Many aspects of a PCB’s performance are determined during detailed design, e.g. making a trace a specific length for timing reasons. Timing issues are also affected by temperature differences between components. Thermal issues with the PCB design are largely ‘locked in’ during the component (i.e. chip package) selection and layout phases. After this point only remedial actions are possible if components are found to run too hot. We advocate a top-down approach starting at the system or enclosure level [Ref. 1] in order to understand the flow environment for the electronics, which is critical for air-cooled electronics. Assumptions made about the uniformity of the airflow in early design that subsequently proves unachievable can have a disastrous impact on the commercial viability of the product and meeting the market window.