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56 DESIGN007 MAGAZINE I NOVEMBER 2024 top of the resist. If this over-plating happens, it creates a mushroom on top of the photoresist. is can reduce the spacing between conduc- tors and create a much smaller gap to remove photoresist from the channels, which could cause electrical shorts. Both issues can make it harder to get the photoresist out from underneath the mush- room cap, and makes it more likely that some of the photoresist hangs onto the panel, poten- tially creating issues with board functionality down the road. ough it is mostly on the fabricator to properly plate the panel and avoid over-plat- ing, designers have available measures to help prevent this type of issue. You should consider whether you are leaving enough space between the traces, how much copper is required inside the plated through-holes, and copper thick- ness on the surface. More copper increases the chances of over-plating and the dreaded mushroom cap. Also, as a general rule of man- ufacturing, traces embedded through a ground plane are more difficult to strip effectively. What if photoresist remains on the board after the stripping process? If resist remains on the board, it becomes more difficult to etch out the copper under- neath, potentially creating shorts or signal issues. If we do not remove all the intended copper, there can be dire impacts to board performance, so it is important to inspect the panels aer they exit the solution. If any sig- nificant amount of photoresist remains, it will stand out. e operator can perform the pro- cess again at an accelerated speed or pressure wash away the unwanted resist. Etching: Removing the Exposed Copper Now it's time to remove the copper under the photoresist. is will define the outer layer features we want to keep. Etching is yet another chemical process and one that uses a lot of spray pressures and continuous flooding of chemical solution over the panels. During this phase, we etch the copper down from the foil height to the laminate. As we etch down, we are also etching underneath the tin laterally toward the traces. e etching chemical solution is highly reac- tive to the copper but mostly benign to other metals on the panel, so we don't have to worry about damaging them during the process. Tin protects the areas where we do not want to have the copper removed. Keep Your Traces Protected Designers can take measures to protect their most important traces from over-plating, especially those with impedance or clocking requirements. Place other copper features around them to help distribute and deposit electrolytic copper more evenly. If you run a critical impedance trace out by itself, essen- tially in the middle of nowhere on the board, the thickness and geometry of the trace become less predictable and carry more potential for that mushrooming effect discussed earlier. Designers should not rely on design rule checks (DRC) to call out this potential issue. " More copper increases the chances of over-plating and the dreaded mushroom cap. "