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The Shaughnessy Report: A Handy Look at Rules of Thumb

Rules of thumb are everywhere. I took an auto mechanics class when I was 16, and I still remember the instructor telling us, “Remember, the cause of the problem is usually the simplest thing.” That holds true for much of life, actually.

There are hundreds of rules of thumb for PCB design. They’re built on design formulas, fabricators’ limitations, and tribal knowledge. Unfortunately, some longtime rules of thumb should be avoided at all costs.

We wanted to get to the bottom of rules of thumb, so we surveyed readers. Here are a few of the interesting answers that readers provided, edited slightly for clarity.

What rules of thumb do you utilize regularly, and why?

• Avoid 90° and acute angle routing to prevent acid traps
• Higher than 1GHz = high speed; lower = I don't care
• 2x dielectric (approximation) thickness for 50-ohm RF traces

When is it time to “do the science” instead of using a rule of thumb?

• For fast edge rates and high-reliability products
• When you need to simulate the BER of DDR4 memory or, even better, DDR5 memory, serial link buses
• If traces are very high-speed, currents are large, and interfaces are mission-critical, especially when simulations or other evaluation aren't being done. If the rule of thumb seems borderline, it's worth a closer look. Trust your gut.

Speaking of which, I’ve heard “Trust your gut” quite a few times while working on this issue. That’s a good rule of thumb in itself, because some rules of thumb have outlived their usefulness.

In the November 2024 issue of Design007 Magazine, we delve into rules of thumb—which ones work, which ones should be avoided, and when it’s time to “do the science.” We have a feature interview with IPC’s Kris Moyer and columnist Kelly Dack, and another interview with Chris Young of Moog Space and Defense Group. We also have articles by Scott Miller of Freedom CAD and Patrick Davis of Cadence Design Systems. And Barry Olney, Joe Fjelstad, and John Watson contribute feature columns.

We also have an article by ASC Sunstone CEO Anaya Vardya and an interview with Jon Smith, a former student in John Watson’s Palomar College PCB design class.

It’s show time, and I’ll be heading to PCB Carolina in a few weeks. Hope to see you there.

This column originally appeared in the November 2024 issue of Design007 Magazine.