Steve

Your answer MAY be in your first sentence! A HI of ~38°C suggests your 
RH may be in the 70% or higher region (HI, by itself is fairly 
meaningless without knowing the temp as well.) So far, this year, we 
have had a HI of 43.4°C for a temp of 38.8°C and a RH of about 75%. A 
few hours of that may hydrolyse flux residues to render them insoluble. 
If I remember correctly, MIL-P-28809 said that thou shalt clean thy 
circuits within one hour of soldering them and not one minute later or 
thou shalt be hanged, drawn and quartered, for just that reason. 
However, this is not the only possible cause. I catalogue 5 pages of 
causes in my book, divided into 5 causal categories. There are 7 bullets 
just for hydrolysis of fluxes alone.

If this were the cause, you MAY be able to dehydrolyse the residues by 
heating to ~150°C by hot air from a shrinking gun. If the residues lose 
their white aspect, you MAY be able to re-clean immediately, but they 
may re-hydrolyse.

Brian

On 22/06/2012 15:58, Steve Gregory wrote:
> 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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