Mr. Marsico:
In many/most(?) board fabricators, the board is on a continuous line, from Microetch, (rinse) flux application, to HASL. This is the optimal situation.. Especially when the Microetch is feed and bleed, thus virtually guaranteeing a good microetch.
But, I suspect strongly that the problem you are having is not due to a bad microetch, but rather to the the lack of such a line, and the board sitting around after microetch, and before flux/HASL... This is exaggerated even more, if the microetch (and thus the rinse afterwards) is in a tank, rather than sprayed.
A tank rinse needs to be multi-stage, and residence time in any tank should never be more than 60 seconds, and 30 seconds is better.
Too much time in the rinse is an invitation to oxide formation, and often it is Cuprous oxide, which is essentially invisible... (I have a theory that all tarnish begins as Cuprous Oxide, and is converted, thus making it visible, by atmospheric Oxygen to Cupric Oxide)
Dewetting is often, perhaps usually(?) caused by oxide on the Copper, and this is a good place to look for causes.
And remember free advice is often not worth what you paid for it...:-)
Rudy Sedlak
RD Chemical Company
"Marsico, James" <[log in to unmask]> wrote:
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]
631-595-5879
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