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46 The PCB Magazine • December 2017 teristics to change. As the junction temperature of an LED increases, the lumen (brightness) output decreases. The output wavelength also shifts with a change in junction temperature. The optimum way to address this heat passively is by using materials with a high thermal con- ductivity to move heat away from the junction as quickly as possible. The dielectric should have high thermal conductivity both laterally (x and y directions) as well as vertically through the base of the device. The circuitry routing for most LEDs is very simple, requiring only one layer of traces. A single-sided IMS would incorporate a thin ther- mally conductive dielectric sandwiched be- tween a copper foil and aluminum heatsink. The dielectric helps to conduct the thermal en- ergy away from the copper circuit to the alu- minum heatsink where the energy is dissipated through convection to the air, or by conduction via a mechanical mounting system. More com- EVOLUTION OF THERMAL MANAGEMENT IN PCBS plex designs will have multiple layers of copper circuitry mounted to a heatsink. Components can be mounted on both sides by routing the aluminum prior to lamination. Very rarely will the aluminum heatsink be used as an electrical ground for the circuit. If electrical contact to the aluminum is a requirement for the design, it is a complex process to manufacture a plated via with connection to the aluminum. The alumi- num interacts negatively to the electroless cop- per bath, and although feasible, few PCB shops offer the process. The easier approach is to use a copper heatsink versus aluminum. Copper is an exceptional thermal conduc- tor (~400W/mK), but cost and weight consider- ations drive aluminum to be the preferred heat- sink for most applications. As discussed earlier, aluminum reacts negatively to electroless copper. This is also true with some surface finishes, for example, electro- less nickel/immersion gold (ENIG). There is an Figure 1: Standard IMS builds. Table 1: Aluminum and copper properties comparison.