Evaluation of the crack-initiation strain of a Cu–Ni multilayer on a flexible substrate

A simple-support bending method was used to evaluate the cracking behavior of a Cu–Ni multilayer on a polyimide substrate. It is found that the crack density of the multilayer does not increase until the strain is less than a critical value (1.54%). The crack-initiation strain is determined to be 0.3%. Theoretical analysis of fracture properties demonstrates that the proposed method is an easy and effective way to evaluate the reliability of a thin film on a flexible substrate.