In my experience, solder balls are normally caused by only a few things. In order of likelyhood:
1) Water based fluxes (VOC-Free)not having the water carrier completely dried from the board before going into the solder. I.E, too fast conveyor speed relative to the amount of heat being applied. Solution: More heat or slower conveyor speed.
2) Too much flux; either alcohol OR water based. This is sort of related to #1. Solution: Put as little flux on the board as possible while still achieving acceptible soldering performance.
3) A combination of flux propensity to generate solder balls AND the board mask type which also has a certain propensity to cause solder balls to adhere to it. Certain fluxex JUST GENERATE more solder balls. You may have even seen certain flux vendors advertising a flux as being a low solder ball producing flux. This gets into flux chemistry which isn't my expertise so I'll leave it at that. The other, Mask type, plays a pretty significant role in the appearances of solder balls. Just to be clear though, a board masking will NOT generate solderballs per se, but rather will cause solder balls that ARE being generated by another source to adhere to the mask and not fall away. Normally, the fix for this is to get a mask type that has a rough texture instead of the flat, shiny types of mask. If you want to know more about this, your board vendor 'Should' be up-to-speed on the issue. Solution: Change fluxes (yeah right! I hear laughter from some of you, I know. ;)) and change the mask type to a more appropriate type to cut down on solder ball adhesion (STOP laughing people!! This is only a suggestion for those who have the luxary to actually do so)
There are a couple more things that can cause solder balls but their likelyhood of being the culprit is small. I'd check these first items before going any farther.
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