Hi John
Dewey must be very busy today, otherwise I would have expected a very bad joke about the name of your company by now.

The problem:
If I'm understanding this correctly, you see it at one site, even if the part isn't there (your pic1 is of an unpopulated PWB).  To me this says it's likely a stencil issue - alignment or size of aperture.  This assumes the same stencil has been used on both runs.

If it was the HASL coating, you should see it across the board randomly - why would HASL care about a particular location?  If it was the paste, same thing.

Good luck!  Let us know what it turns out to be...

regards,
 - Graham Collins

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of John Parsons
Sent: Thursday, April 23, 2009 3:16 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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