Issue link: https://iconnect007.uberflip.com/i/1522641
72 PCB007 MAGAZINE I JUNE 2024 Equipment-wise, the important parameters are conveyor speed, spray pressure, and oscillation. Again, most etchers will have a specific grav- ity control as standard. For cupric chloride the maximum etch rate occurs at about 34°Be (1.306 sg) and the lowest etch rate at 28°Be (1.239 sg). e change in etch rate between these two extremes is approximately 0.4 µm per min./°Be (0.11 sg units). e specific gravity controls that come standard on a typical etcher should be able to hold the set point at ±0.5°Be, so we're looking at a maximum change in etch rate of 0.4 µm/min. Except for the tight- est tolerance work, this should be more than adequate for high-density work. For alkaline etchant, the situation is a little different. e specific gravity is the trigger for the addition of replenisher solution that not only controls the specific gravity but also the pH through the ammonia in the solution and the chloride lev- els from the ammonium chlo- ride added, which is also con- tained in the solution. us, even though the change in etch rate due to specific grav- ity is not much greater than that for cupric chloride, changes in pH and chloride levels will also affect etch rates. For this reason, a much tighter control of the specific grav- ity for alkaline etchant is highly recommended. e price of a controller that will hold a spe- cific gravity set point of ±0.1°Be will be in the range of $5,000–$10,000 but should be consid- ered essential for alkaline etchers, especially for high-density work. For cupric chloride, the main control is the ORP, which basically measures the ratio of Cu +2 ions to Cu +1 ions displayed as millivolts. As the Cu +1 concentration increases due to cop- per being etched, the ORP goes down, and the etch rate drops slightly. Copper will etch when the ORP rises above 500 mv, and etch rates will increase until 600 mv is reached, when the etch rate levels off (at 600 mv, there is almost no Cu +1 in solution). e change in etch rate over this range is about 0.25 µm per min/ for cupric etchers, and pH and chlorides for alka- line etchers. Equipment-wise, the important parameters are conveyor speed, spray pres- sure, and oscillation. Most etchers in use today have some form of spray tube movement to improve chemistry exchange and even out the side-to-side etch profile on the etched panels. Let's take a brief look at each of these param- eters to determine what is needed and where improved control (costs more) can be effec- tive. Before delving into the individual parame- ters, I have one important piece of advice to give on equipment purchases. Most etchers nowadays come with PLC control displays. I have occasionally seen requests for relay electronics, mostly as a cost reduc- tion consideration and, in some cases, unwillingness to switch from a tried and true, easy-to- understand system to a new- fangled computer system. Don't even think about get- ting a new etcher for high- density circuits without a PLC control display. All etching machines will come with a temperature con- troller as standard. Most control specifi- cations that I have seen are in the ±1°F range. My observations show that most are really holding temperature variation to ±0.5°F or less. Testing has shown that the change in etch rate due to temperature for cupric chlo- ride operating at 130°F (54°C) is about 0.5 µm per min./°F. (Alkaline is a little more, but not much.) So, for a temperature control that holds temperature to a ±0.5°F range, the maximum change in etch rate will be, at most, 0.5 µm per minute and will probably be much less than this. If you want to control your tempera- ture even tighter than this, the alternative will be something like an encoder-based AC servo motor control for a couple of thousand dol- lars of extra cost. It is for you to decide if closer control is needed.