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Low Cost Copper Tungsten Heat Sinks Made by Near Net Shape Infiltration
Keywords: copper tungsten , near net shape , low cost
CuW heat sinks have been widely used in the packaging of microwave devices, optoelectronic components, integrated circuits, and many other electronic packaging applications. Their measures of thermal expansion coefficient, thermal conductivity, and electrical conductivity can easily be controlled by varying Cu/W. They can also be matched with semiconductor silicon, arsenic, gallium arsenide, aluminum oxide and beryllium oxide, etc. Since W and Cu have considerably different melting points, it is impossible to melt W and Cu into each other at high temperatures. In other words, no eutectic will be formed. At present, there are two major techniques to make the composite: infiltration and high-temperature liquid phase sintering. 1. Infiltration In this technique, W powders are mixed with a small amount of binders, pressurized and molded into a W compact, and then sintered into a W skeleton. Designed excess Cu is thereafter attached to the surfaces of the W skeleton and infiltrate into the W at temperature higher than the melting point of Cu (typically 1350C). Precision machining will then be performed to get the desired dimensions. An advantage of this technique is that during the process of infiltrating Cu into the W skeleton, most of the voids and interstices between the sintered W particles can be filled by Cu and hardly any eutectic develops. Therefore, critical measures such as hermeticity and thermal conductivity are ideal, especially the thermal conductivity, which can reach 180-200W/mK. The disadvantage associated with this method is that, for each and every piece of Cu/W, thickness of the Cu infiltration overflow is unpredictable and inconsistent. To facilitate precision machining, the W skeleton needs to be at least 0.8mm extra in thickness than the finished product. Machining allowance has to be even bigger when machining parts with mounting holes, steps, pedestals, slot and other 3D features to their desired shapes. The result is a decrease in efficiency and vast waste of W, which sometimes accounts for 80% of the final composite and is a few times more expensive than Cu. Compared to high temperature liquid phase sintering and activated liquid phase sintering; this method causes 10-45% more waste of materials. It is the most expensive technique of all considering the material and manufacturing costs, especially for irregular shaped products. 2. High-temperature Liquid Phase Sintering First, W powders and Cu powders are mixed to the designed proportion. The mixture is then pressurized and molded into compacts 0.2-0.3mm larger in every dimension than the final product, after which it is sintered at temperature higher than 2000 C. To obtain the desired product dimensions, precise machining is performed to remove the excess Cu. An advantage of this method is the efficient use of expensive W, since the machining allowance will be much smaller than required by the infiltration method. It has its drawbacks, too. First, sintering temperature is high, which makes this process costly. Sintering cost takes up the biggest proportion of total cost after material cost. Second, high temperature sintering leads to the formation of eutectic, which reduce the hermeticity and thermal conductivity of Cu/W and Cu/molybdenum composites, the two properties that affect the reliability of integrated circuits. Thermal conductivity of final product attained by this method is only 180-190W/mK. Finally, precise machining surface by surface and piece by piece increases complexity of manufacture and cost. It is especially the case for the fabrication of irregular shaped packaging materials. Accordingly, there is a need for a new process for making Cu/W composite electronic packaging materials that reduces the complexity and cost of manufacture. In this paper, we will present our latest development in near net shape infiltration to make CuW composites. In our process, W powders are compacted to a near net shape and then Cu is infiltrated into the W skeleton. Our new manufacturing method enjoys the comparable performance as that of infiltration and the low manufacturing cost as that of sintering.
Ken Kuang, President
Torrey Hills Technologies, LLC
San Diego, CA

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