PCB Fab: Etching Innerlayers (Waste Treatment)

Waste Treatment

Any discussion of waste treatment in a modern PCB fabrication facility must be based largely on conjecture and simplification. This is because essentially all such waste treatment facilities are afterthought "add-ons", and are nearly futile attempts at doing the almost impossible. Most PCB fabricators in existance today are in compliance with the letter of the law only because they use so much water. In otherwords, much of what passes for heavy metal removal is so marginally effective, that the facility meets the regulations only by effectively diluting the wastes until it can meet the regulations. Much of this is because the regulations are so poorly written that they essentially force this type of approach, because they regulate what is done within the plant, and thus inhibit not just innovation, but even clear thinking. But we digress into bitterness if we continue with that train of thought, and this is a discussion of the engineering possibilities.

The key fact to remember is that Copper, dissolved in water by itself, is a simple salt of an ion that does not interact with the Copper, such as Copper Sulfate, will precipitate cleanly and essentially completely just by raising the pH of the solution to 9.0. This is so simple, that it should make waste treatment a complete non-problem.

The problem arises from the fact that the waste streams from all the processes in most PCB fabrication facilities are mixed together, and consequently waste treatment to remove heavy metals becomes either very difficult, or very expensive, or both. And much of the problem arises from the rinsings from the ammoniacal etch. Although the properly configured etch chamber will not introduce much copper into the waste stream, it will introduce significant ammonia into the waste stream. When this ammonia is mixed with Copper arriving from other sources (microetches are the principal culprit) the ammonia complexes with the Copper, and carries it through the normal waste treatment. Normal waste water treatments, like the addition of Ferrous Sulfate, which are effective on dropping Copper out of other complexes, like EDTA, are ineffective on removing Copper from the ammonia complex. There are effective agents (precipitants) that will remove Copper from Ammonia complexes, but they are extremely expensive per pound of Copper removed.

Chloride ion, is, compared to ammonia, a very weak complexing agent. This means that when the Cupric Chloride etchant is diluted significantly, as it is in waste treatment, the Copper Chloride complexes break down, and the Copper can be precipitated easily by merely raising the pH.

Much of the problem potential with the rinsings from the ammoniacal etchant can be solved by keeping these rinsings separate until after the rest of the waste stream has gone through the clarifier, so the the ammonia will not complex and carry through any of the Copper that is being removed in the clarifier. Unfortunately, in some (most?) jurisdictions, this may imply a seconday waste treatment system just to treat the rinsings from the ammoniacal etch. If this is the case, ion exchange waste treatment may be a good approach.