Good Day Technet:

Here's an unusual situation.  We recently got a lot of PWBs from our
supplier who we've been using for years.  The solder on the boards (HASL)
looks good, thick and shiny.  A couple of weeks ago, after SMT assembly with
a fairly aggressive water soluble solder paste, we noticed that one of the
components (diode) had fallen off and was sitting on the operator's bench.
The pads on the board where the component had fallen off showed bare copper,
while the solder fillets remained on the component!  Thinking that this was
an unexplainable one-time occurrence, the component was soldered back on and
the board went on its merry way.  Well,  a couple of days later, boards from
the same lot were soldered and we noticed that some of unused pads or test
pads were copper only, with no traces of solder.  We then took some bare
boards, all of which looked good, and ran them trough our reflow and in-line
cleaning process (no flux or solder paste).   Low and behold, on about 20%
of the boards there were copper pads with no solder, and a few pads where
the solder was partially peeling off the pads, exposing the copper
underneath (with no traces of copper on the pealed solder).  After
discussions with our supplier, we both came to the conclusion that some of
the boards must have been oxidized to such a degree that solder coated the
pads but did not achieve a metallurgical bond.  I would have thought that
such a condition would result in a de-wet or non-wet condition, but, as I
said earlier, the boards looked great!

Has anyone else ever come across this before?

Jim Marsico
Senior Engineer
Production Engineering
EDO Electronics Systems Group
[log in to unmask] <mailto:[log in to unmask]>
631-595-5879

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