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84 PCB007 MAGAZINE I MARCH 2023 As I've mentioned in previous columns, etching can be the most complex process in the wet processing stages because there are many factors that contribute to your etch rate. Without keeping these contributing factors steady, your etch rate will vary and, therefore, so will your product quality. If you are looking to implement an automated approach to main- taining your etch chemistry, here are the basic concepts you will need to understand. The Structure of Control Most, if not all, process control systems rely on a simple feedback loop. rough this loop, we get a clear approach to process con- trol. In Figure 1, you can see that we initially go through a monitoring stage. At this stage, we look to see if all the desired parameters Chemical Control for Wet Processes meet specifications. If they do, then the pro- cess can continue normally. However, if some- thing does not meet specifications, it will ini- tiate a process to make the necessary changes. With this change being identified, it goes into an "adjusting" process. Aer adjustments, the parameter is monitored again to ensure that it meets requirements. is process will repeat until the desired outcome is obtained. is is only a rudimentary version of the feedback loop, meaning that this can become more com- plex given a certain process. What the process will look like may differ, but conceptually, the processes should remain similar. Implementing Control Now that we have the base structure of pro- cess control, how can we apply it to our etch- The Chemical Connection by Christopher Bonsell, CHEMCUT Figure 1: Simple process control feedback loop.

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