Global PCB Connections: Following DFM Rules Leads to Better Boards
Elementary Mr. Watson: How to Reinvent Your Professional Journey
Fresh PCB Concepts: PCB Plating Process Overview
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.
Share on:
Suggested Items
If You Can Define It Right, You Can Design It Right
12/05/2024 | Andy Shaughnessy, Design007Design engineer Chris Young is known for his optimized design process. As lead hardware engineer with Moog Space and Defense Group and owner of Young Engineering Services, Chris collects data like it’s going out of style, and he leaves nothing to chance. With that in mind, I asked Chris to discuss his views on rules of thumb—which ones work, which ones should be avoided, and how rules fit ideally into the PCB design process.
PCB Layout Rules of Thumb for Consideration
11/25/2024 | Patrick Davis, Cadence Design SystemsJust because a “rule of thumb” is usually based on experience instead of precise facts doesn’t negate its value. For instance, when I told my kids that a good rule of thumb was not to back-talk to their mother, they discovered very quickly how accurate my advice was once they crossed that line. There are a lot of rules of thumb that we rely on daily, including those that apply to PCB design.
Elementary, Mr. Watson: Rules of Thumb—Guidelines vs. Principles for PCB Design
11/26/2024 | John Watson -- Column: Elementary, Mr. WatsonThe infamous "rules of thumb" are simple guidelines that help you make decisions based on experience, not exact facts. They’re like shortcuts we use because they work most of the time. For example, if you want to know if spaghetti is done cooking, a common rule of thumb is to grab a spaghetti strand and throw it against the wall to see if it sticks. I used to do that, except that instead of the wall, I used the ceiling, which drove my mother crazy.
Rules of Thumb for PCB Layout
11/21/2024 | Andy Shaughnessy, I-Connect007The dictionary defines a “rule of thumb” as “a broadly accurate guide or principle, based on experience or practice rather than theory.” Rules of thumb are often the foundation of a PCB designer’s thought process when tackling a layout. Ultimately, a product spec or design guideline will provide the detailed design guidance, but rules of thumb can help to provide the general guidance that will help to streamline the layout process and avoid design or manufacturing issues.
Flexible Thinking: Rules of Thumb—A Word to the Wise
11/20/2024 | Joe Fjelstad -- Column: Flexible ThinkingIn the early days of electronics manufacturing—especially with PCBs—there were no rules. Engineers, scientists, and technicians largely felt their way around in the dark, making things up as they went along. There was a great deal of innovation, guessing, and testing to make sure that early guidelines and estimates were correct by testing them. Still, they frequently made mistakes.