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PCB007-Aug2022

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52 PCB007 MAGAZINE I AUGUST 2022 One of the biggest obstacles that PCB manu- facturers face is etch factor. In the past, I have touched on etch factor in my columns, but to refresh, etch factor is the ratio of downward etch to sideways etch. Etch factor poses chal- lenges to PCB fabricators because it limits PCB design. It can determine how fine a line you can etch, and it can even affect how close together you can have features. For instance, if you wanted to use cupric chloride (etch fac- tor of 3:1) to etch a fine line (3 mils or less), you might not be capable of doing that on pan- els with thicker copper layers. If you try to do that, you will likely receive inconsistent results throughout your panels. Inconsis- tencies will arise because there is a point where the sideways etch will affect metal underneath the photoresist and etch it away. is phenomenon, where metal is etched from underneath the photoresist, is known as "undercut" (Figure 1). In some cases, the undercut for fine lines is so severe that the photoresist will li off the copper surface, thus exposing the metal to be directly attacked in the etcher and creating the possibility for that feature to be completely etched away. What Undercut and Etch Factor Have in Common roughout the industry, etch factor and undercut are terms typically used inter- changeably, despite their differing definitions. is is likely because they are both the result of the one thing that PCB fabricators want to get rid of—sideways etch. Without side- ways etch, etch factor and undercut are the- oretically nonexistent. If you could remove sideways etching from the etch process, the translation between PCB design and PCB fabrication would have no limitations. You could etch down as far down as you would need without increasing the spaces between features and without decreasing the defini- tion and accuracy of features. Unfortunately, sideways etch is not so easily controlled, and it is oen accepted to be a limitation to etching. How Sideways Etch Has Been Challenged Although sideways etch has been accepted as a normal obstacle, there have been efforts to combat it. Perhaps the closest copper etch- The Etch Factor The Chemical Connection by Christopher Bonsell, CHEMCUT Figure 1: Diagram of undercutting on a PCB.

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