Here is what the Merix website http://www.merix.com says in their DFM
section:
Liquid Photo-Imageable (LPISM) Soldermasks
Enthone DSR 3241‡ is applied using the flood screen coating process, while
PROBIMER 52M® is applied via the curtain coating process. Enthone DSR 3241
has a green, semi-matte finish. Enthone DSR 3241 has improved resolution
capability, meaning that it can hold a finer feature, such as a "dam"
between SMT pads. Liquid photo-imageable soldermasks are considered to be
soldermasks of choice for most circuit board product due to their high
resolution, excellent electrical properties and compatibility with surface
mount technology.
Hole "tenting" is available through the via-cap process in which PC401®, a
thermally cured epoxy, is screened over the holes to be tented, after liquid
photo-imageable soldermask is applied. This is an advantage for vacuum
applications after assembly.
Via Capping with Screened Resist
Hole capping is available through the via cap process. On boards coated with
liquid photo-imageable mask, the vias can be screened with soldermask
creating an epoxy cap. Artwork modifications necessary for processing are
performed as part of the initial tooling. A separate design file must be
provided by the customer, which includes only those vias which are to be
capped. The customer needs to provide master pad soldermask and via files,
i.e. soldermask and via pads that are the same size as the outer layer pads.
Via Capping Design Constraints
The maximum finished hole size for via capping is .020 diameter (preferred
drill diameter 0.021 inch).
Generally, the non-test vias are capped on the bottom side of the board. Via
capping on both sides results in raised or broken caps. Therefore, it is not
permissible.
Via caps will have a raised surface of about 0.0024 +/- 0.002 inch above the
outer layer copper pad. This measurement may include solder and/or permanent
soldermask thickness.
Merix will guarantee a minimum of 98% of holes plugged, with open holes
randomly located.
Glenn
Subject: [DC] Plugged Vias
I need a quick verification here, hope someone can help...
I have always been under the impression that when a
drawing specifies "plugged vias", it means the holes
are filled (or at least partially filled) with an epoxy, the
epoxy is cured, and then the vias tented with solder-
mask. In the batch of boards we just recieved, I'm pretty
sure they just used the LPI soldermask to fill the holes,
and the fabrication vendor says "well, soldermask is
epoxy, so what's the difference". I'm not sure LPI glopped
into the holes is cured properly, and in my application the
board is maintaining a pressure seal, no air is supposed
to escape "through" the board, which is why I asked them
to plug the holes in the first place.
Can anyone enlighten me on this point? Is soldermask the
same epoxy as the epoxy used to fill holes? Or has it
always just been soldermask in the holes and I just didn't
know it?
thanks in advance,
Jack
p.s. If plugging really is a separate step, would it help to
supply a separate artwor that just has vias exposed so
they know what to plug? just curious, not a-retentive : )
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