Alkaline etchant is a solution that is used to etch copper. It is an etching solution that consists of copper complexed with ammonia. The etch reaction goes as follows:
Alkaline etchant is often the choice etchant for printed circuit board manufacturers because of its high etch rate, improved etch factor, and compatibility with metallic resists. It can be a great etchant, but if you want to reap all the benefits of it, it will require extra work to maintain. Often, we receive a lot of questions about maintaining alkaline etchant because there are so many different parameters that can have an impact. Here are some tips we have to help you maintain your alkaline etchant.
1. Don't Let the pH Drop
Measuring pH of an alkaline etchant can be tricky at times. Often alkaline etch machines are equipped with pH probes, but, because of the nature of this etchant, the probes can easily wear out and cause incorrect readings. Due to this, it is recommended that you routinely check your pH levels through titration to ensure that your pH remains within the 7.9-8.2 range and your pH probe is reading accurately. If you ever start getting too low in your pH, your will etchant will start “sludging-out” and start forming dark blue crystals in the solution. This will of course cause abrasions on your products, reduce etch rate and plug your nozzles. If there ever is a point where the pH drops too low and it sludges out, it is best to just get a new alkaline solution and start fresh. That is because at the point of sludge-out, the copper-ammonia complex precipitates out into a solid that will not redissolve back into the solution
2. Have Adequate Ventilation
Having ventilation control on your alkaline etcher is highly important. Ventilation in the case of alkaline etchant serves 3 important purposes—safety, regeneration, and pH control.
Having adequate ventilation provides a safety feature to ensure your employees will not be overly exposed to the ammonia fumes this etchant releases.
Regeneration of your alkaline etchant plays a key role in maintaining a consistent etch rate and quality. As you etch more copper, your etchant will have more difficulty etching copper. This is because you are depleting your etchant and forming the product of the reaction. To get the etch rate back, the products of the etch reaction need to be converted back into etchant through a regeneration reaction. That reaction goes as follows:
Ventilation connected to your machine allows you to have an airflow through your machine – thus bringing oxygen into your etch chamber to perform the regenerative reaction. Although having some airflow through your machine is good for the regeneration reaction, too much ventilation will result in a significant drop in pH and cause sludge-out. There is an optimum level of airflow that needs to be established. Since this etchant is ammonia-based and it relies on it to keep its pH high, allowing too much airflow will make it easier for the ammonia to escape and cause the pH to drop.
Although the volatility of the ammonia in solution can make maintaining the etchant more difficult, it provides a simple control to prevent the solution from getting too high in pH. If the pH levels are too much, the machine will just need to increase the ventilation for a small period.
3. Don't Let It Sit
Alkaline is an etchant that almost needs to be constantly in use. If it is allowed to sit for long periods, the ammonia can escape the solution and potentially sludge out. If you are running an alkaline etch process, it is recommended that you do not let it sit for more than 3-4 days. If you need to leave it for longer than 3 days, it is best to pump it out of the machine and place it in drums until you return. This significantly helps maintain the life of your alkaline etchant by sealing it and preventing the ammonia from escaping, dropping the pH, and then causing sludge-out.
4. Have a Replenish Rinse
Replenisher solution is a mixture of ammonia and ammonium chloride. Replenisher solution is important to alkaline etching because it provides a source of ammonia and ammonium chloride for the regeneration reaction.
Replenisher can also help with allowing your products to come out in a clean condition. Alkaline etch machines usually differ from other machines by having replenish rinse systems at the output of the etch chamber. This is because often when panels leave the alkaline etch chamber there is some residual ammonium compounds. These compounds are typically insoluble in water and therefore will not be effectively removed in a normal water rinse.
5. Check Chloride Levels
Checking your chloride levels is also important in maintaining your alkaline etchant. It is possible for you to also sludge-out your alkaline etchant by not maintaining the chloride levels – only in this case the crystals that will form will be in a light blue color. The best way to maintain your chloride levels is to perform routine titrations. If you find that you tend to keep running low on chloride levels, it is usually a sign that you are either not getting enough replenisher solution feeding into your etcher or that something else is diluting your bath. Chloride levels typically drop because something is diluting the concentration. This is a typical case for people that only use ammonia and not a replenisher solution. If your chloride levels ever drop, they can be brought back up by dissolving ammonium chloride crystals into your etchant.
Whether you are currently using alkaline etch or considering it, we hope that you will find these tips useful. If you wish to go into more depth on the chemistry behind the alkaline etch process, or if you want a more thorough guide on the process, we recommend that you check out our technical paper on “Process Guidelines for Alkaline Etching”. If you are ever experiencing issues with maintaining your alkaline etchant, you should also feel free to contact us about your problem – our process team would be happy to help.
