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10 DESIGN007 MAGAZINE I NOVEMBER 2024 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 le 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, fab- ricators, CEMs, and even the occasional uni- versity. Rules of Thumb: A Primer 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 dis- cuss 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. Feature Interview by the I-Connect007 Editorial Team