The shape of your solder joint below a flip-chip or BGA is controlled by the surface tension of the alloy, the activity of flux used, the tension or compression on the component during reflow, as well as the solder mask opening size. If the solder mask opening is too small, the solder joint wetting is limited to a small area of the pad. If the opening in the mask is too large, there is nothing to stop solder from wetting onto the trace next to the pad.