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Date: | Fri, 11 Oct 2002 01:05:19 EDT |
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At risk of repeating myself, and others...
The problem you are having is because the etch is accelerated by the galvanic
action of the Gold/Copper galvanic cell that forms, and jacks the etch speed
WAY up...
This does not happen nearly as much with Tin/Copper because they are similar
in electronegativity (a chemists way of saying they both give up their
electrons with a similar voltage applied) ....and also because the Tin forms
a nicely non-conductive oxide on the surface, which is a blanket to the
electron exchange.
ONLY way to deal with this is to cut exposure time in the ammoniacal etcher.
And this is difficult to predict/preset, as the etch speed of ammonia
etchants can vary a whole lot, because the etch ingredients and parameters
are mostly uncontrolled, except of course for the metal content. This
etchant makes me crazy, and I predict the industry will see a day when both
pH control, and air control (via ORP) will be the norm.
Rudy Sedlak
RD Chemical Company
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