Hi All,

We're having a problem with blow holes on small through hole vias after wave
solder on one of our packs. They are bottom-side test points, so we can't
simply soldermask over the bottom annular ring. I am hoping for suggestions
as to cause, and input to one specific question. I can think of a few
possible causes for this problem, but was looking for other possibilities.
Glass fibers or other protrusions through the barrel, as well as voids, or
other non-continuous areas of plating (including thin plating) are areas
where the PWB may cause problems. Excessive moisture or other chemicals
behind the plating in the barrel of the hole (due to excessive wicking)
could also cause problems, although in this case I would expect to see
separation between the hole wall and the copper plating or at least some
effect on the hole plating. Also the use of a VOC free flux in wave solder
which is not properly preheated (dried) prior to hitting the wave is a
possible assembly cause. Any other moisture introduced into the wave is also
a possibility, although in this case I would expect the issue to not be
confined only to small vias. I have reviewed several cross-sections of
specific blow holes and it appears that the plating is at least a mil and
continuous. No fiber or other defects are found in the hole wall. One thing
that is seen is that the hole wall is rougher than expected for a high
quality board supplier, but adjacent holes of similar roughness exhibit no
problems. Another thing seen on the cross-sections is that wicking is seen
on most holes and on many layers. It is always less than 3 mils in length so
it meets specification, but is more consistently present than I would like.
However, once again, a similar amount of wicking is seen on both good and
defective holes. If this was the cause, I would expect that all the holes
would have some evidence of damage due to entrapped chemicals volatizing
rapidly. I would like your thoughts on the subject and am interested in
other possible causes for this phenomena. Also, I would like to specifically
ask if rough plating in otherwise good hole walls could be a contributing
factor in this defect. As always, thanks in advance for your responses and I
want to extend my appreciation to this forum.

Thanks,
Robert Furrow
Printed Wiring Board Engineer
Supply Chain Networks
Lucent Technologies
978-960-3224    [log in to unmask]

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