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60 I-CONNECT007 MAGAZINE I JUNE 2026 questions that some of us lofty personages with decades of experience forget to put in our tutorials. However, when I checked with our field service team and others in contact with customers, I found that the most frequently asked questions have been about cupric chloride etch chemistries. Therefore, I will cover the very basics of cupric chloride etching; not so much the technical facts but why we give the advice we do. The Old Need for Speed Back in the almost prehistoric days when I started in the industry, the mantra was speed, speed, speed. Get as many square feet through etching per hour as possible, because all those TVs, refrigerators, stereos, newfangled micro-computers, etc., needed circuit boards. State-of-the-art line and width spac- ings for circuit traces were 0.010" (10 mil or 254 µm) for densely packed circuits, and most of the produc- tion circuit designs were most likely double this. A sophisticated PCB would be double-sided with plat- ed through-holes on 0.064" (1.65 mm) thick epoxy fiberglass laminate. There were no such things as multilayers, other than a gleam in a PCB engineer's eye. Components (resistors, capacitors, etc.) were much larger than today (you could actually see a resistor with the naked eye), so a circuit board had to be quite large to fit all the components. Hence, the need to cram as many panels per hour as possible through the etcher to meet the demand for boards. Nowadays, of course, with multilayer boards, chip-on-board, and many other miniaturization technologies, modern boards can be much, much smaller in area to accomplish the same tasks as early boards. The need for raw etch speed has diminished since boards are now much smaller and multi-func- tional, and we don't need as many square feet per month to meet demand, even though extra square feet are needed for multilayer production. I have to admit, however, that I still hesitate to suggest or try new things that might improve etch quality at the expense of etch speed, so ingrained is the need for speed from my early career. The Quality Factor A major concern today is etch quality, which, in the case of using cupric chloride to etch copper circuit lines, means the rate of sideways etch to the rate of downward etch (etch factor) as the line is formed, or how straight we can make the sides of the cir- cuit trace. Ideally, the sides of the trace would be straight up and down, but in reality, they are slant- ed, with the trace being wider at the bottom. When the space between traces is 0.020" (500 µm), the shape of the sidewall doesn't matter, but when the space is 0.002" (50 µm) or less, as they tend to be these days, the shape of the sidewall is extremely important to avoid shorts and crosstalk. The main contributors to both etch speed and etch quality are the concentration of cupric chloride in the solution (specific gravity) and the amount of free acid (hydrochloric acid or HCl) in the mix. The best range of concentrations for cupric chlo- ride etching copper is between 275 grams per liter (specific gravity at 1.2501 or 29°Be) and 370 gpl (sg at 1.3302 or 36°Be). We use specific gravity to define the concentration of cupric chloride as it is easier to measure and control. Degrees Beaume is an old French unit used since the 1600s to measure specific gravity, and it has somehow survived to describe the specific gravity of cupric chloride, ferric chloride, and alkaline etchants. So, the best range of specific gravity for cupric and etching copper is between 29 and 36°Be with 28°Be giving the fastest T H E C H E M I CA L C O N N ECT I O N " At the end of the day, there is no magic chemistry that eliminates the tradeoff between etch speed and etch quality."