Issue link: https://iconnect007.uberflip.com/i/1327102
JANUARY 2021 I DESIGN007 MAGAZINE 23 (CAF) failures as the finer glasses. So we often use 1080 and 2116. The Dk of glass is higher than the Dk of the resin, and there are thick knuckles where the fiberglass thread bundles cross each other in the weave. High speed signals traveling across the weave see the Dk alternating between knuckles and resin between the threads, so the knuckles act as speed bumps. When appli- cable, we select spread weave glass prepregs to minimize this effect (Figure 8). One of the things that happens is that, if we have two of these prepreg weaves stacked on top of each other, as the resin melts, these will start what we call nesting, where they slip over so the knuckles are adjacent to each other. They press in closer to each other, reducing the finished thickness. That's just one of the interesting features about predict- ability for the dielectric thicknesses; we have to pay attention to those things that the simu- lators take into account based on actual mea- surements. Of course, it's not always possible, depend- ing on how difficult a design is, but we do our best to try to maintain best practices to reduce manufacturing processes, which increases yield for our customers, and they can get their boards in a shorter lead time (Figure 9). Ellis: To Richard's point, customers will give us our competitors' stackups that were prototyped here in North America, and then we need to move them to Asia. We'll see things like three different prepregs in this one dielectric open- ing, because somebody in North America, for quick-turn prototypes, is just using whatever material they have on hand to meet customer requirements. But we try to plan just the next step better, so that we're planning using com- mon materials that we know will also transfer to Asia without having to add additional part numbers to a stackup. Dang: Because we do so many boards over in China, there are some classes of material that we actually have more laminates available and qualified for over there than we have here. Something that might be available in China may not be stocked by local distributors, so we often have to request alternate, equivalent materials to support quick-turn prototypes here in North America. We do a lot of pre-analysis on things like that before recommending a stackup. Figure 9: Glass "knuckle" shown in fiber weave effect close-up.