Stress relaxation and creep of 12 to 35 mu m copper foil

Stress relaxation and creep of the electrodeposited and rolled copper foils, 12-35 mu m thick, are investigated near yield stress and near room temperature. The stress relaxation does not obey a logarithmic time law; the creep appears to follow a power function. These deviations from the expected logarithmic behavior are thought to be caused by very small grain size, unstable non-equilibrium defect structure and extensive micropore population (vacancies and vacancy clusters) typical of the electrodeposit. Relaxation and creep are significantly lower for the rolled (than fbr the electrodeposited) foil. Decreasing the electrodeposit thickness has an effect of enhancing relaxation and creep, attributable to a limited nucleation on the cathode surface and consequent generation of microvoids between growth clusters in the vicinity of the substrate. The foil thickness effect on creep and stress relaxation is not observed for the rolled foil, which is prone to embrittlement and stiffening at about 323K.