Steve: A little known, but highly important fact about corrosion: Water is a crucial catalyst for most corrosion reactions, and your humidity issues may be triggering your white residues. If boards are sitting around loaded with flux in a high humidity, this could trigger your problem.
As an aside, we have created some products which should have been highly corrosive, but were completely free of attack. We even put "water absorbers" in the product to deal with absorption from air.
Rudy Sedlak
RD Chemical Company
--- On Fri, 6/22/12, Steve Gregory <[log in to unmask]> wrote:
From: Steve Gregory <[log in to unmask]>
Subject: [TN] White Residues
To: [log in to unmask]
Date: Friday, June 22, 2012, 5:58 AM
Morning All,
Boy, I'm looking for it to cool off here this weekend, the last couple of
days have been brutal! Heat indexes up around 100 F.!
Anyways, ran into a peculiar white residue issue with a few boards we just
got through building. We've never had this problem before (that I've seen)
with the solder paste we use here. Here's a photo of the residue on some
20-mil pitch TSSOP leads:
http://stevezeva.homestead.com/White_residue.jpg
The flux in the solder paste is a water soluble RELO, and we normally have
no problem cleaning the residues. We use 130 F. DI water in a inline
cleaner. Once the residue was noticed, we tried running them through the
cleaner again, no luck. We tried a trick I remembered from a while back to
remove white residues which was to take some of the same flux and apply it,
then heat the board for a bit and run it through the cleaner again, no luck.
It seems the only way to remove it is to mechanically brush it away. Like I
said, we've never had this problem before with this solder paste.
The two assemblies that we are seeing this problem with do have a different
finish than we normally have on the boards we build. Most all of our boards
are gold finished. But one of the boards that we saw the white residue with
was finished with Immersion Tin (.ugh!), the other was HASL'ed with Sn/Pb.
Do you think that had anything to do with it? With one assembly the white
residues weren't on all the boards, only some of them, which is strange. We
hand solder all our through-hole here, and some of the boards may have sat
around for a few days longer than others waiting on their through-hole
before they were cleaned which may explain why some had white residues, and
others didn't.
We only built 34 of these boards and they have already been brushed and
cleaned, so I can't get anything analyzed now. But I'm wondering if any of
you have an idea of what might have been going on here to cause these
residues?
Thanks!
Steve
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