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Solder Balling
The causes of solder balls are numerous and they have always been present on the bottom side of printed boards. It has been the increase of the use of no clean low residue soldering which has focused the attention to the problem. Regardless of the cause, if they do not adhere to the solder mask when leaving the solder wave the problem is mostly eliminated. Selecting the best solder mask is the best solution to making a board design robust. Solder balls are caused by gassing and spitting of the flux on the surface of the wave or by solder literally bouncing back from the solder wave. This is caused by excessive back flow in air or too high a drop in nitrogen environments. Place a piece of white card on the conveyor above the wave and see the effect during production.