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Rules of Thumb: A Primer
November 14, 2024 | Andy Shaughnessy, Design007 MagazineEstimated reading time: 2 minutes
According to Merriam-Webster, the first reported use of the term “rule of thumb” dates to a sermon given in 1658 in England. It seems to have begun with tradesmen using their thumb for measuring, because an adult thumb is generally about 1 inch wide. If you left your tape measure in your other tunic, you could get by with your thumb.
Over the next 366 years, the term has become ubiquitous, particularly in electronics design. Spend 10 minutes searching online and you’ll find hundreds of PCB design rules posted by OEMs, design bureaus, design instructors, fabricators, CEMs, and even the occasional university.
Many industry-wide rules of thumb are based on DFM constraints or formulas, but others are based on tribal knowledge. I sat down with our contributors Kris Moyer and Kelly Dack to discuss the role of rules of thumb, when to employ them, and when it’s time to do the math.
Andy Shaughnessy: Kris, there are hundreds of rules of thumb, and a lot of designers have a love/hate relationship with them. Doug Brooks once wrote about the “no 90° traces” rule, joking that the electrons would fly off if they went around a 90° corner, like passengers on a roller coaster. Some readers thought he was being serious.
Kris Moyer: You hit the nail on the head. These rules of thumb have existed for decades, but designers need to understand when to use them and when not to use them. These are the “It depends” cases.
Shaughnessy: Right. Some rules of thumb are based on formulas, but others are just based on experience, DFM limitations, etc.
Moyer: That’s true. Even if designers are not using formulas directly in their designs, they may be unknowingly using formulas anyway. For example, when designers go into Doug Brooks’ software tool that tells them what trace width they need for a given current, temperature rise, and copper thickness, there are several formulas under the hood. There are things like the reflection coefficient formula for doing signal integrity analysis, voltage clearance formulas, and even footprint design. We have package design consideration formulas.
Whether we all realize it, everything in electrical engineering is formulary. We have an understanding of what formulas are. Designers don't necessarily have to understand the formula directly, write out the formulas, and solve math problems by hand or calculator. But if designers truly understand the mechanics of the formula, they can understand when their tool does something that doesn't seem right. There are a lot of formulary rules of thumb that are just wrong.
Continue reading this interview in the November 2024 issue of Design007 Magazine.
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