Finding and troubleshooting cracked BGA solder connections are a real pain in the neck. Please describe how xray fits into this process, the type of machine used, and how that machine is used in conjunction with other equipment.
As you probably know, x-ray inspection has problems positivly identifying cracks or delamination. This task is more suited to scanning laser acoustical microscopy but the cost is very high. It has been our experience that cracks are associated with the process problems of cold solder joints and/or thermal deformation of the packages and have x-ray image "signatures" associated with them. In other words, as I have been repeating, if the ball bonds are not uniform in size and shape then a process problem is creeping in. A particular signature associated with cracking or delamination can be found in IPC-7095 on page 64.