There have been several postings about plugged / tented vias over the last
several days. It has been stated that you must tent vias to prevent
contamination being trapped in the holes. It has then been pointed out that
tents can crack and allow contaminants in the holes. Plugging with LPISM,
plugging with epoxy mask, before hot air level, after hot air level have all
been discussed with each being the way to do it or absolutely NOT the way to
do it depending on the experience of the author. As a board manufacturer we
have been involved in these discussions with customers for years (not to
mention should it be plugged from the top-side or the bottom side, or both).
Many of them feel passionatly about their position. We, of course, strive to
give them what they want, resulting in our not being able to standardize on
one process. 
  Since the desire for plugged vias is due to the need for vacuum at
in-circuit test, is there not a way to modify the test fixture so the vacuum
isn't required? If that is not feasable, has anyone tried to leave the
soldermask around the vias open, leaving solder on the hole walls after
hot-air level. Then at the assembly level, have a small opening in the
stencil corresponding to the vias. This would partially fill the via with
solder paste and would plug after reflow. Has anyone attempted this? If it
worked it would be able to be performed during an existing process step as
opposed to the extra steps required to plug vias with soldermask.

David Arivett
Cuplex, Inc

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