Hi John 

It looks like a bad profile/paste issue, not the PCB. 

The profile is either to hot to quick and the flux is exhausted before
reflow, or the profile is to long in soak before spike and the flux is again
exhausted before reflow. 

If the profile is ideal then the paste is the problem, it can simply be
expired or a bad batch. The next thing to look at is the handling of the
paste by the operators, it could have been left out to long, reused to many
times, or taken out of the fridge and used immediately, someone has mixed
good paste with old, all comes down to poor paste handling.

A new paste jar from a different batch should be taken out of the fridge and
let sit for a couple of hours to bring it to room temperature before
opening, run a print and see if there is any difference.  

The fact that you have solder balls on the pads and on the solder joint not
reflowing reeks of oxidized solder spheres in combination of a shortage of
flux at spike, even on shifted prints good paste and reflow runs back on the
pad, it may leave a few solder balls behind but not what you are seeing,
that is extreme. 

If this was a PCB problem you would see the via's filled with them to, 

Bob

-----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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