Hans:

I want to direct you to the website http://www.pcbfab.com for an article that
I put together on the chemistry of the ammoniacal etch.  However to answer
your direct question, the ammoniacal etch is a great piece of chemistry, in
general, but is real difficult to give precision etching, and especially does
not like to be shut down.

The parameters that need addressing to make sure the etch is consistent
include:

temperature
pH
Baume' (Copper Content)
ORP, also known as Cuprous ion content, or "Am I putting enough air through
the etcher??"
Chloride ion content
(We're assuming that your etcher is in good shape.....)

This last needs a little further discussion, as you have no control over
Chloride ion content, it comes in the replenisher, and therein hangs a tale.
  I have gotten involved in some of the issues that you are discussing a
number of times, as a consultant, both paid, and as a favor.  We do not sell
etchant chemistry.   I do not know if the conditions that I observed still
apply, but I have observed that the quality control in the incoming etchant
replenisher used to be so poor that getting consistent precision etching was
nearly impossible.

Further, the specifications that the etchant replenisher suppliers put on
themselves were so loose, or irrelevant, that virtually anything that they
manufactured would pass.

Specifically, the etch replenisher should have a free ammonia spec, not just
a pH spec.
It should have a Chloride ion spec, and this should be not greater than +/-
2% of the nominal value.

Then, you might just want to get the tests that they perform to insure that
the material meets their specs, and perform them yourself.

Hope this helps, happy to discuss further, but do not want to clutter up this
forum if there is no general interest.

Best regards,

Rudy Sedlak
RD Chemical Company

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