I've now removed the remaining cap cylinder and unravelled it and the patchy
rust film, or whatever, stops abruptly at where the glass seal was.

Much more interestingly viewing the die bond area to the base with a 30x
microscope. This appears to be a gold metalisation that has been eaten by
snails , if you've ever seen paper that has been in a leaking shed and so
grazed. Roughly linear tracks and then blobs of missing gold that seem to
have shamfered or grazed edges rather than the plating lifting and then
leaving sharp edges. One side of the die seems to have some of this gold as
a sort of powder in a few lines like iron filings over a magnet, rather than
solid metallic tin whisker needles, collecting there. Something seems to
have etched away the gold into a dust and then electrostatically or
magnetically drawn it to the die edge. It still measures 10 ohm so not a
will'o'whisp, frail accumulation . Perhaps I will take some microphotos
before probing the accretion with a needle. These grazing marks are roughly
midway between die bond edge and the edge of the support pillar.

So not an atomic level process of metal migration , so what is the process
that went on here presumably mainly when unpowered and very slow .


--
Diverse Devices, Southampton, England
electronic hints and repair briefs , schematics/manuals list on
http://home.graffiti.net/diverse:graffiti.net/