Legend or nomenclature silk screen inks usually fall in one of two
categories:
   1. One or two part epoxies (thermal cure types)
   2. Acrylates (UV cure types)

Both of these types of inks are designed to be permanent and would be very
difficult to remove without damaging the underlying solder mask.

There is one possibility I can think of in terms of a legend ink you could
remove without damaging the solder mask. Several companies make white and
black liquid, photoimageable (LPI) solder masks. You could use these products
as legend inks by applying thes LPI masks onto the entire surface of the
panel (usually by blank screening), drying, exposing with legend ink artwork
and then developing as normal (usually in sodium or potassium carbonate
solutions) to give your legend ink pattern. After development, DO NOT CURE.
This will make the legend ink only semi durable (the solder mask underneath
needs to be cured) and you could probably selectively remove the legend by
using a sodium hydroxide solution and not damage the mask. A word of caution
would be that uncured legend would probably not survive wave soldering
processes, so you probably need to cure legend after you have made your
alterations before going through the wave (60 minutes at 300 deg F should be
sufficient).

Hope this helps.

Larry Fisher
Dexter Electronic Materials
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