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April 2009

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From:
Mike Fenner <[log in to unmask]>
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Date:
Fri, 24 Apr 2009 13:26:25 +0100
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Looks like the most likely cause is lack of coalescence to me, possibly also
too much paste.
First though some housekeeping to get out of the way: Some water sol pastes
are vulnerable to poor handling techniques, due to their susceptibility to
humidity. All the rules on paste handling  - do not leave lid off jar, do
not keep recycling paste off/on the stencil, life on stencil, time after
print to reflow etc - have to be observed. So those concerns are the first
to be eliminated. Simplest thing to do is just try a fresh jar and 'walk'
the boards down the line no delays.

The next to check is profile. Water sol pastes usually respond best to
straight ramp profiles, long soaks are undesirable. This is because the
paste contains no natural "body" like a resin based no clean, they are all
rheology agents and so are more likely to hot slump. The flux will also be
more likely to exhaust itself with a long profile as there is no protective
resin.

As to why the problems seems amenable to pre-bake, this could be allied to
the house-keeping issues out lined above, excess moisture, but seems
disproportionate/unlikely. Or it could be a surface condition issue causing
the flux components to bleed and carry solder particles with them. The
baking would modify the surface energies and reduce bleeding. His would
therefore be board issue.
Likely there is no single magic bullet and combo solution will fix the
problems

Regards 

Mike Fenner 
Indium Corporation 


-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Parsons
Sent: Thursday, April 23, 2009 7:16 PM
To: [log in to unmask]
Subject: [TN] Solder Ball Problem

I am trying to get to the root of a little solder ball problem.  The problem
is specific to a single part and was evident on the first run of this board
a few months ago.  We are seeing it again on this run although the common
denominator here is that the PCB’s are all from the same production batch.

It appears that the formation of solder balls can be reduced by pre-baking
the PCB’s (100°C for 4-8 hrs) but not eliminated.  The best results are
obtained by pre-conditioning the PCB’s by running them through the reflow
oven first,  letting them cool, then print, populate and re-flow.  The
majority of balls now formed are dealt with at wash.

The PCB’s are simple double sided, HASL finish.  I suspect a problem with
the solder finish (HASL) but what exactly that “problem” is I am not sure.
FYI, paste is water soluble.

http://stevezeva.homestead.com/files/Solder_ball_1.jpg - this photo is of a
board that was pre-baked for about 4 hours @ 100°C, it was then printed and
re-flowed.  It does not matter if the 4 hour bake improved things you can
see that the results are pretty spectacular.

The following pictures are really more of the same except that these boards
have been populated and washed. 

http://stevezeva.homestead.com/files/Solder_ball_2.jpg
 
http://stevezeva.homestead.com/files/Solder_ball_3.jpg
 
http://stevezeva.homestead.com/files/Solder_ball_4.jpg

Any insight would be appreciated.

(thanks to Steve for hosting the photos)

Regards
John Parsons

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