Issue link: https://iconnect007.uberflip.com/i/1524164
74 PCB007 MAGAZINE I JULY 2024 but to draw air into the etcher for the reoxi- dation process to get the Cu +1 complex back to the Cu +2 complex. ere is such a thing as too much venting. Too much venting will pull more ammonia up the stack and contribute to the pH problem. To set the proper venting the draw on the vents on the etch chamber or aer the replenisher rinse should be set so there is negative pressure in the etch chamber. is can be checked by closing the vent at the etcher entrance and holding a rag with the end soaked in hydrochloric acid to the etcher entrance. In the presence of ammonia, the rag will give off a white vapor. Open the vents until the vapor is just drawn into the etch chamber and no more. is should be enough to keep the ammonia fumes in the etcher. e vent at the entrance of the etcher should be set so any stray fumes are captured but not enough to interfere with air- flow into the etcher. Aqueous Ammonia Use aqueous ammonia (30% ammonium hydroxide) to raise the pH. is will still cause some drop in the specific gravity of the etchant but not nearly as much as using replenisher solution. Aqueous ammonia should be avail- able from any industrial chemical supply com- pany in various size containers. Industrial grade is okay; it doesn't need to be laboratory grade. Ammonium Chloride Crystal Get some ammonium chloride crystal to control the chloride content without affecting the specific gravity of the etch solution. Weigh out the amount of crystal you need to bring the chloride level back into spec and place it in a bucket. Draw some hot etchant from the etch sump into the bucket and stir until the crystal is dissolved, then pour it back into the etcher. Just dumping the crystal into the etcher will cause most of it to settle to the bottom and take a significant time to dissolve. is should also be available from any industrial chemical sup- plier as well in 20- to 50-pound bags. Again, industrial grade is good enough. no copper in it and excess amounts of ammonia and ammonium chloride (to replace losses of those two commodi- ties during the etching process), keeping everything in balance. e system works well as long as there is a steady supply of copper to be etched. If the supply of cop- per is not steady, such as small lots of pro- totype panels with long periods of time between lots, or resistor cores with not much exposed copper to be etched, then the system begins to break down. • pH: e pH continues to fall, even when the system is not in use, because the ammonia molecule is very small and vola- tile and will continue to escape no matter how well you think everything is sealed. To get the pH up again and avoid the dreaded sludge-out (low pH causing the copper to crystalize out of solution) more replenisher solution is manually added to the system. Unfortunately, the amount of replenisher that needs to be added to get the pH back to spec lowers the specific gravity to the point where the etch rate is significantly changed. It then becomes nec- essary to etch more copper (either scrap or dummy panels from the plating shop) to get the copper levels back up to spec. • Ammonium chloride: Now you are using up the ammonium chloride without replace- ment since the specific gravity is too low to trigger the replenisher flow and the pH continues to fall again. You must manu- ally add more replenisher solution to both raise the pH and replace the chlorides which, of course, drops the specific grav- ity again and so on until everything is com- pletely out of control. What can be done? Here are a few things that could help keep the alkaline etch system under control. One of the most important and least under- stood parts of the system is venting, which is needed to not only control ammonia fumes,