I am assuming that you are talking about aqueous processible LPI solder
masks, so a few suggestions would be:

1. Bake the boards for 30-60 min at 250 deg F to really dry the boards. At
this point the mask is very, very dry and is more impervious to absorbing
moisture from aqueous processes (think in terms of a bone dry Chamois that is
very difficult to wet out when it is dry, but absorbs water easily when it
becomes damp)

2. Oxide processes tend to be highly alkaline (>13.0 pH) and operate at high
temp (up to 160 deg F). Try to keep the exposure of the mask to a minimum in
this type of environment. Aqueous LPI masks are developed is alkaline media
and are much less resistant to alkaline solutions than to acidic ones (in
fact, aqueous LPI masks can be stripped after they are cured in highly
alkaline, high temperature solutions). High alkalinity, high temperatures and
long exposure times are not your friends, so try to minimize all of these.

It is possible that the aqueous LPI masks will never work well in your
process. There may be other alternatives for masks:

1. One or two component thermal cure epoxy solder masks are highly impervious
to both highly acid or alkaline solutions. The down side here is that the
resolution capabilities of these masks are not as good as the LPI's.

2. There is a  LPI  solder mask from Ciba Geigy called Probimer 52. This mask
is developed in a solvent (not aqueous) and is highly impervious to both
highly acidic and alkaline solutions. Only about 20 shops in North America
have this system installed (system costs >$500,000); however, there are
several services in the USA that process Probimer 52 for PWB manufacturers.
They are:

     A. Solder Station One
          Santa Ana, CA
          Mr. Odilon Cardenas
          
     B. Independent Technology Systems (ITS)
          Bloomington, MN
          Mr. Don Paterson

     C. Radian Technologies
          Austin, TX

I hope this information helps. Please contact me if I can be of further
service.

Larry Fisher
Dexter Electronic Materials
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