From your description you are using water throughout the line and not a
semi-aqueous process. If so, can you confirm your paste flux type? RELO
seems a bit unusual for a straight water wash type.

If you have that excess heat and/or humidity in your assembly area you might
have part of the answer. 
Leaving the boards in that environment after applying flux and before reflow
could be setting flux reactions on the tin surfaces but probably not on the
gold. Also if you are batching stuff then this could explain intermittency,
work will be hanging around for short to long times before processing,
giving sufficient variations for stuff to happen. 
Possibly the mix of cored solder flux with the paste is a factor (which
might not be the case at lower temps/humidity shorter times).

130F/54C could be too hot and provoke some secondary flux reactions esp.
with delayed cleaning. Some older water wash formulations can give
uncleanable spotting in straight water. 

Conversely if the flux is a RELO part cleaned just warming to >70C could
cause the residues to (seem) to disappear. Part removed resin will appear
white  - easily visible - and on warming will remelt to clear. Not so
visible.  This can  also improve cleanability on subsequent processing, it
sort of sets the clock back. 

Mixing flux types  from say hand soldering odd forms or through hole parts
even if cleaned and leaving around for a  while before processing and
cleaning again is a good way of making white residues. 
If the first cleaning is not 100% but looks OK, then the second cleaning
process can struggle on the modified residues and show them up.




Regards


Mike Fenner

Bonding Services & Products
M: +44 [0] 7810 526 317
T: +44 [0] 1865 522 663
E: [log in to unmask]

 


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Steve Gregory
Sent: Friday, June 22, 2012 1:59 PM
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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