10

Solder Balling
Solder balling during wave soldering has always been around but the elimination of cleaning after the soldering operation has made it more visible as a process problem. In the past the solder balls were washed off the board surface during cleaning, out of sight out of mind ! Solder balls are caused by a number of process parameters. On the example board the position of the balls is random. This type of defect is normally caused by spitting from the surface of the wave which is associated with wave soldering parameters. If the solder is falling a distance from the printed board as the wave separates the solder can literally splash back from the bath If the preheat is incorrectly set or the quantity of flux applied increases the evaporation of the solvent from the flux may be affected. Using a glass plate over the wave should show up the gassing problem. Ideally there should be minimal bubbles visible below the glass when it contacts the wave. The compatibility of the resist and flux should be examined; often the mask can contribute to solder ball adhesion.