X-ray imaging is a very effective tool for detecting solder bridge under BGA’s. But why is it that in some instances an electrical short exists under BGA even when the balls are perfectly spherical and are separately formed? The culprit is the solder mask registration of the PCB.  During the design stage when routing under BGA care must be taken to have the separation between the BGA pad and traces not violate the registration accuracy of the solder mask layer. The worst case scenario is when the trace beside the pad is exposed . But even if the trace wall , and not the surface , is exposed there is a strong possibility of formation of short during the ball reflow.