ed

My comment addresses the PINK RING issue
of your problem  

Although the various specifications state that
pink ring is a process indicator and not a failure
mechanism - remember that it indicates that 
there is a problem or there would not be any 
pink ring. 

That problem is: At some point in time the
bond between the prepreg and inner layer was
breached and chemical solutions were allowed
to invade this breach.  

Whether or not this becomes a failure mechanism
depends upon several things such as: did the 
subsequent electrolytic plating "heal" this breach
and completely prevent any voids at this location.

Ionic contamination within a board, if hydrolized by
moisture, will migrate if under a bias. 

My controversial opinion is that all boards with pink
ring are suspect for wedge voiding and contamination 
within the board.  Some boards will not degrade due
to their use and others will.  I am not certain if this is
related to the laminate system (polyimide is more
hydroscopic than epoxy), but it sounds reasonable.
The problem is you cannot microsection every hole 
when you see pink ring conditions.  If "pink ring" extends 
down associated conductors of affected pads, I think 
you have more than pink ring,

Susan Mansilla
Robisan Laboratory
317-353-6249