Issue link: https://iconnect007.uberflip.com/i/1327102
66 DESIGN007 MAGAZINE I JANUARY 2021 together and squashing out some of the resin. All those manufacturing processes change the raw material; you've taken the shelf stock into something else once they've gone through the pressing and curing processes. That's really the big difference compared with an assembly of a product versus the fabrication of product. You're actually doing stuff and changing the materials in the PCB manufacturing process. Holden: The one thing that is important to stackup that we haven't covered, is the fact that we're now going to need more accurate dielectric constant dissipation factors for differ- ent prepregs in different cores and the different materials, and we're going to need it measured out not at one megahertz or 10 megahertz, but at 10 gigahertz and above. I don't know if the material suppliers are going to step up and actually provide the technical information about their materials, because it's not simple. Gaudion: It's not simple, but some of them are. In our material partner program, we deal with 20 or 30 different material suppliers, and some do a really good job of giving data at 1, 10, 20 gigahertz so we've got different frequency data. In that case, some have more frequency data than we can cope with. It's an interest- ing area where some of the guys that want to push the high frequency area, they're actually working very hard on qualifying the material. There's been an improvement on that, I would say, in the last five years. There are some well- known material fabricators, if you ask them, they've got good art applications people who will give you VNA data and TDR data, and actually will have extracted the Df up to 10, 50, or even 70 gigahertz, say for automotive applications. That's a big improvement. We've done some experiments where we bake the boards, and you can see the insertion loss fall off once you bake them; some per- form better than others. In terms of high-speed digital, one thing that's a bit more forgiving is that you're looking at the new high-speed serial comms chipsets. They have adaptive pre-emphasis and equalization, and dynami- cally, the silicon can be a bit more forgiving. At the lowest speeds when you're worried about reflections, the chipsets tend to send a higher load and you have to deal with the reflections. But at the ultra-high speeds, they're using pulse-shaking techniques and things like that, which gives a bit more wiggle room so they can do a bit of dynamic tweaking to minimize the error rates. Shaughnessy: Martyn, do you have any final words of advice for designers dealing with stackup issues? Gaudion: I think it's the same as I always say: Speak to your fabricator and if you're not in close contact with your fabricator, then use one of these value-added brokers who has fabrication people who understand stackup themselves. They are invaluable to work between you and a fabricator. And other than that, establish a direct connection with the fab- ricator to go through any stackup issues. Don't just assume that the material you get off the stock shelf is what's going to end up in the fin- ished product because it gets processed first. That would be my conclusion. Shaughnessy: Good stuff. Thanks for talking with us, Martyn. Gaudion: Thank you all. DESIGN007 For more information, read Martyn Gaudion's I- Connect007 eBooks Secrets of High-Speed PCBs, Part 1 and Part 2.