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October 2017 • The PCB Magazine 27 2%. We can talk about the chal- lenges with keeping those toler- ances that tight, like how the di- electric thickness and the etching characteristics in plating are real- ly the key parts of the fabrication process." Yogen Patel added, "We know that most manufacturers control impedance close to 10% most of the time. To control to under 5%, they charge so much extra mon- ey. There are three main differ- ences between the conventional process as we call it and our pro- cess. First, our press is resistive- heating so the outer layer copper becomes a heater, and that's why we don't have to press at 350 PSI—we use 230 PSI—so the di- electric is very consistent across the whole pan- el. And then we drill, clean the holes, and pan- el plate copper over the whole board, which is the second difference. This makes for very con- sistent coper thickness over the whole panel and in the hole as well. And the third major difference is our positive-acting electrodeposit- ed photoresist, where the areas exposed to UV light develop off. Then we etch in cupric chlo- ride. In this way, we get very uniform conduc- tors, and that's how we get controlled imped- ance to close to 2%." Continuing, Sunny added, "The main issue on the board fabrication side is consistent lam- ination, making sure that the board shop has a good, consistent way of measuring at differ- ent weaves and resin contents. If a PCB manu- facturer is pulse plating, the consistency will be pretty good, but that's generally one of the ar- eas where most will have difficulty—with cop- per plating and copper etching as well. So, peo- ple with more direct imaging and pulse plating technology, or our way of doing things, will get you finer impedance characteristics. But a lot of it comes back to the engineers themselves, dis- cussing stackups and what they want in the end with their PCB supplier—having that thorough conversation with the engineer on the board side to ensure that what they want is what they're going to get with the stackup. Those are the two main things: The process has to be con- trolled and consistent, and there has to be a clear communication with the design engineers." Mark Thompson had more to say about interaction with the design engineer: "I agree with you guys re- garding the negotiation process. If a customer approaches you be- fore they have a design ready, and their engineer has just told them, 'I have 8-, 10-, and 12-lay- er boards, and I need to get a slew of impedances from you, and a dielectric stackup with ef- fective dielectric constants for each of the subsections,' there are several questions that must be asked at that time. A lot of us come up with an impedance checklist that asks all those basics, such as: What's the mate- rial type? What's the copper weight? Where will the impedances reside? What are the thresh- olds in tolerances (e.g., 100 ohms, 120 ohms, 75 ohms)? And what are the intolerances asso- ciated with it (2%, 5%, 10%, etc.)? At a stage where there has been no layout done and no ac- tual artwork exists, it's difficult from a fabrica- tion side to crawl into the customer's head and picture what it is he's trying to accomplish." "But if you're dealing with a fabricator that has not dealt with coplanar waveguides prop- erly, they'll come back and say, 'Cosmetically, we want to route this thing out and don't want to see any burring, so we're going to hack back that launch pipe three or four thousandths,' which is doing a huge detriment to you as the end user. Obviously, in a coplanar waveguide, the launch needs to reside right at the part edge, and the end-user is well aware that the exposed copper there is really what you want to see. You really want to see it actually meeting with the Z-axis and you don't want to trim it back in any way." Thompson then shared an experience in which a material manufacturer shipped in very consistent material up to a point and then sud- denly all the impedances were off; it was linear and all in one direction. After cross-sectioning, they discovered that the various laminate ma- terials were all ~1 mil thicker than previously EXPERTS DISCUSSION: SIGNAL INTEGRITY AND IMPEDANCE CONTROL Sunny Patel