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18 DESIGN007 MAGAZINE I SEPTEMBER 2020 Feature by Dr. Johannes Adam ADAM RESEARCH We all know that the temperature of compo- nents and materials in a PCB must not exceed a limit between 100°C and 150°C. When the environment is already hot or when heat dis- sipation is impeded, it is important to be aware of this as early as possible so that appropriate action can be considered and initiated. Com- pliance with the thermal specifications is not everything, but it is one of the necessary steps in electronics development. Simple Temperature Predictions: Likely Wrong What are some methods for predicting the temperature of a PCB? You could take the data- sheet of the component and search for tem- perature information (e.g., for the maximum power dissipation P max or the junction-to-ambi- ent thermal resistance R ΘJ-A ). Both are in rela- tion to each other via T max ≈ T A + P max x R ΘJ-A . The formula is simple but, unfortunately, not to be recommended because "ambient" really includes everything around the component, from the pad to the vias, and the design of the layers up to the air. We can easily imagine that R ΘJ-A cannot be a "material constant" of the component. On a good thermally conductive copper plate, the same heat source (the com- ponent) would have a much lower tempera- ture than on a plate made of FR-4 and thus a different J-A thermal resistance. The PCB on which the datasheet is based plays an impor- tant role. The test environment for the datasheets is usually based on a PCB with a minimalistic design, according to JESD-51 [1] . A review with examples is given by Edwards and Nguyen [2] . Due to the deviation between the real environ- ment and the test environment, R ΘJ-A must not be used for temperature predictions (Wong [3] p. 4). RΘJ-A is used rather as an indicator for comparing the thermal properties of different components, which are tested under compara- ble environmental conditions. There is another often misused version of thermal resistance, derived from the Fourier differential equation of heat conduction, namely that for a plate according to R th = thickness / (area x thermal conductivity). This simplification is only valid Thermal Management: Don't Believe the Datasheets

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