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Mark Thompson has been in bare board fabrication for over 30 years. He is now laying out printed circuit boards at Monsoon Solutions, a high-tech design bureau near Seattle, Washington. With Mark’s extensive hands-on knowledge of PCB manufacturing, he brings a unique perspective to PCB design.
In this discussion with the I-Connect007 editorial team, Mark shares what’s important from a process engineer’s point of view, and how to stay on top of evaluating and benchmarking your manufacturing process, along with insights from his new role as a designer.
Barry Matties: Mark, now that you’re a PCB designer, after decades in fabrication, what is the most surprising thing you’ve learned?
Mark Thompson: Oh, gosh, there are a number of them. For years I preached things like, “Don’t design at minimums due to etch compensations based on copper weights.” But now I find myself living that because I’m running out of space. I’m designing it 4/4 and then thinking, “Well, gosh, if I do that, now I’m going to have to etch compensate it on half-ounce copper, and it’s going to be four and a half and three and a half, which means they’re going to be relegated to three-eighth ounce copper foils.” And that might be a problem for power functions. It might be a problem for the part itself, and it may not live with that. So that’s one of the things I’ve learned.
I’ve also learned a lot about component placement and footprint design. We’ve talked about footprints before and how critical it is to get your footprints right. And truthfully, the footprints themselves are pretty much the same. What changes is the courtyard, the distance around the component that you’re actually putting on the board, and that distance is dictating how much space you have. There could be a tall part, and you might need to have a human, have a finger go inside there and actually reach in and hand solder something in on a very tall part. So that becomes an issue, and that’s one of the other things that I’ve learned.
In fact, it’s almost like I blanked out my 25 years at Prototron and 30+ years in PCB manufacturing when I started designing. I have to completely rethink the game and deal with a whole lot more variables. Designing is certainly 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 considerations do you have?”
In conjunction with that, there are also power 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 recently, 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 geometries and the available space, I’m going to have 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 pouring polygons. That’s a big one.
Matties: Right. Is there anything that you used to preach out there 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 structure that goes right to the edge of the board and needs to go right to the Z-axis edge of that particular board, and it has to stay that way.
Matties: That’s interesting. Thank you for sharing 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 conversations I’ve had with the designers here at Monsoon about fabrication-related issues. Frequently 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.
To read this entire interview, which appeared in the April 2021 issue of PCB007 Magazine, click here.