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44 PCB007 MAGAZINE I DECEMBER 2022 Considerable progress has been made in the field of consumer and industrial electronics; today's devices are reasonably priced and effi- cient, yet small and portable. However, these advances create challenges for the manufactur- ers of the components necessary to produce these smaller, more portable devices. PCB designers, for example, must attempt to squeeze increasingly more amounts of informa- tion onto a smaller board. In practical terms, this means that circuits are becoming more com- pact; circuit lines and the spaces between them are becoming finer. As the distances between these lines decrease, copper overplating is more likely to have a deleterious effect on photoresist stripping. e number of unstripped residues tends to increase at the very small dis- tances between conductor lines on these new circuits. Under these conditions, conventional photoresist strippers are ineffective and the amount of photoresist remaining on the substrate aer stripping increases. Complete photoresist strip- ping is needed to avoid shorts on costly fine-line PCBs. In addition, some fabri- cators lack proper control of the plating process. Coupled with thinner resist film, this leads to overplating—and overplated copper and/or tin will entrap resist under the heavily plated features. Overplating and Entrapped Resist An example of overplating is shown in Figure 1. e overplated condition, cou- Revisiting the Art and Science of Photoresist Stripping pled with tight spacing, will make clean resist stripping a major concern. Another example of overplating and entrapped resist is shown is Figure 2. Of course, part of the issue here is tighter line-and-space requirements. is issue appears primarily in higher-density circuit designs driven by min- iaturization. ese difficult circuit geometries can result in overplating the copper, which increases the instances of resist residues. is in turn leads to various defects. If the photo- resist stripping is incomplete, the photoresist le behind will protect the underlying copper from etch. is results in a circuitry "short," where copper bridges an area between two cir- cuit lines that were meant to be isolated from Trouble in Your Tank by Michael Carano, AVERATEK Figure 1: Plated copper "mushroomed" up over the resist.

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