Issue link: https://iconnect007.uberflip.com/i/1525368
28 PCB007 MAGAZINE I AUGUST 2024 etch uniformity or etch rate, so there is even more room for variance than the manufacturer suggests. e same is true for developing tempera- ture. e manufacturer recommends a pro- cess temperature between 80–90°F (27–32°C). Again, etch testing has revealed no difference in etch uniformity or etch rate for test panels developed at different temperatures within this range. is allows a ±5°F (±2.5°C) varia- tion from an 85°F setpoint without affecting the development quality. ere are similar wide process windows for solution strength and spray pressures, so expensive upgrades to the developer equip- ment manufacturer's standard temperature, pressure, and speed controls aren't necessary. Some opt for a pH monitor or controller to more efficiently determine when to put a new bath in or to extend the bath life to reduce the time lost changing the bath. A newly mixed bath will have a pH range of 11.3 to 11.4. e pH will drop rapidly to around 10.8 when the developing process begins, then drop more slowly until the pH reaches about 10.5. At this point, it is a good idea to change the bath. Using the pH to control a dosing unit to add water and fresh developer solution will extend the life of the bath and allow longer times between bath changes. A pH setpoint of 10.7 for adding fresh developer will keep the pH between 10.6 and bath to have an ORP of about 130 mv, drop- ping quickly to between -10 mv and +10 mv as the copper etching process begins. How can knowing the ORP of your alkaline etch bath be useful? If the ORP drops below the -10 mv range and continues to drop, it's a warning sign that there may be these potential problems: • It may be that the ventilation system is not drawing enough air into the etch chamber for the regeneration reaction. is usually happens when etching a high volume of copper in a short period of time. • If the ORP comes back up aer stopping the etching, this is an indication of the problem. e etch rate slows down as the ORP drops, so this warning might prevent under-etched product. ORP readings have indicated the need to open air spargers at the bottom of the etch sump to provide extra air for high-volume etching systems. • Low ORP values in an alkaline etch bath can also indicate that the chloride levels in the bath are low, and there's not enough ammonium chloride to react with the oxy- gen in the air to re-oxidize the Cu +1 ions. Here, it might be a good idea to check the chloride levels in the etchant. ORP mon- itors are relatively inexpensive, and ORP probes are very reliable, so it might be worth considering this as an add-on. Developing your panel aer exposure is one of the most forgiving steps in the whole cir- cuit board production process. For instance, the manufacturer of the dry film etch resist I use recommends setting the conveyor speed for the developing process so the clean resist breakpoint (the percentage of distance through the developing chamber where the unexposed resist is visually gone) is between 50–60%. For my developer, if I set the con- veyor speed to a 55% breakpoint, I can have a ±5 inch per minute speed variation with- out affecting my developing quality. Etch test- ing has shown that panels developed at 40, 50, 60, and 70% breakpoints have no difference in " Developing your panel aer exposure is one of the most forgiving steps in the whole circuit board production process. "