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98 PCB007 MAGAZINE I JANUARY 2021 Introduction While troubleshooting everyday processing issues, final etching touches on many down- stream processes. These include surface prep- aration, imaging, and copper surface quality. There are concerns with the etching process it- self and how process issues and operating pa- rameters impact the circuit formation quali- ty. In this month's edition of "Trouble in Your Tank," the subject of etch-outs, undercut, and line width reduction will be presented. Overview-Etch Factor and Undercut Electrical engineers and circuit board de- signers are not thrilled if, as fabricators, you don't give them the line width and spacing (as well as the circuit geometry) that was initial- ly intended. Issues such as undercut and over- etching will detract from the circuit trace quality. However, the constant battle is mini- mizing undercut while ensuring the unwanted A Process Engineer's Guide to Etching Defects, Part 3 copper is completely removed during the etch- ing process. Let's discuss the difference between the un- dercut and etch factor. The undercut is the dif- ference between the width of the developed re- sist line (it could also be the design line width) and the final width of the etched circuit across the top. Expressed another way, undercut is the amount of copper that was removed laterally as the etchant did its job of eliminating unwanted copper as it etches downward (Figure 1). The etch factor is the ratio between the dif- ference in the widths of the copper trace at the most expansive and narrowest parts of the fea- ture after etching is completed and the metal's thickness (copper) etched. The etch factor can be easily expressed as a ratio by dividing the copper's thickness etched downward by the amount of copper removed laterally. Please note that Figure 1 describes conditions related to print-and-etch (typically known as Trouble in Your Tank by Michael Carano, RBP CHEMICAL TECHNOLOGY Figure 1: Illustration of undercut (AW = adjusted width in artwork if allowed).

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