And here's why you can't see most solder joint cracks with x-ray:
Lead and tin are very good at stopping x-ray radiation. A BGA solder ball or termination solder joint is relatively thick with respect to the x-ray beam and a crack is usually perpendicular to the beam. In this case a normal intensity beam is NOT going through the same thickness of material irrespective of the presence of a crack or not. If you turn up the beam current, you just white out everything. 

Now if you turn the sample so your beam is parallel to the crack you might think you can see a hairline crack, but you still can't.  If you are 5000 feet up a 10000 foot mouuntainn range you can't see through the mountains even though there is air between each pair of peaks. Take a complimentary-shaped mountain range and bring it down upside down so that it is 6 feet above the first mountain range. You even more so can't see through the mountains. If you move back so you're not crushed and then bring the two mountain ranges together the volume of material you are trying to see through is only a tiny fraction thicker. And again you can't see through the mountains.  You eyes can't tell the difference between the 6 foot "crack" and no crack. Same for the x-ray system. 

Now if the "crack" in your solder joint is as big as a barn door, that is the teeth of the solder crack are open so much you could literally visually see through from one end to the other, you would be able to see it by x-ray, but in most cases this would just be redundant. 
Bev
RIM

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