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18 SMT Magazine • March 2015 Thermal issues with a PCB design are mostly determined during the component selection and layout phases. After this point, only remedi- al actions are possible if components are found to run too hot. Addressing thermal issues early in PCB design, starting at the system or enclo- sure level to understand the flow environment critical for air-cooled electronics, can streamline the process. Assumptions about the airflow uni- formity in early design that subsequently prove unachievable can have a disastrous effect on the commercial viability of the product and meet- ing the market window. by John Parry MenTor GrAphiCS CorporATion How to Streamline PCB Thermal Design Begin Before Placement and Layout Substantial work can be done well before layout is completed within the electrical design flow. A simple representation of the enclosure [1] can provide information about the air flow pro- file over the board. Start by smearing the total board power over the total board surface, which will provide a temperature map that will show any hot regions that are caused by a badly dis- tributed air flow. Treat the board as a block with an isotropic conductivity of between 5 W/mK and 10 W/mK to optimize enclosure-level air flow ahead of the PCB design. Components inject heat locally into the board so the heat flux density into the board below a component will be higher than the av- erage for the board. As a result, the local board Feature

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