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28 The PCB Design Magazine • December 2017 by Dr. John Parry MENTOR When designing a PCB, thermal issues are often locked in at the point of selecting and lay- ing out the chip package for the board. After that, only remedial actions are possible if the components are running too hot. Assumptions made about the uniformity of the airflow in these early design stages can mean a disaster for the commercial viability of a PCB if those as- sumptions are incorrect. A different approach is needed to improve reliability and to optimize board performance. This approach starts with considering the overall flow environment, which is especially critical for good operation of air-cooled elec- tronics. Start early, keep it simple, and focus on collaboration between mechanical and electrical design. If you are the mechanical en- gineer responsible for the thermal integrity of the product, you can provide as much useful feedback as possible to the electronic engineers about effects their choices will have on the ther- mal issues in their PCB design. This collaboration entails advising with package selection and the positioning of com- ponents to best use system airflow for cooling. Layout and package selection usually are deter- mined by electronic performance and cost; how- ever, temperature and cooling can inevitably af- fect operation and cost so the consequences of design choices on thermal performance should be made as clear as possible, as early as possible. Before Placement and Layout First step is to optimize the enclosure-level airflow. Begin with a simple representation of the enclosure [1] to provide information about the airflow profile over the board. Smear the to- tal board power over the total board surface to get a temperature map indicating any hot re- gions caused by badly distributed airflow. You can treat the board as a block with an isotropic thermal conductivity between 5 and 10. The re- sults at this stage will not be affected by which- ever value you choose in that range. Do not use the board temperature to esti- mate component temperatures at this stage be- cause components inject heat locally into the FEATURE

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