Hi Kelly,
I have been plugging vias for years now and experienced most the problems you have been seening. Especially what I call "little volcanoes" created by outgassing. We feel we have established a process that provides pretty good results on standard .062 material. I have not really tried to plug anything up to .080. Please feel free to contact me direct. If you are interested I can be reached at 1-800 889-1138 or [log in to unmask]
Ed Cosper
Graphic Electronics Inc.
Tulsa, OK 74008
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From: Kelly M. Schriver
Sent: Thursday, October 01, 1998 8:54 AM
To: [log in to unmask]
Subject: [TN] LPI Filled Vias ?
Good morning TechNet!!!!!
Here's a challenge for the board fabrication guys and gals:
Most of our board designs go into severe environment service. For the past
eight ten years or so, we have been tenting vias with thin dry film solder
mask disks, followed by an LPI overcoat of the entire board (both sides). The
purpose of all of this is to seal up all exposed conductive surfaces,
particularly those under component bodies, prior to assembly. This leaves
only the termination pads and plated thru holes which receive thru hole leads
exposed. This has worked out to be a very successful process.
We arrived at this combined mask application after having tried a number of
different via fill approaches and just simple dry film tenting. None of these
were fully satisfactory (bumps, open voids, ruptures, incomplete fill, not
process durable, too costly, not repeatible, etc.). The dry film plus LPI
also turned out to be the only approach that ALL of our board vendors could
handle reliably.
Now I am revisiting the process. We have run a few samples using LPI as a via
plug as well as the overall solder mask. These have turned out to be very
durable in our assembly process. Microsections indicate small voids, less
than 50% of the via volume, which are not a problem to this application.
Board thicknesses range from .040" to .080". Finished vias range from .008"
to .016" dia. (This covers multiple designs.)
Now, here's the big question: Could you produce class 2 and 3 boards with
LPI plugged vias which will not rupture or expose metal during my soldering
and cleaning processes? (Assume that worst case will not exceed four solder
and cleaning cycles. Cleaning agents may include DI water, long chain
alcohols or citric acids) Total mask thickness buildup over pads should not
exceed .0015", in order to not interfere with our stencils.
For anyone who is actively performing this process or assembling CCAs using
this process - I would greatly appreciate your inputs.
Regards - Kelly
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