Galvanic effect can explaine your first observation very well. Genral
speaking,  you should not see too much difference  using Sn vs Sn-Pb as
metal resist thru etching. As far as metal activity, Sn is more active than
Pb and also it's grain structure may tend to be more porus. Some people even
concern about pin hole thru dry film striping when use Tin plating. But
overall pure tin plating works just as good as solder for SMOBC process in
terms of plating-etching and solder stripping.

-----Original Message-----
From: Nelson, John <[log in to unmask]>
To: [log in to unmask] <[log in to unmask]>
Date: Friday, January 08, 1999 9:15 AM
Subject: [TN] Galvanic effects


>When I image a board using nickel/gold as an etch resist on part of the
>panel
>and solder as an etch resist on another area, when I etch in alkaline etch
>the
>copper near the solder area etches much slower than the copper near the
>nickel
>gold.  The electropotentials of the metals are different enough to explain
>the
>differences in etch rate.
>
>My question is this; when we use tin as an etch resist we seem to get much
>more
>undercut than when we use solder as an etch resist.  Can this difference in
>copper
>etch rate be explained the same way?
>
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