Hi Matt,
I agree with the vendor that they 'built to print'. I also agree with
you that the excessive soldermask is the cause of poor gasketing and the
subsequent poor print. Special tricks with the stencil, mentioned by
others, is one option. (By the way, Bill Coleman makes a good stencil).
Ultimately, I think you need to get the board built the way you want it.
Everybody translates 'tented via' differently. Some board shops will
simply spray coat over the via and consider that tented. Others,
fearing their screen print process will entrap air, will plug the via as
a secondary operation before screening on the soldermask.
There is ALWAYS wiggle room for interpretation with any requirement. I
would suggest the definition of terms is clarified and additional
restrictions of maximum soldermask thickness over copper is included to
your 'print' (or other purchasing agreements).
Although that is still not enough. I've recently had to add wordage to
our purchasing agreements that soldermask 'bumps', (due to pooling,
splatter, via drips, etc.), exceeding a certain thickness within a
certain distance of SMT pads, are not allowed.
Whatever you do, don't tell the board shop HOW to build the board
(unless you think you know how to run their factory better than they
do). Do tell them your requirements for the final product and let them
decide how to build it.
For example, do you really need the vias tented or plugged? If not,
don't specifically tell them to tent or plug the vias. If your ICT
draws a vacuum to pull the board flat, then indicate a requirement
concerning the maximum amount of air that can be drawn through the board
(which implies plugging) and let them figure out how they want to meet
that requirement. (They might opt to reduce the drill size, plate the
copper extra thick, selectively plug vias, the list goes on...) If you
are trying to stop solder shorts to the via, tell them a minimum solder
mask web width (some shops will tent the vias) and let them figure out
how to meet that requirement. If you're trying to improve soldermask
adhesion during BGA rework by making the solder mask web extremely wide,
again, tell them the minimum web width.
-Ryan
-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Matt Kehoe
Sent: Friday, March 09, 2007 10:01 AM
To: [log in to unmask]
Subject: [TN] Paste Volume, off contact vs non off contact
We have been processing a family of boards for 3 years. No issues. New
PCB
Vendor and the boards suddenly will not print very well. Sloppy printing
definition. Heavy heavy deposition.
Closer inspection reveals that the soldermask is built up .0012-.0019
over
the vias, preventing the stencil from gasketing. Vendor claims they are
built to print.. Opinions?
Pictures at ;
http://www.sipad.com/ssiqa/3942vias.pdf
http://www.sipad.com/ViasBuildup.pdf
The question we are trying to answer is, if your boards came in with
mask
this thick over the vias would you feel confident in running them
without
modifing the stencil? Would you expect a significant increase in solder
volume?
mk
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