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 ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. If you need assistance - contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ##############################################################