maximum trace temperature on FR-4
This is not my area of expertise so I cannot point you towards any MTBF data but I work with tools that routinely operate in environments up to 175°C so I can tell you from experience what I watch out for.  


Voltage regulators with thermal protection.  These usually cut out about 125°C.
Solder.  Depends on the lead/tin ratio this can vary greatly.  You have to check the rating on each type but you can typically get solder that doesn't melt until around 220°C
Components vary greatly.  You have to read the datasheets carefully.  For instance, a microcontroller might be rated to operate at 125°C but that is 100 MHz.  If you only run it at 12 MHz it might run much hotter.  There is a good article here that talks a bit more about this.


I haven't experienced any trouble with traces separating from the boards before these other failures occurred.