I strongly disagree with Larry Fisher's comment that LPI solder mask is
superior to tenting vias with dry film solder mask:

1. Regardless of the application method (screening, spraying or curtain
coating) there is no assurance that the hole is filled and not simply
tented by virtue of the surface tension of the liquid mask. The
mechanics of the three application methods which apply 1-2 mils of wet
film on a surface will not put 32 mils of solder mask in a hole ( 1/2 of
an .062" thick board.

2. The solvents in the polymer must be removed before the polymer is
cross-linked.  I know of no method that will remove the solvents from a
1-2 mil film and a 62 mil plug in a via.  The fact is, the solvent is
not driven out of the mask in the hole. A skin forms over the surface of
the hole and locks the solvent in.

3. LPI solder mask requires exposure to UV light through the mask. 
Exposure units are set up for exposing 0.5-1.5 mils of dry solder mask. 
It will not penetrate 31 mils into the via.

4. Likewise, final curing of the mask is based on a 0.5-1.5 mil thick
film - not a 62 mil plug.

When the plugged via goes through HASL, the solvent in the liquid mask
vaporizes and either blows the cured mask off the surface of the via or
worse yet, cracks the surface of the solder mask.  Those vias that do
not have the solder mask blown off are not filled with solder mask -
they are tented with air trapped inside.  The tent may or may not fail
during wave soldering.

We have had some success in plugging vias with a thermal cure solder
mask (like SR-1000) and then applying the LPI mask in a conventional
manner.  However, this is a very labor intensive process and does
require additional cleaning to remove any residule thermal curred mask
from the surface of the board. Again there is no way of knowing whether
the hole is tented or filled.

Others have suggested filling the vias with solder.  The concept of hot
air solder leveling is to put a level surface of solder on the board
without filling the holes.  The hot air solder leveler can be adjusted
to prevent the solder from being blown out of the holes (most of the
time).  However there is a significant variations in the solder
thickness of component holes and surface mount pads. 
 
Hans Rohr

**************************Reply seperator****************************
[log in to unmask] wrote:
> 
> Plugging via holes with LPI solder mask is actually superior to trying to
> tent them with dry film solder masks for a number of reasons:
> 
> 1. When you plug with the LPI masks, you completely fill the holes with the
> solder mask polymer. Therefore, the corrosive fluxes cannot enter the via
> holes and cannot come into contact with the metallized hole where corrosion
> problems and, hence, reliability problems can occur.
> 
> 2. No dry film solder mask supplier can guaranty 100% perfect tented vias and
> there is always a chance that some of the tents will crack or break. When
> this happens, fluxes will enter the via holes and there is no way to remvoe
> these flux residues. The end result is the potential for corrosion and
> reliability problems.

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