In response to the questions below:

1.  Moisture contaminated solder paste may cause solder balling.  The
moisture may splatter during reflow, leaving solder balls behind.  Improper
reflow profile or uneven heating may also cause it.  If the temperature ramp
rate is too high on some areas of the board, this will increase the
probability of solder balling.  Solder balling may also be caused by excess
oxides in the solder paste, inhibiting the solder from bonding together
during reflow.

2.  Probably not caused by solder mask unless it is moisture laden.

3.  Solder balls may be eliminated with Near Equilibrium Heating.  With Near
Equilibrium Heating the zone temperatures are very uniform and the set point
temperatures are not too high, but very close to the desired board
temperatures.  The CUREFLOW is designed so the ramp up rate is controlled
initially where the board temperature is very low.  This reduces the chance
of solder balls forming.  Minimizing the exposure of the solder paste to high
temperature and humidity also reduces the chance of solder balling.

4.  Could not find anything in the IPC standards on solder balling.

Regards,

Loren Hillukka
Process Engineer
ETS, Inc.
[log in to unmask] (e-mail)
612-553-0051 (voice)



>Good day all,
>
>I have a question relative to solder balls and hope that someone can
>shed some light on this issue.
>
>1.  What causes solder balls?
>2.  Could solder balls be the result of incompatiability between flux
>and soldermask?
>3.  How do you eliminate solder balls?
>4.  What is the IPC acceptability standards for solder balls?
>
>Any feedback would be greatly appreciated.  Thanks to all that offer
>assistance.
>
>Best Regards
>
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