Now, maybe I'm wrong here, but I do believe that 50 microns = 2 mils.  That 
doesn't seem very thin to me....  The problem I believe with extremely thin 
dry films is usually pinholes and some sort of  difficulty with the carrier. 
 The other problem with a dry film solder mask is getting conformity over 
relatively high traces with a very thin film.  You may hear from some df 
solder mask people on this one.

There were some recent lengthy discussions on plugging vias.  Best go to the 
web site and browse on that subject.

Patty
 ----------
From: TechNet-request
To: TechNet
Subject: Relative issues concerning the tenting of vias
Date: Monday, October 28, 1996 11:12PM

As the industry heads towards a point where surface mount technology and
dense circuit topologies are the norm it becomes imperative to revisit
the issue of to tent or not to tent.
With liquid photoimageable solder masks still being an unreliable means
of providing a uniformly conformal coating over copper features, is dry
film the answer?  Have strides been made of late in this technology
which allay the inherent problems of process control, media thickness,
and contamination due to oily residue buildup and its subsequent promise
of sundry other problems such as delimitation?  Has MIL SPEC 2000 (is
rev C out yet?) acquiesced in the use of either the dry film or lpi
tenting of vias as an acceptable practice?
I have seen boards produced in the orient with dry film thicknesses as
small as 50 microns.  Are these available in the US or is their use
shunned as being too thin?  The suppliers I have spoken with cite
minimum thicknesses in the >1 mil range.  This lays bare other problems,
such as potential tomb stoning while providing the opposite benefit of
creating a solder well around SMT lands.  The latter stated benefit is
of dubious value if paste stencils and application coupled with fine
tuned temperature control bogs down production and outweighs the
reliability bonus, if any.
It is a confusing issue and all the literature I can find seems to be
horribly out of date (the latest being an excerpt from PC FAB magazine
dated 1991.  I am reasonably certain things have progressed since then,
but where is the data?
If you have any insights into this well masked dilemma, I for one could
stand the relief.
[log in to unmask]

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