Issue link: https://iconnect007.uberflip.com/i/1348195
MARCH 2021 I DESIGN007 MAGAZINE 15 of time, and they'll give the information for the trace and space, as well as the stackup. Now, bear in mind, if the fab- ricator comes back and says, "Hey, I'm going to have to change your copper weight," that's a red flag. e designer has to say, "Wait a minute. Now, if you're going to reduce my two-ounce to a half-ounce, you're digging into my cur- rent-carrying capacity of the design." at ends up being very problematic. Netlists are another area where things can happen. Parts will change, and you'll have to re-annotate the design. You've got to bring in the new parts and update the schematics, if we don't get an updated sche- matic. If they just send us something on the proverbial paper napkin saying, "Here's what we're going to do, and we need to update these particular parts," and the designer doesn't re-output the IPC netlist or properly call out intentional shorts, it's going to get a stop at the fabricator. Matties: So, what you're talking about, again, is just sloppy workmanship? Thompson: at's generally the whole thing, yes. We always talk about the four DFs: DFM, DFA, DFT, and, last, DFX, design for excel- lence. If you're always keeping those in mind, you're always going to have a decent design, but the design is not everything. It's like Jen said, if you don't have a good kickoff meet- ing, you may get sent down an avenue that you don't want to be going down. Matties: But the right questions are based on years of experience, I would think. Kolar: Yes, they really are, and I think there's the nature of the design process. So, one of the most common sources of error for us is that, from the engineer's perspective, they're working the sche- matic. ey say, "I'm just going to change the value of this part." e problem is they don't actually update the full symbol. ey may update just one thing in the sche- matic such as the value, but they don't actually update the footprint name. ey don't update the part number. en, that doesn't end up getting propagated into the board where we rely on the footprint name. en we're guessing, and then they're having to go back to the PCB designer and make sure, "Hey, are you actually using the right package, and, by the way, is that package available?" Or you end up cobbling together a BOM manually based on changes in the schematic, and those aren't properly making it through the design. Matties: Have you taken a job, looked at the time that you spend start to finish and found that you spend, say, 80% of your time doing detective work and planning, and about 10% of the time designing the layout; then maybe 10% of the time wondering what the hell just happened? Kolar: I have not done the actual analysis on that. But it really varies by customer and by type of project, and some projects are very complicated, and you're going to necessar- ily need to do a fair amount of upfront plan- ning. Others are straightforward, and some things can be figured out along the way, but I would definitely say, without fail, if you start doing placement, especially routing before the schematic is pretty locked, or at least before the schematic's fairly mature and mechani- cal is locked, then you're guaranteed to have trouble. Mark Thompson