Hi Michel

even though most people feel that as long as the temp is below the T/g then
everything is fine in PCB land, however some long term studies I did showed
that in fact everything is not fine in PCB land. When you run a FR4 pcb
hotter a couple of problems arise, first the hotter you get it the more it
starts to carbonize, the longer you keep it at a high temp the worse things
get. one study showed with as little as three months at 100 deg C lowers the
dialectric voltage rating as much as 30 percent. the temp your talking about
is right on the border line for FR4,even high temp stuff a switch to BT
epoxy or polyimide may be in order , polyimide is 2 to 3 times the price but
BT epoxy gives you a higher operationg temp with only 1.2 X the cost. The
damage to the base lam is based on time at temp and max temp the hotter and
the longer the more damage is done, I have seen even 80 deg c type temp turn
a fr4 board brown over the years. I have advocated for years that 160 deg
FR4 has a max long term operating temp of 100 deg C.  i would suggest you
talk with some of the laminate tech guys  ( not sales type) and see what
they say.
robert tarzwell   megadawn.com

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