This is solder that is ejected during the vacuum step. If the voids in the liquid solder are too large, parts of the solder are pulled out when the vacuum is applied and remain on the component as splashes or balls. Solder balls can be minimized with gentler pressure curves (soft vacuum). However, the actual root-cause is usually the formation of large voids. These defects can be caused by contamination of the surfaces or a faulty soldering profile and should first be elminated before the vacuum settings are optimized.