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CTE-Matched Wafer-Level Coldplates
Keywords: water cooling, CTE, wafer-scale package
Water cooling is used for high power devices ranging from laser diodes to high-performance CPUs. In the case of lasers, which have power densities of 500W/cm2 and above, microchannels are used for their high rate of heat removal. In other, lower density coolers larger scale designs may be used with adequate effectiveness. Water cooled coldplates are most commonly constructed of copper or aluminum alloy and are individually bonded to the device. A disadvantage of the use of these metals is their much higher coefficient of thermal expansion (CTE) compared to the semiconductor. This mismatch can result in stress on the devices during soldering or operation. It can also stress the solder joint, encouraging voiding. One solution is to attach the chip to an cooler made from low CTE material such as tungsten-copper or molybdenum-copper. We have designed, modeled, and fabricated prototype CTE-matched coldplates for wafer level packaging. Such assemblies would allow dozens to hundreds of water-cooled package bottoms to be bonded to devices at the wafer level, then to be singulated and connected to water flow. We present thermal performance data based on computational fluid dynamics modeling as well as flow, thermal resistance, and thermal effectiveness tests. We show that the device junction temperature can be maintained below 60C even for power density above 200W/cm2. Finally, we propose application for which these devices will be effective and economical.
Greg I. Rudd, Chief Technologist
Spectra-Mat, Inc
Watsonville, CA

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