Last year I took multi-layer cards with BGA on them to Nepcon West to put
X-ray equipment to the test.  I had known defects under these BGA's, not
bridges, but solder cracks that caused intermittent problems.  I gave the
boards the companies and asked them to find the solder joint crack.  I
tried Glenbrook, 4-.Phi (HP), and Nicolia(spelling?).  Non of them could
find the opens!  The problem with X-ray on BGA is they can see the gross
defects like bridges, but on a BGA with 50 mil spacing, if you get bridges
you have BIG problems.  The biggest problem we had was solder cracking due
to moisture in the component causing popcorning or the joints cracking due
to stress and heat in wave solder (the BGA on top side trapped heat, we
placed Kapton tape on bottom side under the BGA to resolve the problem).
The X-ray equipment cannot find these cracks from a top down view.  You
would need a cross section view.  Therefore, we did not purchase an X-ray
system, but choose to spend the money on an ICT upgrade that would check
every solder connection on the BGA.

Good luck,
Brad Kendall

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