Etched foils used in heater manufacturing do have standard resistivities that reflect the properties of the metal alloy. Lower resistance heaters will be made commonly of different thicknesses of 70-30 Copper-Nickel Alloy which has a resistivity of 180 ohm-square-mil/foot (same as 225 ohm-circular-mil/foot used in another example). Higher resistance heaters will commonly be designed in different thicknesses of Inconel which has a resistivity of 490 ohm-square-mil/foot. Both of these alloys have a common property that they have a low TCR, i.e. resistance is relatively stable across temperature.
There are other foils used to make heaters that may have desirable properties for other applications. In fact, it is possible that the heater could be made of all copper materials with a fine pattern where you want heat and larger traces where you want it to act like a flexible circuit. Copper has a resistivity of 9.2 ohm-square-mil/foot and is much more temperature sensitive than the other alloys. So if the heater resistance is low enough, it could be designed on copper.
If you are using copper, calculate the current that will be flowing through the traces and design the flex lead per the conductor nomograph charts in the flex circuit design guide for the conductor temperature rise that you can allow. If you are using another alloy, the power density in the enlarged trace must be the equivalent of the power density in the copper trace used in the nomograph.
For a 1 oz copper, the trace width needs to be near 0.15" for a 10 dC rise at 2.5 amps. At 1/2 oz copper, the trace width increases to near 0.35".
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