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DECEMBER 2022 I PCB007 MAGAZINE 75 540 to 560 mV. It is best to avoid overshoot- ing ORP past the value of 600 mV because typ- ically there are not enough Cu +1 ions in a solu- tion to absorb all the chlorine used in regen- eration. is can lead to a chance of chlorine gas forming and being released into the atmo- sphere, which can be dangerous—so it is best to avoid high ORP values. If you are too low on ORP, you need to add more regenerating components so the regeneration reaction can replenish the Cu +2 ions. If your ORP is too high, you can remedy this by simply running panels. However, if the ORP is too high and causes an unpredictably high etch rate, it is best to find some way to introduce copper into your etch bath to bring the ORP back down. Bringing the ORP down can mean running a piece of scrap copper through the machine or dissolv- ing copper powder into the solution. If you are finishing a shi and you notice the ORP is running low, you can leave the etchant to rest during a break or overnight because as it sits, it will react with the oxygen and increase the ORP again. Free Hydrochloric Acid Content Another critical factor to monitor in your CuCl 2 is free hydrochloric acid (HCl) content. For CuCl 2 to etch efficiently, there needs to be a detectable amount of excess acid. is is because CuCl 2 is relatively insoluble in water on its own, and it needs to be in a highly acidic solution to prevent the CuCl 2 solids from pre- cipitating out of the solution. e HCl also helps remove the copper oxide layer that is present on copper panels. CuCl 2 is not efficient at removing this, but with HCl present, that layer gets removed, allowing for easier access to the copper surface. Without HCl, the etch rate of CuCl 2 is cut in half. Typically, the higher the HCl levels, the faster the etch rate. Although higher HCl content increases etch rate, it comes with the drawback of increasing undercut. Since there are some drawbacks to higher HCl contents, it is recom- mended to keep the free acid in a range of 1 to 2 N HCl. One major consideration with free acid content is the difficulty of keeping a close eye on this parameter. is is because there are a lot of technicalities in trying to measure it. e only established, reliable method of doing this is through routine titrations. Conclusion Once you know the basics of maintaining CuCl 2, and both the spray pressures and total etch time you need to meet your PCB speci- fications, running your etching process should be fairly easy. To summarize the parameters discussed in this column, see Table 1 for the recommended parameter ranges for cupric etchant. PCB007 Table 1: Summary of CuCl 2 parameters and their recommended values References 1. "Process Guidelines for Cupric Chloride Etch- ing," Chemcut.net Christopher Bonsell is a chemical process engineer at Chemcut. To read past columns, click here. Figure 3: Nernst equation for ORP in CuCl 2 etchant. E o is the standard potential of your electrode, R is the gas law constant, T is temperature (in Kelvin), n is number of electrons transferred per molecule.

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