The Influence of Cyclic Torsional Bending on Fatigue Morphology of Copper Foil

Abstract

The fatigue property of copper foil on Flexible Printed Circuits (FPCs) is a critical factor for the miniaturization and the life of electronic devices. In recent years, as FPCs are adopted for wearable devices, evaluation methods with complex stresses which can predict fatigue properties on actual applications are required. In this report, torsion tests were carried out as a practical method. The influence of cyclic torsional bending on fatigue morphology of copper foil was evaluated. It was found that the rolled annealed copper foil with cubic recrystallized texture showed longer fatigue life than the electro deposited copper foil. Some FPCs were buckled depending on the test conditions. The copper foil with buckling discolored into a periodic X pattern. Although the copper foil without buckling discolored uniformly in the line direction of the circuits in FPC, the degree of discoloration varied depending on the line position. Since persistent slip bands were observed in the discolored areas, it is said that the discoloration state represents the fatigue morphology. The stress distribution by simulation is good agreement with the fatigue morphology.