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by Rudy Sedlak
Everyone knows that caustic soda strips resist, but nobody uses them. This in spite of the fact that it is inexpensive and easily waste treatable. That's because the list of all the things that we want a resist stripper to do (and not to do) is as long as a bad day on the electroless line.
More than that, we want the stripper to do all these things at a constant rate throughout the life of the stripper. Most of the time everything that a stripper does stays more or less the same, except the stripping speed. But this is not always the case. Frequently it looks like the anti-tarnish takes a holiday when the stripper is only partially used, and this is not because it is used up. As a stripper is used, it picks up copper, and the dissolved copper accelerates the corrosivity of the stripper. An anti-tarnish system that is great when a product is new can look as if it is consumed as the product gets older. In fact, it is merely not effective enough to deal with the increasing copper content of the stripper.
Spray strippers demonstrate this effect more than soak stripping, because dissolved Oxygen from the air accelerates the corrosion. However, soak stripping can more readily cause problems with immersion plating of Tin on to the Copper.
All of this is needed, while trying to keep a tight control of stripped resist particle size. Most resist strippers use Monoethanolamine (MEA) and Choline (Choline is the stuff that gives the stripper the fish odor) as their two principle sources of horsepower for stripping. The problem is that these two stripping "drivers" are not used at the same time. All the Choline is consumed before any of the MEA is consumed. Choline is the answer to our prayers in stripping, except for one tiny detail, it is very expensive. Which is why many strippers today strip very quickly for the first 10-15% of their life, and then settle into a slow stripping pattern. These strippers contain a small amount of Choline which is consumed during the initial few panels. After this Choline is consumed the remaining panels are stripped with the MEA remaining.
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