Steve, I worked at a place about 12 years ago where white residues would appear every few months, exhibiting the kind of behavior you describe. 

We worked up a scheme to control the manual additions of saponifier to the cleaning machine, so as to minimize the sum of the squares of the excursions, over time, from the target specification. The purpose was to bring the saponifier concentration under control as well as we could, and maybe reduce costs. As it turned out, saponifier expense dropped by about 65% and the white residue problem seemed to go away. I did not do any statistical analysis to see what is the probability that an event expected every 2-3 months does not take place for a year, but all concerned believed the frequency of occurrence of the white residue problem had been, at worst, greatly reduced.

I wanted to send white samples to Foresite, but that analysis never happened.

It's cooling off a bit around Pittsburgh, as I found when I rode my bike in this morning, so you should be getting some relief in Allentown in a few hours at most.   Louis Hart

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Friday, June 22, 2012 8:59 AM
To: [log in to unmask]
Subject: [TN] White Residues

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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