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Forwarded message:
Subj:    Re: GEN: Blind via Fill
Date:    96-12-08 15:10:48 EST
From:    Dryfilm
To:      [log in to unmask]

It is extremely difficult to completely fill blind vias without having resin
squeeze out on the surface of the panel.   The practice of allowing the resin
to flow onto the surface and removing it by sanding is fairly common.  There
are some potential dangers in this process, such as removing too much copper
from the plated surface.  This method rarely results in consistantly filled
holes;  some will be even with the surface, while some will have a slight
depression; others may remain above the surface.  

Generally, a slight depression at the via hole (<1mil) is acceptable.  The
makers of Pacothane make a product called Paco-Via.  Paco-Via is a
conformable release sheet used in place of Pacothane or Tedlar on a blind via
layer.  It conforms to the hole, and completely prevents resin from flowing
onto the surface of the layer.  The via holes will fill nearly level with the
surface, thereby eliminating the need to sand the panel to remove resin.
 Although this product is a little more expensive than Pacothane, my
experience with it has been excellent and I believe the cost is justifiable
from a quality and manufacturing perspective.  

Mark Dowding
Regional Technical Specialist
Insulectro

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