The guru is right as always - is that why he deserves the title?

Pyralux is an acrylic film. Unfortunatley, it is the only practical
adhesive system used to laminate flex and rigid/flex circuitry. It is
not like a resin based system that is thermosetting and cross links to
ensure innerlaminar integrity. Plus, the stuff has a Tg of "room"
temperature and "cold" flows with a mind of its own.

When a flex circuit is laid up and laminated, a certain amount of
delamination usually is assured. You cannot get the Kapton and adhesive
to conform to circuit trace edges, thus "soda strawing" is effected
adjacent to them extending to the next dielectric thickness. This can be
the source of further, unacceptable delamination. The answer here is
using lighter copper weights (thicknesses) as half ounce copper for
example.

No repairs are possible. The only way to ensure rework unnecessary, is
to prevent it by assuring designs and suppliers are qualified.

Earl Moon

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