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116 I-CONNECT007 MAGAZINE I FEBRUARY 2026 After the tin plating, the circuit image on our panel is a silvery matte color. Areas without copper are blueish or purple in color, including channels on the board. Those blueish-purple areas mark photoresist areas. The next step in production begins with removing the photoresist. Stripping Away Photo-resist Photoresist is acidic in nature, so we submerge the panels and spray them with an alkaline solution to dissolve it. It risks the overall finished product if the stripping process is not completed correctly. During pattern plating, it was possible to plate over the top of the resist. Over-plating creates a mush- room cap on the photoresist, reducing the spacing between conductors and creating a much smaller gap to remove photoresist from the channels. This can cause electrical shorts. Both issues can make it harder to get the photo- resist out from underneath the mushroom cap, making it more likely that some of the photore- sist hangs onto the panel and potentially creates issues with board functionality down the road. If resist remains on the board, it's more diffi- cult to etch out the copper underneath, poten- tially creating shorts or signal issues. Not remov- ing all the copper we intended can lead to dire impacts to board performance, so it's important to inspect the panels after they exit the solution. Any remaining photoresist will stand out. The operator can perform the process again at an accelerated speed or pressure to wash away the unwanted resist. Etching: Removing the Exposed Copper Removing the copper under the photoresist will define the outer layer features we want to keep. Etching is yet another chemical process that uses a lot of spray pressure and continuous flood- ing 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 laterally underneath the tin toward the traces. The etching chemical solution is highly reactive to the copper but mostly benign to other metals on the panel, so we don't have to worry about damag- ing them during the process. Tin protects areas where we don't want copper removed. Etching is one of the most complex and artistic components of the manufacturing process, with many variables that can impact the process, includ- ing how fast the panels move through the solu- tion, whether pressure washing was required, the chemical makeup of the solution, and its tempera- ture. When running a panel through the process, especially with elements like impedance traces, the operator needs to accurately evaluate what comes out of the process. It takes experience and know-how to recognize when the etching process has been properly completed. With isolated traces, potential for mushroom- ing, and the subtleties of etching, you can see how design and etching interact. Designers need not be experts on the etching process, but they should understand how it will respond to a design. It makes sense to design defensively and avoid potential pitfalls. I recommend visiting your fabri- cator to observe the SES process to see how they navigate the inherent variables. Even though the process can be complex, SES typically goes off smoothly, but there are outli- ers. They can be design-related or a function of the manufacturing process. All is not lost if you've C O N N ECT T H E D OT S Keep Your Traces Protected Designers can take measures to protect their most important traces from over-plating, especially those with impedance or clocking require- ments. 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, essentially in the middle of nowhere on the board, the thickness and geometry of the trace becomes less predictable and carries more potential for the mushrooming effect discussed earlier.