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

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The rapid development of high-density power electronics has led to remarkably challenging thermal issues. Over the past several decades, transistor development has followed Moore's Law, which states that device sizes decrease exponentially over time.[1] Although advanced power semiconductor technologies have delayed the need for aggressive cooling by several years, the heat flux from the power device has risen significantly, approaching 500 W/cm2. This level is beyond the capability of conventional heatsinks used for silicon-based devices, which can achieve only about 20 W/cm2 when maintaining junction temperatures below 150°C. Therefore, novel technologies must be developed for thermal management. One such technology is microchannel cooling.

The plot in Fig. 1 compares commercially available water-cooled heatsinks, including two microchannel designs, which exhibit dramatically better performance than conventional centimeter-scale channel designs. However, these microchannel heatsinks are limited to small footprints in terms of manufacturability and scalability, thus they have yet to see widespread use in power electronics.

Fig. 2 displays the fractions of the thermal resistivity attributed to different individual layers for a typical module. In the case of microchannel cooling, the largest contributor to the resistivity is the thermal grease layer between the baseplate and the heatsink.

Integrated Microchannel Heatsink

Because of the increased importance of the conductive resistance of the stack, a better solution would be to eliminate some of these layers from the structure. To that end, a team from GE Global Research designed an integrated heatsink with a series of microchannels fabricated directly into the bottom copper layer of the active metal braze (AMB) substrate. This concept, shown schematically in Fig. 3, has advantages in reducing both the convective and conductive resistances of the module.

By using microchannels, the convective thermal resistivity is reduced dramatically. In addition, this stack removes the baseplate solder, the copper baseplate and, most importantly, the thermal grease from the conductive path. As seen in Fig. 2, this results in the elimination of the two largest resistances in the structure. The overall result is a reduction of the total stack resistivity by a factor of two when compared to the best microchannel heatsink in Fig. 1.

An integral microchannel heatsink should be systematically designed for optimum thermal response. This process consists of first-order analysis, detailed 3-D computational fluid dynamics (CFD) simulations and experimental validation.

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