Kevin,
     I had this same problem 2 years ago when I was implementing BGA and
was not able to find someone who could basically do a cross section image
to show cracks. I did not buy an X-ray system because I had known opens
(cracks) and I took them to NEPCON and none of the X-ray machines could
find them.  The only way we found to do this check was to send the boards
to IBM for a dye-coloration test.  This is a destructive test!  They
basically dip the entire board in a special dye, then literally rip the BGA
from the board.  Any cracks will be noticed because dye will cover the
joint.  Any joint that had to break will obviously show solder and not dye.
This is very expensive, but gave us the most insight to where our problem
was.  I then did some testing on my own by ripping the BGA from the board
and looking at each joint under high magnification.  If the pad was gone,
obviously the joint was good.  If the joint was dull and jagged appearing
that I had broke the joint it was most likely soldered.  In some cases the
pad from the BGA was stuck to the joint, again a good joint.  But if I saw
a shiny surface or grainy surface that did not look as though the joint has
been broken by force I took that to be an open due to a cracked solder
joint.

     As far as a machine to check production boards for cracks without
destruction, I could not find anything on the market at that time.  To my
knowledge there is still not a way to do that.  I have talked to the 4pi
rep and Nicolet rep and they have not made progress on that.  Good Luck.

Brad Kendall
Hella Electronics

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