Greetings.

I've encountered a number of different reasons for plugging vias, most
of which have been cited by others.  The most successful method I've
seen is to use a wet screenable soldermask or a product called ViaPlug
to fill the vias prior to LPI.  In order to ensure filling, you need to
screen it from both sides, but it depends on your application.  If
you're doing it for vacuum reasons, one side is probably enough.  Doing
it prior to LPI would eliminate the possibility of it coming loose
during assembly.  Filling from both sides should eliminate worries about
moisture entrapment and oxidation.  Unfortunately, it's extra process
steps and bake times no matter what you do.

I think ViaPlug is made by Landwerke Peters.

Bill Anderson

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