Thanks to all who have replied so far.  I always try and anticipate the
obvious and cover my bases when I write one of these posts but as usual it
seems that I have left out some information.

While the photos are of small areas of the PCB this is for the purposes of
providing detail only.  The problem is evident across the entire PCB, dips,
quads, discrete, everywhere.  The spheres  are visible around the complete
circumference of a pad, not just one or two sides as I suspect would be
associated with a misalignment during printing.

While the focus has been on the solder ball problem some have noticed, and
it is certainly evident across the PCB that pad wetting is not ideal.

To repeat from the initial post, a pre-conditioning step of running the bare
board through the oven all but eliminates the solder ball problem.

John

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Parsons
Sent: Thursday, April 23, 2009 11:16 AM
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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