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20 SMT Magazine • June 2014 FR-4 in the DK2 is five times higher than that in the DK6 the heat is conducted into the cool- er at a remarkably lower temperature because the thermal vias are in intimate contact with the cooler. This also leads to a steeply declining temperature in lateral direction outside of the LED chip. Significantly lower as on the first two PCB types are the temperature distributions on the two IMS type samples. In spite of a heat flux den- sity of more than 50 W/cm 2 the maximum tem- perature of the LEDs are lower than 40°C in case of the IMS1 sample and even lower than 33°C in case of the IMS3 sample. This can be explained by the higher thermal conductivity of the dielec- tric (in IMS1: 0.5 W/(mK), in IMS3: 4 W/(mK)). Advanced Thermal Management Solutions: Cavity Boards Introduction Another possibility to reduce the thermal resistance of the PCB is the use of cavities. Local depth reduction (through various meth- ods and technologies) has long been applied to achieve a number of design and/or application linked results. In this section the advantages of PCBs with special cavities regarding to advanced thermal management performance are presented. Cavity Formation The production method of PCBs with cavi- ties presented in this paper is based on patent- ed technology [5] , which enables the removal of multiple layers at varying depths. The specific depth is achieved by the application of a paste on the release layer with subsequent relamina- tion of the entire board. A laser cutting process then trims and cuts at the predetermined shape to separate the relaminated layers from the re- lease layer. The final steps are "cap removal" and paste stripping (Figure 6). What remains is the solder footprint pattern. Diverse surface finishes and also application of solder mask can be employed in the cavities. ADvAnCeD THeRMAL MAnAgeMenT SOLuTIOnS continues feaTure Table 1: Ir-thermography measurement results and evaluated thermal resistance values. figure 6: Schematic process flow of cavity formation.