We've seen some similar "long term failures".  We were able to do failure
analysis on the boards and found ionic contaminants on the innerlayer that
were very high in the arc'ed region.  We looked for CFF but have not
conclusively observed this phenomenon.  Unfortunately, all the parts that we
looked at had already arc'ed so if there was CFF it was blown away.

Suzanne Nachbor
Honeywell
6129573923