The failure of this type is not uncommon when using the high temp.
polyimides, Keramid 601, (and some of the loweer temp ones) which tend to be
brittle especially when the resin is not reinforced in power/ground  planes
where the lands have been removed and unreinforced resin from the prepreg
fills the area between the hole wall and the copper cladding on the ground
plane.  A careful horizontal microsection often reveals radial cracks
extending from the hole wall to the ground plane in these resin rich areas
and after electroless plating these partially fill with copper and if not
initially shorted will form a path for migration.  The thicker the copper
cladding, the more likely for this to happen.  And when getting into thin
metal core, 0.007in. thick,  and using the brittle resin, it is sure to
happen. 

Phil Hinton 
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