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22 SMT Magazine • June 2014 We considered the spatial distribution of these losses (an exponential approximation of the decrease of the loss intensity with increas- ing distance to the LED surface) with an "onion shell" model [6] . The test assembly is assumed to be mounted on a heat sink with a constant temperature of 25°C. Thus, at the assembly's bottom face, Neumann boundary conditions were considered: TC = 25°C. Further bound- ary conditions on the remaining surfaces were considered as natural convection and radiation assuming an emissivity of one. Three-dimen- sional steady-state thermal simulations with the finite element method using a multiphys- ics software package with materials data were made for all set-ups of the considered packag- ing concepts as shown in Figure 10. Figure 11 depicts the thermal model of an FR-4-DK set-up as shown in Figure 10(b). In order to reduce the necessary node num- ber without loss of accuracy, the hollow ther- mal vias were replaced by cylinders filled with ADvAnCeD THeRMAL MAnAgeMenT SOLuTIOnS continues feaTure figure 9: Cavity in cavity board with high reflective cavity walls. figure 10: build-up of: a) iMS; b) fr-4 dk; and c) cavity board (led-in-cavity); d) material parameters used for thermal simulation.