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28 SMT Magazine • March 2015 to have useful lives in excess of 30+ years, they are finding out that the actual LED assemblies can fail in as little as 5–6 years. Initial analysis is pointing to the significant X and Y axes CTE differences between the sol- der joint, copper circuitry layer, thermally con- ductive dielectric, and the aluminum. The net result of the CTE differences is a shear effect be- ing created that can eventually disrupt the sol- der joint, which results in operational failure. Depending upon how accurate this information is, it could mean the start of a whole new ap- proach to PCBs for LED applications. Below is an abstract of a white paper writ- ten by Thomas Tarter from Package Science Ser- vices, which performed initial testing on carbon fiber and graphite based materials provided by Stablcor Technology Inc. The carbon fiber con- straining cores (CFCC) materials evaluated are carbon-fiber and/or graphite reinforced epoxy cores to aid in heat dissipation, rigidity, weight reduction, and CTE control. These cores can be used independently or in conjunction with cur- rent MCPCBs to produce functionally improved heat dissipation while reducing the CTE mis- match currently present on LED assemblies. abstract: Introduction and model Parameters (by Thomas Tarter) Thermal performance for PCB structures are investigated in the form of steady-state finite- element models of various stack-ups of com- monly used materials for LED applications. The models show the effect of materials used in the stack up including FR-4, aluminum, cop- per, graphite and CFCC. The goal of the study is to compare relative thermal behavior of typical boards modified with the enhanced core mate- rials. The materials are inserted into standard PCB stack-up configurations as an added or re- placed layer. Models are solved for maximum temperature on a 25 mm x 25 mm coupon with a 2 mm x 2 mm-square heat source. The stack up resembles substrates known as 'metal-clad' where the dielectric and topside copper are lam- inated directly onto a metal substrate. In addi- tion, FR4 boards are used as a worst-case com- parison. A simple stack up is used as shown in Figures 1 and 2. Figure 1 shows a typical stackup for a metal-clad assembly. Figure 2 shows the stackup with an added CFCC. Variables used in the study include mate- rial properties and layer thickness. The primary variables are top side copper thickness/weight, dielectric thickness and base material thickness. Table 1 lists the ranges for geometry and ma- terial properties. The heat source is simulated as a planar load, directly on the surface of the top-layer copper. One watt is applied over a 2 mm x 2 mm square area in the center of the coupon. The models are solved in natural con- vection with an ambient temperature of 30°C. TImE TO DITch hEavY mETaL FOr SOFT rOck? continues Feature figure 1: Typical stackup. figure 2: Inserted CfCC.

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