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Design007-Nov2024

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NOVEMBER 2024 I DESIGN007 MAGAZINE 57 not le in the solution too long, the copper we want will be relatively unscathed. You may wonder why we would want to remove the tin. Aer all, it is a conductor. But tin has a propensity in electronics to grow tin whiskers. ese are dendritic growths of tin that, over time, can cause failures. So, leaving tin on the traces can result in a sleeping dragon that will wake up and create issues with board function down the line. At this point, we have a fully functioning circuit board, one that could move on to pre- assembly, but also one we have to protect. Oxygen and copper are not the best of friends, so there is still more to do—namely the appli- cation of solder mask and silk screen. Keep an eye out for my next article on that subject. But if you can't wait, listen to the solder mask and legend episode of On the Line with… To hear our in-depth conversation about SES, please listen to episode 9. DESIGN007 Matt Stevenson is vice president and general manager of ASC Sunstone Circuits. To read past columns, click here. Read Matt's book, The Printed Circuit Design- er's Guide to … Designing for Real- ity or listen to his podcast here. Most DRCs do not look for large gaps between traces, instead focusing on the traces that are too close together. Etching has both complex and artistic com- ponents of the manufacturing process. ere are numerous variables that can impact the process, including how fast the panels move through the solution, whether pressure washing is required, the chemical makeup of the solution, and its temperature. When run- ning a panel through the process, especially when there are elements like impedance traces, the operator must be able 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 mush- rooming, and the subtleties of etching, you can see how design and etching interact. Designers do not need to be experts on the etching pro- cess, but they should understand how it will respond to a design. It makes sense to design defensively to help avoid potential pitfalls. I recommend visiting your fabricator to observe the SES process and get a sense of how manu- facturers navigate the inherent variables. Even though the process can be complex, SES goes off without a hitch most of the time. But there can be outliers. Sometimes they are design-related, and sometimes it's a function of the manufacturing process. If we complete the process and it does not produce the desired outcome, all is not lost. If we've under-etched, we can take measures to correct the issue and recover the panels. On the other side of the spectrum, over-etching usually leaves us with scrap. The Second Strip in SES Now it's time for the second strip of the SES process: e panels go through another chem- ical stripping process, this time with a nitric acid-based solution that aggressively attacks and dissolves the tin. Copper does react a lit- tle with nitric acid, but as long as the board is " Etching has both complex and artistic components of the manufacturing process. "

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