Issue link: https://iconnect007.uberflip.com/i/1361971
12 PCB007 MAGAZINE I APRIL 2021 to drop down to an internal layer. And by doing that, it means I'm going to have to have a via. I have to account for that via width and that pad size associated with that via all the way down through the stack. I'm also dealing with pour- ing polygons. at's a big one. Matties: Right. Is there anything that you used to preach that you would now say, "Hold it, I've rethought this now that I'm on this end, and let's do it this way?" Thompson: I'll give you an example of that. As a manufacturer, you want things looking nice and clean. If they've got metal at the edge, you're going to try to clip it back. Where do you not clip it back? You don't clip it back on an RF launch, a coplanar waveguide type struc- ture that goes right to the edge of the board and needs to go right to the Z-axis edge of that par- ticular board, and it has to stay that way. Matties: at's interesting. ank you for shar- ing that. With your fabrication background, you're in a really unique position as a PCB designer. Thompson: I am. I can't tell you how many con- versations I've had with the designers here at Monsoon about fabrication-related issues. Fre- quently they'll ask me what the minimum hole size with a particular pad size is. And they're very simple questions, but they're very helpful if you don't know the answer. Matties: You're also a PCB fabrication process engineer turned designer. When you look at a shop from a process engineer point of view, how do you go about benchmarking your process? Thompson: It's all application derived. What's my application? Am I building a cellphone where I'm going to have a very, very thin barri- er of copper much like an mSAP process, some- thing like that where it's a very specific process with very rigid constraints on manufacturing, In fact, it's almost like I blanked out my 25 years at Prototron and 30+ years in PCB man- ufacturing when I started designing. I have to completely rethink the game and deal with a whole lot more variables. Designing is certain- ly not point-to-point connecting the nets. It is so much more than that. It's understanding those nets; it's understanding power. What's the very first thing I look at when I'm looking at a board? I'm looking at power functions. I ask questions about mechanicals. When we do a kickoff call with a customer, the very first thing I say is, "What sort of mechanical consid- erations do you have?" In conjunction with that, there are also pow- er functions. If they have considerations for high-current power, then I've got to make sure that the trace width is going to be large enough. I'll use a site like Saturn and I'll pre- calculate how wide that trace needs to be, and then I'll be able to preplan things. In fact, that's one of the things that I've learned here recent- ly, very intimately: When I'm placing parts, I allow myself enough space to drop vias to be able to do interconnects. Because if it's a six- layer board and I don't have very much room on the outer layers based on the part geome- tries and the available space, I'm going to have Mark Thompson