The Hitchhiker's Guide to PCB Design

The Hitchhiker's Guide to PCB Design

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63 N ow that we have established the PCB stack-up is more than just a thickness diagram, we have laid a foundation for routing and utilization of copper planes within the layout. Layout is usually thought of as merely a job of "connecting the dots" by those who do not understand the complexities of PCB design. This perception is true to a very small extent, but PCB layout is far from simple. After routing and completion of the layout, when the PCB goes on to be manufactured and assembled, the dots which a PCB designer has connected will have turned into copper connections which must carry electrons. The copper and its surrounding substrates must be properly configured to be manufactured and assembled together with a myriad of components which must operate by using those electrons. There are thousands of things which can go wrong. In Ian's design in chapters one and two, the auto-router routed two-hundred and fifty-six nodes of ground connections using 3 mil trace widths. This simply never would have worked—Ian's circuitry needed solid power planes to provide power and enhance performance. Ian allowed the software defaults to have their way with his design which proved very costly for all project stakeholders from a time and expense standpoint. Even if Ian's design had made it all the way through the manufacturing processes, the electronic circuitry within the board would not have performed as intended. Without knowledge of key design fundamentals, relying on automated features (like the auto-router in default settings) can cause a serious design wreck. Left to its own devices, an auto-router may pack all the signals together, causing a great degree of cross-talk, destroying their ability to perform There are five important topics to consider during completion of the layout: Design Rules To successfully complete a PCB layout, it is very important to set the design rules to match the estimated producibility class of the design. PCB design rules can be set in most layout tools which allow generic settings to be saved. Design Rules Checking (DRC) tools will check against these rules in real-time, providing you feedback as you go. Having this real-time feedback is invaluable as a designer. Trying to keep track of all the rules and how they interrelate manually is not a task you want to take on. Chapter 8 Routing and Planes

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