I find that most of the customers that insist on plugged vias do it
     for purposes of loaded board testing.  Apparently there is a
     significant improvement in the vacuum drawdown during this process.

     Others design their vias "covered" to take advantage of the improved
     soldermask dam between the vias and nets adjacent to them.  By
     "covering" I refer to mask intentionally designed on the vias but
     without the intention of getting the mask in the hole.

     "Tenting" of vias is a term that I still often see used but really is
     only appropriately used with dry film soldermask because its thickness
     and texture has the strength to bridge over the via holes nicely with
     a single soldermasking operation.  For the most part it is not the
     mask of choice in fine pitch SMD design because its thickness and
     durability does not lend itself to the resolution required in fine
     pitch, which is why we see it so rarely.

     Mike Mikucki



______________________________ Reply Separator _________________________________
Subject: Re: [TN] Help! Plugging vias
Author:  Matthew Park <[log in to unmask]> at SMTPLink-Hadco
Date:    9/24/98 8:13 AM


Chiman!,

What is the customer's purpose of plugging vias completely with
solder mask?  I don't think it is possible.  Maybe some fab guru's
can elaborate on it.

 We tent all via holes from the bottom side to prevent solder
pluggings during wavesoldering process.  It does a wonderful
job with LPISM, even though holes are not plugged with
soldermask.   Our via size is from 0.4 - 0.6mm.  If via is bigger
than 0.7mm, it does plug the hole with solder sometimes.

Try DFPISM (dry film), it does excellent job of tenting small and
large vias with soldermask.

regards
Matthew S. Park
NII Norsat International Inc.


>>> Chi-man Chan <[log in to unmask]> September 24,
1998  2:59 am >>>
Currently we have a big problem on plugging vias with solder
resist. Our
customer requires that vias must be completely filled with solder
resist. (i.e. light cannot go through). The via size is 0.3-0.5 mm.
We
use liquid imageable solder resist. I have tried to print with 43T
then
32T open screen wet on wet on both side of the board. It does
not work.
The vias seems to be blocked with resist after screen-printing.
However,
some vias open again after track dry and developing. I have
reduced the
thinner from 5% to 2%. It does not work and causes a lot of
bubbles
among tracks. In order to completely plug these vias, we now
print these
boards with dot pattern before solder resist  printing and after hot
air
solder leveling. Some vias are very close to SMD. It is a very hard
and
time-consuming job. I think there should be an easier way to do
this
job.

Any feedback is appreciated.


Regards,

Chan Chi Man
[log in to unmask]

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