Hi Dave,

> Hi Hans! The answer to your problem is elementary!
>
        Good, I like simple answers.

> Well, I have a guess anyway.
> There have been a number of industry cases documented on the appearance of
> tin
> and/or lead oxides on various areas of circuit board assemblies coming out
> of
> cleaning processes. This is most commonly called "white residue" and has
> been
> associated with rosin fluxes. Many of the reports found the root cause was
> the
> cleaning process (typically water wash processing) was too hot or the
> circuit
> boards were being introduced to the cleaning process too hot (i.e. right
> from
> the wave to the cleaning process). The temperature and the tin and/or lead
> oxides interact, the oxides come out of solution from the cleaning water,
> deposit on the boards and are virtually impossible to remove. You have
> both
> ingredients present - RMA flux and if I read between the lines very hot
> circuit
> boards. Try letting the assemblies cool off prior to being introduced to
> cleaning and/or make sure cleaning solution temperature is in the 140-150
> range
> (assuming that you are water washing).
>
        You bring up dam good points . . . but, unfortunately for me, it's
not quite that simple.

        We definitely let the assemblies cool down.  I wait until it is
about room temp say 30 - 40 minutes at the earliest before hitting a
solution but I'm shooting for within two hours of soldering so the flux
doesn't get hard(er) to remove.

        In fact, we had a problem cleaning them because the shop was waiting
a week or two before trying/expecting to get the boards really clean.  Until
then they didn't see the need and this process wasn't clearly defined (it
will be by the time I finish though).  Their philosophy was 'The next step
will get the assembly dirty so why bother cleaning it really well - ever?'

        Our Aqueous Cleaner is set at 130 degrees F in both the wash and the
rinse sumps and while it might contribute to the corrosion, it can't be the
cause.  I see corrosion before it even goes through the Aqueous Cleaner.
During an initial clean using a spray or soak of Chemtronics Flux Off Rosin,
or a mixture of isopropyl alcohol and Trans-1, 2 Dichloroethylene it appears
when I dry it off with shop air.  At that stage it hasn't even hit the water
yet.

> Also - oxidation is not necessarily corrosion unless you are talking about
> iron.
>
        Very good point.  I never took the corrosion series in grad school
(my focus was on microelectronic manufacturing) and I only got 'volunteered'
as an assembly/cleaning engineer a couple of months ago.

        Hans

> Good Luck.
>
>

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