Issue link: https://iconnect007.uberflip.com/i/1529118
NOVEMBER 2024 I DESIGN007 MAGAZINE 13 of thumb, or my default condition, is that I will stick with X. If X doesn't work, how do I add analysis around X to modify the condition to meet the requirement? Rules of Thumb: Too Many to Count? Shaughnessy: Does anyone know how many PCB design rules of thumb exist? Dack: ere's a rule of thumb for every work- flow process, so, potentially, there's a lot. Moyer: I have a flowchart that I use in my classes that has the typical design workflow process and rules. We determine width based on current and temperature rise or character- istic impedance, clearance based on voltages, via size based on aspect ratio, hole sizes, pro- ducibility level, and performance classification rule settings. en we have material selection which, as far as stackup, is based on operating temperature range as well as environmental structural needs: Is this a high vibration shock board? Do I need a thicker board? Another cal- culable thing is the quantity of nets. Dack: Additionally, there are rules of thumb for outer layer, inner layer, current carrying capacity, component placement and orienta- tion, density of outer layers, and whether it's topside or bottom side. Moyer: en there's the human interface, which I talk about in my class. Let's say I have a bunch of push buttons or switches that I need to put on an industrial controller. I can put the switches 100 mils from each other, but if I make a little array of switches that are 100 mils apart for an elevator, what happens when the elevator mechanic has large fingers or is wear- ing gloves and tries to push a button? Buttons might get pushed for floors 1 and 5 at the same time. So, I'll have to spread those buttons out about a half-inch more from each other. en there's environment, which includes temperature, vibration, salt, fog, moisture, acid, extreme chemicals, and pressure. Are you designing for the bottom of an oil well a couple of miles below the surface of the earth where there are extreme pressures? Is it for deep sea exploration where things might implode, like the guy who tried to take a submarine tour to the Titanic? Dack: at's a great example, Kris. Everyone in that submersible died because the inven- tor didn't design for environment correctly. e use of the product was not summarized into specific, design-for-environment speci- fications and documentation. Rule-of-thumb design and documentation philosophies are not appropriate when human life is at stake. When a product prototype is getting close to the point where the design is complete, there must be proven, verified materials and process evaluation backed up with specific fabrication notes, dimensions, tolerances, and operation parameters. Moyer: Does this design require prohibited materials? NASA oen prohibits silicone for optics and so on. All of that is part of the fea- sibility study. ere's environment, standards, and regulatory compliance. Do you have FAA, AMA, DOT, or DoD requirements? e rule of thumb is part of the feasibility study: Ensure that you have met regulatory compliance. Is your standard default condition acceptable, or do you need a modification because of regu- latory compliance? at's where the rule of thumb comes in.