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14 SMT Magazine • June 2014 thermal vias in thin board constructions. In an- other approach advanced thermal management solutions will be presented on the board level, exploring different buildup concepts (e.g., cav- ities). Advantages of cavity solutions in the board will be shown, which not only decrease the thermal path leading from the high-pow- er component through the board to the heat sink, but also have an impact concerning the mechanical miniaturization of the entire sys- tem (reduction of Z-axis). Such buildups serve as packaging solution and show an increase in mechanical and thermal reliability. Moreover, thermal simulations will be con- ducted and presented in this paper in order to reduce production efforts and to offer opti- mized designs and board buildups. Introduction Modern power electronics is using power components such as MOSFETs, IGBTs, GTOs, high brightness LEDs and many more. Due to the enormously rapid advances in semicon- ductor technology, particularly in the realm of high-power applications, the trend is going to smaller components with even higher switch- ing speeds and higher current densities. In gen- eral a strong miniaturization trend for whole modules can be seen. With these trends and increasing power loss densities, the thermal performance of an assembly becomes one of the most important quality factors in electronic packaging. New materials and innovative approaches in PCB substrates are required to meet the required re- liability levels. Interest in power electronics has grown dra- matically in the last few years, with an increas- ing need for electric power management and control (smart grid), renewable energy genera- tion and control (wind power, photovoltaic, fuel cell, etc.), electric transportation, and the desire to improve operating efficiency of heavy systems (trains, industrial motors, electric ve- hicles, etc.). Power electronic converters are found wherever there is a need to modify the volt- age, current or frequency. These range in power from few milliwatts in mobile phones to hun- dreds of megawatts in HVDC (high-voltage, direct current) transmission systems (Figure 1). Usually we think of electronics in the frame- work of information, where speed is the prima- ry interest. In the context of power electronics improved efficiency and lower power losses are important. Thermal Resistance Definition of the Thermal Path and Thermal Resistance For steady-state considerations most fre- quently used measures for the thermal perfor- mance of an electronic module are either the junction temperature TJ of the semiconductor device with the significant power loss or, even more common, the thermal resistance Rth. The latter has to be defined by the temperature dif- ference J along a thermal path as e.g. (1) where TC denotes the temperature of the interface of the case of the module and a cooler, and the power loss (Ploss) causing the tempera- ture difference (2) Equation (2) is a useful practical approach to describe the thermal performance of a power ADvAnCeD THeRMAL MAnAgeMenT SOLuTIOnS continues feaTure figure 1: range of power electronic applications. (Source: ineMi Technology roadmaps, Jan 2013)