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June 2014 • SMT Magazine 25 ADvAnCeD THeRMAL MAnAgeMenT SOLuTIOnS continues ity of a plugged via is practically the same as the open one with same hole geometry, with plugged vias the perpendicular temperature drop across the PCB can be reduced significant- ly because they can be placed directly beneath the LED chip without the risk of solder soaking. The respective temperature functions along the perpendicular symmetry axis are compared with each other in Figure 14. feaTure mal vias are placed only in a pad range apart of the LED chip instead of beneath, the path for a major part of the heat flux is comparatively long. This leads to LED chip temperature be- tween 32°C and 41°C depending on the choice of geometry variation (via count, drilling di- ameter, Cu thickness etc.). In this context the feature of the plugged via technology should be highlighted: Although the thermal conductiv- figure 12: results of thermal simulations; detailed views of temperature distribution in vicinity of the LeD. figure 13: results: Temperature profile along the symmetry axis in out-of-plane direction (Z-axis) of a LeD-in-cavity set-up.