistic pressure differences, selecting a 50-m-wide, 300-m-deep, 1-cm-long passage, which experienced a 30-psi drop with a 0.66 LPM water flow. Through the use of this microchannel, a heated device could dissipate 790 W/cm2 while only experiencing a 71°C temperature rise, as verified by subsequent experimentation. With this remarkable result, it appeared that this would provide a new solution to thermal management of high heat-flux power electronics.

Surprisingly, this improved cooling technology coincided with another important milestone in the history of electronic devices, as Intel released the 80286 microprocessor in 1982. This chip marked the first widely used commercial application of CMOS processing technology in the semiconductor industry, which was thermally significant because these devices only consume power in their switching state. Hence, the power dissipation of typical electronics chips decreased dramatically, and the need for aggressive cooling technology was reduced. As a result, microchannels were largely ignored commercially soon after their invention.

In spite of the lack of interest from industry, microchannels still garnered much academic study in the subsequent decades. The fascination grew particularly strong after a series of papers in the early 1990s raised questions about fundamental fluid dynamics in channels of this scale. These results ranged from unusual laminar-to-turbulent transition[3] to remarkably higher heat-transfer coefficients[4], which implied that this technology might be even more promising than previously suggested.

A flurry of research was seen around the world, with hundreds of studies each year of these phenomena — even leading to the organization of conferences solely devoted to the subject such as the ASME International Conference on Microchannels and Minichannels. Ironically enough, most of the surprising results were later attributed to experimental errors or faulty assumptions[5], but this fact did not quell the renewed focus on microchannels. Regardless of these academic pursuits, microchannels were bound to become more prominent once again due to Moore's Law.

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