Hi Hans! The answer to your problem is elementary! 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). Also - oxidation is not necessarily
corrosion unless you are talking about iron. Good Luck.

Dave Hillman
Rockwell Collins
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"Hinners Hans Civ WRALC/LYPME" <[log in to unmask]> on 11/03/99 04:12:18
PM

Please respond to "TechNet E-Mail Forum." <[log in to unmask]>; Please respond to
      "Hinners Hans Civ WRALC/LYPME" <[log in to unmask]>

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cc:

Subject:  [TN] The Case of Corrosion after Wave Flow Soldering Affair



Hi All You Sherlock Holmeses (sp?) out there,

I have a strange mystery which intermittently appears, so put on your
Sherlock Holmes' hat and grab Dr. Watson.

When we wave flow solder one particular board, during our cleaning process
(which can be moments afterwards), we might see tin and lead oxides
covering/surrounding the pins.  When we do see it, it's real bad.  DeOxIt
cleans it off well.  Any better corrosion removal suggestions out there?  My
ultimate goal is to prevent it (well duh).

We use Kester 186 and 186-18 RMA flux, it's a Kester 60/40 tin lead solder
in an old Hollis wave flow soldering machine.  We also use a latex solder
mask, how that might cause corrosion I don't know but there it is.  The only
remaining culprit might be very old (from Hollis) tinning oil, aka dross
inhibiting fluid.  The solder is tested by Kester, Option C, and are well
within MIL-STD-2000 & J-STD-001 specs for the 12 contaminants they check.
The boards are fluxed by brushing it on with a camel hair brush and then
baked in an oven - the wave only has bottom side preheat.  (I'm working on
reviving either the Hollis wave or the on-board foam fluxer, honest.)  Since
we are a low volume shop we might run the wave two or three days a week and
the remaining days it is turned off.  Is something wrong with the startup
routine?

The corrosion shows up around the pads of tin & gold plated connectors.  It
is one of those gremlins which appears and disappears without any apparent
pattern, just when you think it is gone . . .  bam!  The last analysis also
showed sulfur - don't know if that's an essential piece to the puzzle or
not.

We are checking to make sure the boards aren't corroded before soldering but
right now that's an assumption.  At least, a visual inspection doesn't show
anything obvious.

Anybody seen and defeated this Moriarty before?  We've not a moment to lose!

Hans

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