We experienced intermittent corrosion after wave solder at land patterns
that had been protected by temporary latex solder mask. We discovered
that the natural latex, low ammonia, types were less conducive, but
would still produce corrosion.
The variable in the process seems to be thickness. The mask should be
applied in as thin a layer as possible. Thick mask must trap ammonia
under the mask. Baking a thick layer of the mask seems to make the
problem worse. Since the mask skins over the board  and protected land
pattern are baked in ammonia corrosion results.


    Date:
        Wed, 3 Nov 1999 22:12:18 -0000
   From: Hinners Hans Civ WRALC/LYPME <[log in to unmask]

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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