Issue link: https://iconnect007.uberflip.com/i/1284035
SEPTEMBER 2020 I SMT007 MAGAZINE 67 we did. We talked to our assemblers to ask, "How much pullback do you want from a typ- ical part? If we know it overhangs two milli- meters, how much do you want us to call out? How much do you want us to call that there shouldn't be tabs near that?" Johnson: You're talking about the point where the PCB connects to the rest of the system at the point of interface. This becomes a close conversation with mechanical as well. Kolar: It's a huge conversation with mechani- cal. If you're just looking at it from the panel- ization perspective, you could build just about anything. You just may have to fixture it and do it one up, which adds a lot of cost, time, manual handling, and assembly risk. As you get to large volume production, it may not be practical. That's something for them to think about. We see that people consider, "Do we add a coax cable to a board? Do we do that in a circuit?" That's a trade-off that you may have signal integrity versus manual labor trade-offs. Johnson: Structural and enclosure issues, as well as cabling, etc., mean you're carefully considering the three-dimensional space into which the electronics go. There needs to be a lot of communication around that. What would you suggest as key guidelines? You have already touched on flex in the board, and that can be an unpredictable source of reliability issues. Again, that may or may not show up in tests until it's in the field. What are some of the design guidelines one should follow? Kolar: Dan, you've done many of those kinds of designs. A lot of it is knowing what questions to ask the customer, especially for us because we're not the ones that are specifying those mechanical requirements. We need the cus- tomer to tell us, "We're worried about shock and vibration. This part will be in this environ- ment, and it's really fragile. We're really con- cerned about its reliability." We might push back if they're having us put things really close to the board edge, especially like a BGA or something that we're really worried about being right by the board edge. Dan, what thoughts do you have? Warren: That one's difficult because it's just about years of experience. I don't think about it too much anymore. With a recent design, one of the things that came to mind was the sheer number of connectors and putting too many of them on one edge because you'll end up with a lopsided profile, especially if it's a not lab board. For lab boards, often the customer doesn't really care as long as they can power on one end, and they can access everything easily around the edges. Sometimes, they'll want it all on one edge just for simplicity. But when you explain to them that they're going to have to one-up it and they're not going to be able to panelize it very well because of the pro- file, the weight, and the thermal profile, a lot of times, they'll come back and say they really don't need that there. For production, most customers I've seen lately seem to have a lot better grasp than they used to on the ramifications of loading one side versus the other. For this particular board, we're doing the first cut for production on it. All those overhanging connectors are now gone. That was just for the lab board. For the production board, they've taken a much more conservative approach to it so it can be panel- ized to a larger panel without too much has- sle. It is a lot of working with the mechanical engineer. If you see something that just looks weird, question it. Most of the time, they'll say that is how they want it, but it never hurts to verify. Kolar: And you can also always ask the assem- bler. On those prototype boards, everybody's pretty good about getting DFM from the fab shop, but they're not so good about getting DFA from the assembler. For larger production builds, that's standard practice that you'll go to both. You'll often have the assembler define the panel that will be used. When we're doing something that's a production build level, the fab shop has usually worked with the assem- bler, and they've predefined an exact panel drawing and can provide a DXF. We then input