Dry film tent covered with LPI is a great method of tenting - but you may
limit your purchasing department because many board shops do not have a
vacuum laminator. (Specialized equipment to apply dry film soldermask with)

Another possibility is a plugging ink.  Most (all?) shops should be able to
do this.  Plugging inks have very low volatile content and is forced into
holes with standard silk screening techniques.  One problem with this it
they don't always fill and you can end up with a plugged, non filled hole.

One consideration if your board shop is using standard soldermask for
plugging is that mask may plug then in the drying/curing process will 'pop'
like a volcanoe.  This can entrap chemistry and potentially lead to a latent
defect.



>From: "Whittaker, Dewey (AZ75)" <[log in to unmask]>
>Reply-To: TechNet E-Mail Forum <[log in to unmask]>,              "Whittaker,
>Dewey (AZ75)" <[log in to unmask]>
>To: [log in to unmask]
>Subject: Re: [TN] plugging vias
>Date: Thu, 18 Nov 2004 13:30:36 -0700
>
>Although I'll hear some in"tents" criticism, I'll mask my inhibitions and
>answer anyway.
>Since we can tolerate no broken tents at any time, we presently cap the via
>on both sides with a dry film Solder Mask, followed by a final cover coat
>application of a LPI Solder Mask.
>Other people have had success filling the via with a solder mask material
>or
>solder followed by a LPI Solder Mask.
>Other people are using a plated-over filled via with or without a final LPI
>Solder Mask application.
>PWB thickness, the compatibility of the chosen materials, the
>filling/plugging process, tolerance for voiding and surface sink will have
>a
>bearing on the acceptability of the process and product for your given end
>use environment.
>Dewey
>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]]On Behalf Of Dee Stover
>Sent: Thursday, November 18, 2004 12:52 PM
>To: [log in to unmask]
>Subject: [TN] plugging vias
>
>
>Hi all,
>
>Besides the process of tenting vias what other options are available for
>plugging vias on a board?  Solder? Epoxy?  ????  What sort of cost
>differences are we talking about.
>
>When using the tenting method have any of you experienced breakage in the
>mask barrier, during assembly or rework intervals?
>
>We are experiencing is degradation of the vias that exist under quad
>packages after rework.  Especially if quad packages are on both sides of
>the board, directly over each other.  Remove one for rework the flux goes
>through the vias and takes up residence under the quad on the opposite side
>of the board.  Our dunk type ultrasonic cleaner doesn't seem to be able to
>wash the flux etc. out from under the quads.  We do not work with high
>volume of anything so any equipment recommendations should be on the
>moderate side.
>
>Any experience on these matters you can pass along will be greatly
>appreciated.
>
>
>
>Dee Stover  [log in to unmask]
>Sr. Tech Associate Elect
>National Optical Astronomy Observatory
>950 N Cherry Ave
>Tucson, AZ 85719
>www.noao.edu
>520-318-8489
>FAX 520-318-8303
>
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