n mm 2 even after storage for one year at 30 u00b0 c here 30 u00b0 c corresponds to a temperature above the average annual temperature in japan the expression u201c continuously u201d used for the storage period before the copper foil is fabricated into fpcs means that the foil is usually stored continuously for one 2 year at the most with a tensile strength of 300 n mm or more the copper foil has no wrinkling or other trouble during fabrication there is practically no problem therefore when the copper foil is capable of retaining a tensile strength of not less than 300 n mm 2 when allowed to stand at 30 u00b0 c for one year such softening property corresponds in terms of the half softening temperature obtained by annealing for 30 minutes to a temperature of 120 u00b0 c or upwards however when the half softening temperature by 30 minute annealing exceeds 150 u00b0 c the copper foil is sometimes not softened by the heat treatment either after the roughing plating and cutting into a desired size or at the time of adhering to the resin substrate that is why the half softening temperature by 30 minute annealing is specified to be in the range of 120 150 u00b0 c if the resulting fpc is to have an enhanced flex fatigue property the copper foil must itself have an enhanced flex fatigue property the copper foil is incorporated in a.