Smartphones and tablets require very high flexibility and severe bending performance ability of the flexible printed circuits (FPCs) to fit into their thinner and smaller body designs. In these FPCs, the extraordinary highly flexible, treated rolled-annealed (RA) copper foils have recently used instead of regular RA foil and electrodeposited foils. It is very important to measure the Young's moduli of these foils predicting the mechanical properties of FPCs such as capabilities of fatigue endurance, folding, and so on. Even though the manufacturers use IPC TM-650 2.4.18.3 test method for measuring Young's modulus of copper foils over many years, where Young's modulus is calculated from the stress–strain (S–S) curve, it is quite difficult to obtain the accurate Young's modulus of metal foils by this test method. The S–S curves of copper foils always exhibit a large degree of scattering. In order to cope with the issue, “Resonance method,” using the resonance frequency of a specimen, is proposed to measure a much more accurate Young's modulus. The comparison is made between IPC TM-650 2.4.18.3 and the resonance method in view of calculation of Young's modulus accuracy. It is found that Young's modulus values measured by the resonance method were closer to theoretical values than those measured by the conventional method. In addition, the experimental data of fatigue life are used to support the accuracy of Young's modulus values measured by the resonance method.
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