David,
     
     Test fixtures can be modified so that vacuum is not required but it is 
     costly, requires additional tooling, and is not standard with the test 
     equipment.  I have seen companies solder paste the vias so that they 
     fill and plug during assembly refow but have seen inconsistancies in 
     plug and via-fill height.  It can also cause solder to drop through 
     the via causing solder bumps (or balls depending on the far-side via 
     pad geometry) which hang on the far side and can cause vacuum problems 
     as well.
     
     Test is not the only issue for via plugging.  We require flatness on 
     ground areas of the boards which have ground via arrays on them.  The 
     resulting solder bumps during reflow or wave solder cause problems in 
     test and in assembly (emi enclosures, chassis mating, etc.).
     
     Frank Hinojos
     Watkins-Johnson Co.


______________________________ Reply Separator _________________________________
Subject: plugged vias
Author:  David Arivett <[log in to unmask]> at INTERNET
Date:    6/16/97 2:43 PM


     
     
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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