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34 SMT Magazine • March 2015 The conclusion is that using CFCC in the stackup greatly improves the thermal perfor- mance of low thermal conductivity materi- als. The results suggest that a FR4 board with a single, thin layer of the composite material can approximate the performance of metal-clad boards, reducing cost, space and weight. Further heat removal can be achieved with the CFCC materials by increasing the through-plane ther- mal conductivity with the addition of thermal vias coupled from the topside Cu layer through the composite planes. cTE control The thermal benefits are only one part of the solution. The CFCC materials are thermally and electrically conductive low CTE (2–6ppm/ o C) carbon fiber sheets encased in typically 1 oz. Cu cladding. When embedded in a multi- layer PCB, the low TCE of the carbon fiber core, coupled with the high tensile modulus (25MSI) of the carbon fiber, constrains expansion of the high CTE of the conventional PCB laminate layers improving long-term reliability. Table 3 shows the thermal conductivity, CTE and me- chanical properties of the composite material and some commonly used materials. To realize the benefits of the material for CTE control lay- ers should be inserted near the surface of the stackup. If the stack is a single-layer structure then one layer of material can be used. For higher layer-count boards or packages two lay- ers can be used for a balanced stackup. Recent test results have shown that a single layer of TImE TO DITch hEavY mETaL FOr SOFT rOck? continues Feature CFCC material reduced the CTE of an FR4 board from over 17ppm/K to 10.7ppm/K, close to a 40% reduction. Summary: Back to Yash From Tom's analysis, it seems that there are comparable thermal performance results via alternate stackups using a variety of materials. However, when putting long-term reliability into the mix, it's critical to consider CTE prop- erties in materials selection. While the data does not present discrete conclusions for LED applications, it does suggest that composite ma- terials such as CFCC can provide both a ther- mal management solution and a dimensional stability solution that has not yet been present- ed through conventional materials. That being said, everyone's application has its own unique characteristics. As such, it would be wise to de- vise tests that fully simulate the conditions un- der which your products operate. SmT Table 3: Mechanical properties. Yash Sutariya is president of Alpha Circuit Corporation. for more information, or to contact Sutariya, Thomas S. Tarter is the president of package Science Services llC in Santa Clara, California.

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