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June 2014 • SMT Magazine 15 assembly if TJ and TC are isotherms and the entire heat flow from TJ to TC equals Ploss. It should be noted that these conditions are not always fulfilled. In small silicon transistors and diodes the rather small temperature gra- dients within the junction can frequently be neglected. However, as can be seen by a more detailed thermal investigation of a GaAs high- power transistor, depending on the considered semiconductor device, there are tremendously high temperature differences within the junc- tion itself [1] . Another frequently underestimat- ed danger to misinterpret equation (2) arises if the case temperature is not enough uniform. Reaching a uniform case temperature over as large an area as possible is one major concern of thermal management on the PCB level. A further approach to describe the thermal resistance Rth is shown in equation (3). It can be seen that the thermal resistance can be mini- mized by reducing the length d of the thermal path or by increasing the thermal conductivity l of the material as well as by increasing the area of the contact pad A. As already stated in the introduction there is a trend in minia- turization of the power components, so there is no chance to increase the area. For this only the two first possibilities can be used to improve the thermal management of the system. That means the length of the thermal path through the PCB should be as short as possible, and the material between component and heat sink should have a thermal conductivity as high as possible. (3) Motivation for Thermal Management The main reason for deficiencies of electri- cal systems beside dust, vibration and humidity is by far the impact of temperature. Therefore an efficient thermal management concept on the PCB is crucial for the reliability of power electronic systems. As an example we take high-power LED ap- plications, which are likely to dominate in the next years residential and commercial light- ing, signaling and vehicle headlights due to efficiency and extended lifetime. LEDs that range from 500 milliwatts to as much as 10 watts in a single package have become stan- dard, and researchers expect even higher pow- er in the future. Thermal management is of critical impor- tance for high- power LEDs. More than 60% of the electrical power input is converted into heat and built up at the junctions of LED chips due to non-radiative recombination of elec- tron-hole pairs and low light extraction. If that heat is not removed, the LEDs run at high temperature, which not only lowers their efficiency, but also makes the LED more dan- gerous, less reliable and shortens operating life [2,3] . Thus, thermal management of high-power LEDs is a crucial area of research and develop- ment. In this paper results of simulations and measurements of different LED-modules will be shown and should serve as representative of thermal solutions for general high-power appli- cations. Standard PCB Technology for Thermal Management Overview and Short Description Effective heat removal can be based ei- ther on a short heat conduction path to a heat sink perpendicular through the PCB (e.g., thermal vias) or by a conductor layer acting as a lateral heat spreader (extended thermal pads) or a combination of both. There are many different and well known build-ups for these heat removal concepts on PCBs. Thick copper approaches on PCBs guar- antee a very good lateral heat spreading effect due to the excellent thermal conductivity of the copper and are very well used to reduce hot spots. Insulated metallic substrates (IMS) are also state of the art and widely spread for thermal issues in electronic systems. An IMS consists of a metallic base material (mostly aluminum or copper) with a thickness of about 0.5 mm to 3.0 mm. On the metallic base material there is a thin dielectric layer (about 30 µm–150 µm) with a high thermal conductivity (0.5–8.0 W/ ADvAnCeD THeRMAL MAnAgeMenT SOLuTIOnS continues feaTure