andre,
 solder balls could be a result of incorrect placement of components by the
placement machine. Also depending upon what placement equipment you are
using, also check to see how far the nozzle is comming down to place the
component. I am assuming you are checking your printing registration, when
the board is processed through the printer. A good check would be to to
inpsect board after printing, and then after placement.
Also, where are you finding the solder balls? Is it along a particular area
on the board or at random places. If you are finding them at random
places,and you have done the inspection before the reflow oven, you might
be introducing the board and paste to initial high soak temp's which is
causing the moisture in the solder paste to boil and explode. What is your
processing window between the processes? How long does a board sit in the
environment before it is reflowed?
If the solder balls are at a specific location at all times then it may be
a design issue.
Have you checked your paste? Some paste vendors, due to out of control
processes might produce pastes with very fine solder balls , which lend
towards solder balls?


Thanks

Nasir

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