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What is the maximum temperature you would recommend for an isolated trace on the surface of FR-4? (I.e.: not an inner-layer track.) Is there an IPC recommendation? Does it really matter if it is past the glass transition temperature? (What is this value for FR-4? Wikipedia says "above 120C".) I mean, even if it's soft, will the track still stick with high reliability? 'Suppose what I'm looking for is a MTTF vs. temperature (vs. trace area) set of curves, but experienced guesses are encouraged.
This is useful when calculating trace widths vs. temperature rise. I've always limited traces to the lower of about 100C or whatever the connected parts can manage.
[For the Mech-E, Chem-E, and Mat-E: I guess the question can be boiled down to the intersection of shear stress between copper and FR-4 caused by differences in thermal expansion, which would be related to track width, and the decrease in shear modulus as temperature rises.]
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