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Design Rules: For Your Own Good

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 required to design one of today’s PCBs. But with a set of well-defined design rules, a designer can execute the most complex PCBs on the first try.

I have to admit that I had no idea that designers had to contend with so many design rules for each design. Designers tell me they routinely have to set hundreds of design rules for the more high-tech designs. How does a designer manage all of these design rules and avoid over-constraining the design, which adds unnecessary 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 approach 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, fabrication, signal and power integrity, thermal, and assembly. With high-speed boards, you may have to set up matched-length, differential pair, DDR, and spacing constraints. And that’s just the tip of the iceberg.

Today’s more powerful PCB design software tools all feature constraint editors that make setting and managing design rules about as simple as it’s going to get. But setting up design rules is still a time-consuming part of each design cycle.

Is there a model for us to follow? Our friends in the IC arena figured out design rules years ago. Of course, that’s a whole different ballgame with only a handful of manufacturers serving that market. But those manufacturers laid down the law. “If you want to work with us, here’s what you’re going to provide us, every time.” That took care of that problem. But that may not work for our segment with thousands of PCB fabricators worldwide.

To read this entire column, which appeared in the May 2019 issue of Design007 Magazine, click here.