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62 I-CONNECT007 MAGAZINE I JUNE 2026 etch rate downward, but also the fastest sideways etch. Running at an sg of 36°Be gives a slower etch rate but a much-reduced sideways etch. Recommended free acid levels are between 1N and 3N where N is the normality of the acid in the cupric chloride solution and describes the concen- tration of the acid. A 1N solution is 36.5 gpl HCL. The free acid level is important for two reasons. One is to keep the cuprous chloride byproduct of the etch reaction in solution so an accurate reading of the ORP for regeneration can be made. The second reason is that the free acid prevents the formation of oxides on the surface of the copper during etching. The etch rate would be halved without the free acid. Within the recommended range of free acid concentration, increasing the concentration gives faster downward etch rates but also faster sideways etch rates. The free acid level is important, and many smaller shops I have visited don't have the means to monitor it; they just estimate. The equipment to measure it is not expensive, and it doesn't take a trained chemist to use it. I always recommend that a corner somewhere in the shop be cleared and a bench set up to measure free acid at least once a shift. Running at the lowest recommended specific gravity and highest recommended free acid level will give the highest etch rate, at around 0.002"/ min. (50 µm) but a sideways-to-downwards etch ratio of about 1 to 1. Even in the old days this ratio was unacceptable. Running at the highest specific gravity and lowest free acid concentration drops the etch rate to about 0.001"/min (25 µm), but the sideways-to-downward etch ratio improves to 1 to 4. That etch rate was too slow for the old days, so a compromise was reached and the recommended specific gravity and free acid levels were stated as 32°Be and between 1N and 2N. This gave an etch rate of around 0.0015"/min. (38 µm) at the higher acid concentration and an etch ratio of around 1 to 3 sideways to downward. The 32°Be, 1N to 2N recommendation for cupric chloride operating parameters is what is recom- mended in most etching guides, but still favors etch speed over etch quality. I feel that nowadays etch quality is at least as important, if not more so, than etch speed, so I run my cupric etcher at 36°Be specific gravity and 0.8N free acid and accept the lower etch rate in favor of the improved etch quality. This is what I recommend to those who ask. Higher specific gravities (40°Be is close to the saturation point) slow the etch rate without improving the etch ratio, while acid levels below 0.8N also slow the etch rate without improving etch ratios very much and, in addition, become harder to control. In answer to the next anticipated question: Yes, alkaline etchant is reputed to have better etch ratios than cupric, and this is true if the etchant param- eters are kept in the middle of the recommended range for pH, chloride concentration, and specific gravity. One can expect etch ratios of 1 to 4 and etch rates of around 0.0024"/min. (60 µm). By running at the lower end of the ranges for pH and chlorides and higher end for specific gravity you can get etch ratios of about 1 to 5 and a slower etch speed of 0.0018"/min (45 µu). It should be noted, however, that etchant control gets very tricky at these settings. At the end of the day, there is no magic chem- istry that eliminates the tradeoff between etch speed and etch quality. The old mindset of "faster is better" worked well when circuit traces were large and throughput was everything. Today, with much finer features and tighter tolerances, quality deserves equal consideration. Careful control of specific gravity and free acid levels is not simply housekeeping; it directly affects the balance between productivity and circuit quality. Perhaps that is the real lesson from revisiting these older chemistries. Even with all the advances in PCB technology, the fundamentals still matter, probably more now than ever. I-CONNECT007 Don Ball is a process engineer at Chemcut. To read past columns or contact Ball, click here. T H E C H E M I CA L C O N N ECT I O N