Bend stiffness of copper and copper alloy foils

Loop stiffness and permanent deflection in bend testing of tough pitch copper foil and three kinds of copper alloy foils have been evaluated as bend stiffness. The copper alloy foils, in particular precipitation-hardened NK120 and C7025, exhibited high loop stiffness. High Young’s moduli of the alloy foils, which were confirmed by a cantilever resonance method, are considered to increase the elastic loop stiffness. The NK120 and C7025 also showed small permanent deflection in a cantilever bend test. The deflection, in general, decreased as 0.2% offset yield strength increased. However, the permanent deflections of NK120 and C7025 foils were reduced by stress-relief annealing, although the annealing lowered the yield strength in tension. Change in residual stress at the surface layer of the foil by annealing is considered to have increased the bend stiffness.