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30 The PCB Design Magazine • December 2017 board, which means that the heat flux density into the board below a component is higher than the board average. If the temperature of any section of the board is close to the maxi- mum component case temperature, when you later refine the model to represent the compo- nent heat sources discretely, the component temperature limit will then likely be too high. Make a Best-Guess Estimate for Component Power Make a best-guess estimate of the individual power budgets for the main heat dissipating components that will be used in the design, and the approximate size of those packages. Then you can describe these packages as footprint heat sources in your simulation, which will smear the remainder of heat uniformly over the board surface. The system architect will already have some idea of what key components will be required, where they will need to be positioned, and their size, etc. For example, some components may be used that were selected for another product or retained from the previous genera- tion product. Run Thermal Simulations as Early as Possible If possible, include some form of 3D com- ponent model in the simulation before com- ponent selection is finalized so you can share the thermal results that can then be considered as part of the package-selection criteria. Some chips are available in more than one package style, and not all package styles perform equal- ly well from a thermal point of view. Adding a heatsink later may be unnecessary if the design team has access to this information early. Case temperature or junction temperature is the key measure to indicate whether the design is acceptable from a thermal perspective. At this stage, however, we are still only working with a rough estimate of individual component tem- peratures. The simplest 3D component model that can be used is a conducting block. (The 3D thermal simulation software FloTHERM includes material properties that provide case temperature prediction for different package styles.) For plastic components, a thermal con- ductivity of 5 to 10 is recommended [2] , and 15 for any ceramic components. A thermal con- ductivity of 5 will give you a worst-case figure for case temperature. If you have the 3D model, the effect of the component on the local airflow and any down- stream components can be considered in the simulation. For example, large components can shield smaller, lower-profile components from cooling air. The wake formed behind a compo- nent is where the same air gets recirculated, so any components in that region are likely to be hot. Try to align any rectangular components so that their long side is parallel to the primary flow direction. This practice reduces the overall pressure drop because the flow encounters less of an obstruction and produces a smaller wake, which minimizes the effect on downstream components. Share Thermal Results Among the PCB Design Team Now, you can start to feed information about the PCB's performance to the design team. Al- though the simulation is relatively coarse at this stage, the airflow distribution over the board and the resulting board temperature map can illustrate what you have for available cooling air and what that may mean for component temperature. When you share these initial results, empha- size that these nominal component case tem- perature values are subject to change because they are based on: • an assumed layout • rough power estimates • uncertainty about package selection • unknown layer stackup and copper distribution within the PCB, and • preliminary heatsink size and design (if already known to be necessary) This early model is useful for investigating the effect of component placement on the tem- perature of a component and its neighbors. Ad- justments can be made easily and the model re- run in a matter of minutes. The results will give some indication as to which components, if any, might need some form of heatsink, which STREAMLINING THERMAL DESIGN OF PCBS

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