The ceramic BGA packages we use have the 90/10 ball. The plastic
BGA package expands more under heat, so the balls have to be more
pliable to conform to the changing package surface.
     As far as Xrays go, the eutectic (collapsing) balls are a much
easier test; normally when there is a defect, it is pretty severe,
and easily visible to the Xray (HP 5DX in our case). The ceramic
packages are much more difficult to test wel; the diameter of a good
image vs a bad can be only 5 percent of the radius.. 8^(
     Hope this helps.

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