Issue link: https://iconnect007.uberflip.com/i/1529411
NOVEMBER 2024 I PCB007 MAGAZINE 17 but he has never had to realize it in physical form. An example is when you see asymmetri- cal mechanical tolerances. Tolerance is often a significant challenge between design and fabrication and something that can add cost to a printed circuit board. Mechanical tolerance issues are the most common challenge about which layout peo- ple are unaware. I see it frequently, espe- cially in the automotive industry where they are used to working with different mate- rial types, not just fiberglass epoxy laminate. If you want a tolerance that is smaller than ±0.005" or 0.006", you will run into the issue of consistently achieving your tolerance on large volumes and you are locking out manu- facturers. e minimum anyone might guar- antee would be ±0.003", which is a cost-driv- ing minimum. But I see designers put it in. Do they really design less than ±0.003" tolerance? You have no idea what kind of discussions I've had over the past eight years. I believe some of them come from completely different areas, like metal engineering. ey are used to differ- Figure 2: Cross-section of a stackup for comparison. ent manufacturing, maybe to even more pre- cise machines. It is something I run into again and again. Of course, many designers don't even know where their board will be fabricated, and every manufacturer has slightly different tolerances. When the data package gets into the fabricator's front-end CAM department, they do their adjust- ments, put the job on hold, and ask questions. Exactly. If the customer is not prepared to relax their tolerance to a minimum ±0.005", they drive up their costs considerably. What about providing a stackup vs. allowing the fabricator to create the stackup and simply build to the requirements? is is a case-by-case situation. For example, if a customer does high voltage on their PCB, they need certain minimum distances between lay- ers. It is non-negotiable. If you just need imped- ances and nothing else, then leave it to the fab- ricator. But if I see a 12-layer PCB, I expect the customer to provide a stackup. If you're design- ing a PCB with 12 layers, you need those 12 lay- ers for a reason. Usually, it will entail complex routing, maybe microvia technology. Of course, manufacturers are always happy to provide you with stackup suggestions. We also need a com- plete drill map, for exam- ple, with back drills or back road blind and buried vias detailing what type of drilling is required and from which layer to which layer. I had a customer who was trying to do shield- ing. He had a four-layer PCB, and the second layer had a very high impedance of 75 ohms. He wanted shielding on layer one so the signal inside wouldn't prop-