I am experiencing many solder balls when using my selective solder machine when leads are fairly close together. Using a no-clean flux (3.4% solids). I have changed the volume of flux with no change in amount of solder balls.
Higher temperatures are the primary reason that there are more solder balls with selective soldering than in wave soldering. Possible causes are: * Solder mask type is the most important factor ... Solder resists becomes stickier at higher temperatures. * One flux may be more prone to produce solder balls than another. * Too much flux * N2 adjustment is too high * Preheat is too low