A few things you might look into are the age / storage of the paste and the wait time between printing and reflow. If the flux dries out you may get solder balls. 

As far as whether or not it is acceptable for the balls to be left on the board, I would suspect that it depends on the product application. 

Blair

>
>-----Original Message-----
>From: TechNet [mailto:[log in to unmask]] On Behalf Of Forrester, Michael (H USA)
>Sent: Wednesday, November 30, 2011 10:39 PM
>To: [log in to unmask]
>Subject: EXTERNAL: [TN] Real World Acceptability of Solder Balls.
>
>I have a PCA 6"X4" with a 144 pin LQFP on it.  The lead spacing is 0.5
>mm which leaves about 0.2 mm space in-between the pins.  The board goes
>through SMT reflow and a selective wave.  We are seeing various degrees
>of solder balls/spheres.  The balls we see are basically two sizes
>40-45um and 70-80um.  The solder paste is a size mesh 3, which would
>explain the smaller balls.  At this time I am not sure where the larger
>ones are coming from?  We see around the LQFP any where from 2-5 balls
>and literally hundreds of balls.  The solder paste is a no-clean and
>then the board is cleaned.  The reason for using no-clean is because of
>the process wait time from reflow to cleaning.  The vendor does not want
>to leave an aqueous flux on the board for an extended period of time.
>
>My basic question is: Is there an IPC specification(or general rule of
>thumb) on the acceptability of solder balls?  I understand that the
>presence of solder balls is a process indicator and that IPC-610 states:
>
>Solder balls are not entrapped, encapsulated or attached or can become
>dislodged in the normal service environment.
>
>Does this really mean that if I find a single ball that is not
>entrapped/attached, this is a reason for rejection?  Is this practicable
>or is there a spec./industry standard that says something like if you
>have X balls smaller than X in an area X, this is acceptable?  In the
>real world, are companies holding the zero tolerance on solder balls?
>Thank you in advance.
>
>Best Regards,
>
>Michael Forrester
>Sr. Product Engineer
>CSMTPE
>
>Siemens Healthcare Diagnostics
>101 Silvermine Rd.
>Brookfield, CT  06804
>PH: (203) 740-6452
>
>
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