My understanding is that heating FR-4 above its Tg does permanently change
the material (unlike the same process for a thermoplastic, where the
hydrogen bonds re-form, some covalent bonds in a thermoset like epoxy are
irreversibly broken upon heating above Tg), but my own experience is that
the change for any one excursion is small, and that you can do it a few
times without significantly changing the properties of the laminate.
Tim Reeves

-----Original Message-----
From: michael [mailto:[log in to unmask]]
Sent: Wednesday, July 12, 2000 7:15 PM
Subject: Re: Glass Transition Temperature (Tg) Questions


Epoxies are thermosetting. Surely this process is not indefinitely two-way?
Or
should I say: beyond what temperature is the process no longer two-way (and
what
is the technical term for this temp.)?

Michael Fennema

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