Issue link: https://iconnect007.uberflip.com/i/1529118
12 DESIGN007 MAGAZINE I NOVEMBER 2024 Kris Moyer: You hit the nail on the head. ese rules of thumb have existed for decades, but designers need to understand when to use them and when not to use them. ese 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: at'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' soware tool that tells them what trace width they need for a given current, tempera- ture rise, and copper thickness, there are sev- eral formulas under the hood. ere are things like the reflection coefficient formula for doing signal integrity analysis, voltage clearance for- mulas, and even footprint design. We have package design consideration formulas. Whether we all realize it, everything in elec- trical engineering is formulary. We have an understanding of what formulas are. Design- ers 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. ere are a lot of formulary rules of thumb that are just wrong. Shaughnessy: What's one of the first rules of thumb a designer might use in a typical design? Kelly Dack: We analyze the incoming data and do a feasibility study. We must determine whether we have enough given board area for the amount of components on the bill of mate- rials, and whether that will turn into a single- sided or a double-sided assembly. Moyer: It's a density formula. For example, you have five square inches of board, but when you add all your parts together, including foot- prints, it's eight square inches of footprint. It won't all fit on one board. Dack: We look at land patterns, exit vias, and density of routing. Do I need to go to HDI? In terms of workflow, that's the first thing. Once we figure out the outer layer density, then comes stackup. An experienced designer can use some rules of thumb and estimation to come up with an initial solution proposal. Moyer: It's a rule of thumb plus formula. I'll give you an example for an eight-layer stackup: We have four signal layers and four plane layers. Do I have enough layers to meet all my char- acteristics, power distribution, and X, Y, and Z? Do I need to add more layers? Do I realize, "Uh oh, I have a weight limitation because I'm sending this into space. Can I remove a couple of layers to get my weight down?" You start with the rule of thumb, and then you analyze. Marcy LaRont: Are you talking about a rule of thumb plus formulary analysis on top of that? Moyer: Exactly. At every step of the process, rules of thumb can also be thought of as default conditions. When I first start a design, my rule " Whether we all realize it, everything in electrical engineering is formulary. "