Last Week I asked for some design help, and generally asked: "Should there
be a soldermask dam between a soldermask land and the feed thru via
connected to it?"

A quick Summary of Responses:

- If you do not have a solder dam between the pad and the via, the solder
will wick away, leaving the joint starved for solder.

- Assuming .050" pitch and a .050" grid: There should be a soldermask
there, but it isn't necessary.

- Do an experiment.  Put solder on a pad and heat the pad and via and see
for yourself.  For a .030" long trace, there should be no wicking.
However, If your SMT patterns get below 25-mil pitch you may want to
reconsider and add solder mask.


Other comments were received. Thank you for your responces.  At first the
information was a little confusing, but a clear statment emerged.
1) Soldermask dams should be present.
2) The longer the trace between the surfacemount pad and the feed thru via,
the less important the soldermask pad.
3) The closer the via to the pad, (the shorter the trace), the more
important the soldermask dam, for wicking concerns.

In our application, the trace between surfacemount land and via pad is
about .005" - .008" long.  This is the width of a normal soldermask dam.
This is why the original design did not have soldermask dams.  I have
recommended that the soldermask clearance for the vias be .010" SMALLER
than the via pads.  This will allow soldermask up on the via pads, and
maintain a soldermask dam.  (An idea gleaned from previous technet
discussion.)

Conclusion:
In the places you most need soldermask, it is nearly impossible to get it
there.  (Is this Murphy's Law of Soldermask?)


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                           George Franck
  PWB Product Assurance                     Phone (703) 560-5000 x2648
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