BGA defects observed with x-ray inspection fall into catagories. One catagory is the obvious defects; i.e. soledr bridges, missing balls, excessive solder voids.
The second catagory is the more subtle "potential" defect. This is where variations in the ball bond x-ray images are observed. Variation in size and variation in shape. These we call signatures. These signature can indicate package deformation (popcorning and potatoe-chipping), incomplete reflow and collapse, cold solder joints and possible loss of contact. Examples of these signatures can be seen in IPC-7095 Standard.