Sam,

     you wrote:

     >I have an application with a maximum ambient temperature of 70C. I am
     >using copper planes on FR4 to help cool some power resistor leads.
     >The copper planes can hit 115C. Is this a problem? Is there a rule of
     >thumb on how hot FR4 can continuously be operated at?

     How about specifying a higher Tg (glass transition temperature) epoxy
     glass eg 135C? Our suppliers tell us this is commonly available at
     similar cost, and Tg is one of the best ways to specify epoxy glass
     boards these days - after all, not all epoxy glass is FR4. We would
     not want to design 120C/125C Tg epoxy glass for anything over 110C
     operating temp.

     Hope this helps,

     Clive ffitch
     MBUK
     Stevenage, England

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