Everything test fine, until they are packed away, then unpacked and 
installed?  Intermittents around the BGA?

Look into handling issues.  How are the boards packed for storage?  Are there 
mounting holes near the BGA that might be flexing the baord at install?  What 
about connectors that require significant pressure, or a snap in, to install?  Or 
simply careless handling?

If the joints are well made, they should be able to handle a decent amount of 
stress.  You still may have ENIG issues weakening the joints.  If the ENiG is 
good, you still may have reflow issues weakening the joints.  Has anyone 
looked at a section of the joints?  What about a dye & pry?

We fought a similar problem with BGAs on a difficult board.  We identified and 
corrected reflow peak temperatures and cooling rates, and a couple of 
handling issues.  We've built 10's of thousands since then, still using ENiG 
(from trusted vendors - important!!!), with very good yields.

pete

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