You can't stretch a liquid over a hole the way you can stretch a dry film resist.

When you try to mask right over the hole like it isn't there (with the DP-10 process)
you get a variety of different apperances of holes:
1) Most holes will fill with mask from one side and appear plugged.
    The other side will have mask up to the hole and have bare copper on the wall of the hole.
2) Some holes will have an opening in the mask all the way through.
    These will likely take solder at HASL or gold or whatver your final finish is.
3) Some holes will look like they plugged but will have a tiny pinhole in the center of the plug.

The problem is that holes that are plugged on one side or partially plugged will trap liquids from
the pre-clean processes before solder and will be liable to fallout later for etch-outs opens.

Other coating processes like curtain coating or spray coating are not likely to plug the holes at all.

----------
From:   [log in to unmask][SMTP:[log in to unmask]]
Sent:   Friday, December 12, 1997 7:42 AM
To:     [log in to unmask]
Subject:        [TN] Liquid Photo Imagible Solder Mask and Vias

We are changing from dry film solder mask to LPI solder mask on all our
through-hole PCB's (LPI is already used on our relatively small number of
surface mount boards but we don't have long term field failure data on
these).  With respect to vias we are leaning toward covering them with LPI
as we now do with dry film (i.e. not tenting with dry film, not plugging
with LPI, epoxy etc.--just applying LPI in a single operation using the
same solder mask artwork we now use which covers the vias).  It may be
relevant to know that many of our boards are not used in temperature and
humidity controlled environments.  We do conformal coat most of our boards.

In a recent inquiry on this general subject I received a variety  of views
on the relative value and problems involved in plugging vias.  I would
appreciate any views on the above option--just covering vias with LPI with
no special operation to plug or tent.

Some of the earlier replies suggested partially filled vias cause more
problems than totally uncovered vias.  Has anyone tried the above option?
Problems?  No problems?  Would it be useful to minimize via diameters?  Is
there any useful literature on this subject.?

Thanks to all of you who replied to the earlier inquiry.

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