The Hitchhiker's Guide to PCB Design

The Hitchhiker's Guide to PCB Design

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Page 76 of 115

77 Always review the route path for each net to make sure it has been run as efficiently as possible. A net analyzer is included as a feature in most layout tools and can help. Circuitous routing paths or routing paths which double-back on themselves are a scourge to efficient use of routing area and can have negative effects on performance. A net analyzer can list and sort nets within the design by length and highlight each route path individually starting with the longest first. A net analyzer is an effective tool for catching wayward critical route paths, so they can be shortened. Routing common traces between components using modern PCB tools goes quickly and can be quite fun. It can be tempting to get going on the "easy" routing before firmly establishing the routing and locking critical routes first. There is a saying in the PCB design industry: "90% of the routing on a PCB takes only 10% of the time, while 10% of the remaining routing can take 90% of the time." This perspective describes a designer's experience toward the completion of a layout when the fun of routing turns over to an ever-increasing challenge. Suddenly there is a realization all the fun routes have eaten up all the routing space. The routing challenge will most certainly turn into defeat and a complete do-over if the remaining 10% of the routing (mentioned in the saying) includes any critical signals. Keep routing fun and challenging—consider critical net noise, clearance, and establishing shortest routing paths first. At the same time, consider avoiding adjacent layer parallelism and eliminating circuitous routing paths. Avoid complete routing do-overs by routing critical lines first. Conclusion It might be easy for those who do not understand PCB layout to sum up the process as simply "fitting the parts on the board and hooking them together." The intent of this chapter has been to give a new designer some basic routing points to consider before, or instead of, simply activating an auto-routing routine. Successful routing and plane utilization has a direct effect on circuit performance. Implementing set-up, routing and establishing copper planes in a PCB layout can really make the difference between a dot-to-dot hook-up technician and a designer. Metaphorically, this comparison may be likened to the steps a skilled photographer would implement to create a classic photograph. Without understanding how to properly setup and manipulate the subject matter, lighting, and exposure controls, a camera operated by the click of novice may only yield a cheesy snapshot. Tips for Routing and Utilizing Planes: F F Become very familiar with the design rules setup for your layout software. Understand how routing can be controlled or constrained to route based upon rules defined and imported from the schematic or set them manually before routing begins. F F Set spacing constraints for individual lines and traces, not only for fabrication processing, but for electrical performance. F F Create design templates to save time.

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