Issue link: https://iconnect007.uberflip.com/i/1327102
20 DESIGN007 MAGAZINE I JANUARY 2021 transferring between sites, and we request cus- tomer approval for this (Figure 5). Ellis: Figure 6 shows what Richard is talk- ing about. With laser microvias, stackups get really complicated, quickly. This one has a L1–12 through-hole. It's got another mechanical through-hole creating a buried via in the middle of the stackup in the first L4–L8 sublamination, and then it also has all these microvias, L1–4 offset (>0.5mm BGAs) and L9–12 stacked (<0.5mm BGAs). Basically, the way we build this board is through the four lamination cycles starting with the L4–L8 sublam which entails laminat- ing the layers, drilling, plating, and etching. The next sublam repeats the processes, adding layers above and below the first sublam, lami- nating, drilling, plating, and etching, and then adding more layers above and below again, and so on. Every lamination cycle adds mul- tiple processes, resulting in additional costs. So a good designer works to reduce lamination cycles. These are the kind of examples that we see, where customers have a requirement, they've planned for what they think works, but then process and geometry really don't work when we put all the different features together, and we request they adjust their design. Shaughnessy: Right. It seems like they would want to have some communication for some- thing like this. Ellis: Yes, to plan ahead. Another common one that I'm actually working on right now is that we have a board that was designed per IPC-class 2, but somebody slapped an IPC Class 3 fab note on it and then expects us to achieve it. But we can't, because Class 3 has larger design rules—namely, larger annular rings that take up space in a fixed design and thicker copper plating on the hole walls and surface that affects outer layer etching—than Class 2. It's more a fabrication than a stackup issue, but that is a common problem that we've seen. Figure 5: Aspect ratio measurements for typical through-hole vias and microvias.