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8 DESIGN007 MAGAZINE I MAY 2019 Rules are a part of life, but most of us don't like following the rules; I certainly don't. But I think we can make an exception for design rules. Many of you remember simpler times when you didn't need any stinkin' rules. Those were the good ol' days. Try to design even a simple board now without some constraints. Like most rules, design rules came about for your own good. And no single designer could possibly remember all of the constraints re- quired to design one of today's PCBs. But with a set of well-de- fined design rules, a designer can execute the most complex PCBs on the first try. I have to admit that I had no idea that de- signers had to con- tend with so many de- sign rules for each de- sign. Designers tell me they routinely have to set hundreds of de- sign rules for the more high-tech designs. How does a design- er manage all of these design rules and avoid over-constraining the design, which adds un- necessary cost and complexity? One thing that struck me while working on this issue was how little agreement exists about best practices for design rules. If you ask a half-dozen designers to explain their ap- proach to design rules, you might get a half- dozen answers. It reminds me of the Wild West. You may have design rules for schematic, layout, fab- rication, signal and power integrity, thermal, and assembly. With high-speed boards, you may have to set up matched-length, differen- tial pair, DDR, and spacing constraints. And that's just the tip of the iceberg. Today's more pow- erful PCB design soft- ware tools all feature constraint editors that make setting and man- aging design rules about as simple as it's going to get. But set- ting up design rules is still a time-consuming part of each design cy- cle. Is there a model for us to follow? Our friends in the IC are- na figured out design rules years ago. Of course, that's a whole different ballgame with only a handful of man- ufacturers serving that market. But those man- ufacturers laid down the law. "If you want to work with us, here's what you're going to pro- vide us, every time." That took care of that The Shaughnessy Report by Andy Shaughnessy, I-CONNECT007 Design Rules: For Your Own Good

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