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JULY 2022 I PCB007 MAGAZINE 69 tion. e most common cause of chloride loss involves people utilizing only ammonia instead of the typical ammonia-ammonium chloride blend in the "replenish rinse" section. Using only ammonia effectively dilutes the chlo- ride content and causes the concentration to fall out of the desired range. Chloride content should remain within the range of 160-165 g/L. If your chloride levels drop too much, that will result in the formation of light-blue crystals. To bring chloride levels back up, you can sim- ply dissolve ammonium chloride crystals into your etchant. Optimized Air Flow To have a steady etch process, it is impor- tant to maintain the regeneration reaction. Maintaining a steady regeneration reaction relies on having a sufficient supply of the lim- iting reagent, oxygen, to keep up with pro- duction. Since oxygen must be pulled into the etch chamber through the ventilation system, some additional challenges come into play. is is because the air flow needs to be opti- mized. With too little air flow, you may not have enough oxygen to sustain your etch rate. Too much air flow will cause your ammonia to leave the etchant and cause the pH to drop— potentially leading to sludge-out. In cases of large-scale PCB production, sometimes ventilation is not enough. In these cases, additional air sparging in the etcher sump is needed. Air sparging is a method to feed oxygen into an etchant by bubbling com- pressed air through the bottom of the etch bath. It has some of the same challenges as the normal ventilation approach. Too much sparg- ing can drive excessive ammonia out of the etchant, but too little may not be sufficient to keep production steady. Finding the optimal air flow can be a trial-and-error process, but with a careful watch on pH and etch quality, you can find the optimal amount. Conclusion Chemistry control for alkaline etchant has always been a challenge. Most of that difficulty comes from not having the proper understand- ing of how alkaline etchant works, and what it needs to stay stable. Now that we have cov- ered the main troublesome areas, you are bet- ter equipped to maintain and get the most out of your alkaline etchant. PCB007 References 1. "Process Guidelines for Alkaline Etching," Chemcut.net, October 2001. Christopher Bonsell is a chemical process engineer at Chemcut. To read past columns, click here.