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66 PCB007 MAGAZINE I JULY 2022 In my last column," Etchants of the Indus- try: Cupric vs. Alkaline," I compared cupric chloride and alkaline cupric chloride. In that column, I mentioned that alkaline etchant is the most used etchant for PCB fabrication. It is used because it provides a high etch rate, improved etch factor, and compatibility with metallic resists. Although it has some great benefits, it has the drawback of being difficult to control. e etching chemistry requires a delicate balance, and the parameters it needs to stay within are relatively tight. Not only are the margins for error small but falling outside these parameters may have consequences. If you are not careful enough, you could hit the point of "sludge-out"—a point where your etchant will undergo an irreversible precip- itation reaction. Sludge-out can dramati- cally reduce the effectiveness of your etchant, and it can cause severe damage to your products and etching equipment. To pre- vent sludge-out from happening to you, here is a short guide to understanding the chemistry control for alkaline etchant. Understand the Chemistry Utilizing alkaline etchant requires a thor- ough understanding of the chemistry taking place inside the etching machine. Inside the etch chamber, two reactions are the focus: the etch reaction and the regeneration reaction. ose reactions are shown in Table 1. How the reactants are incorporated into the reactions are shown in Figure 1. Don't Sludge-Out: A Guide for Alkaline Etching The Chemical Connection by Christopher Bonsell, CHEMCUT Table 1: Etch and regeneration reactions for alkaline cupric chloride. Figure 1: Process flow for an average alkaline etch machine. Sources of ammonia (NH 3 ) and oxygen (O 2 ) can vary, but the methods shown in the diagram are the most common.