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.

The effects of Co and P on the discontinuous precipitation (DP) behavior of CuNiSi system alloys have been investigated using Cu4.5 mass%Ni1.1 mass%Si alloy (base alloy), Cu4.0 mass%Ni0.5 mass%Co1.1 mass%Si alloy (Co alloy) and Cu4.0 mass%Ni 0.5 mass%Co1.1 mass%Si0.05 mass%P alloy (Co + P alloy) aged at 430, 460 and 490°C. In all the alloys, slightly before attaining peak agehardening within grains, DP cells nucleated at grain boundaries and grew into the grains ahead of reaction fronts. The growth rates of DP cell for Co alloy were slower than those for Base one. Moreover, the trace addition of P to Co alloy (Co + P alloy) considerably retarded DP reaction. The kinetic analyses of DP using the Turnbull (T) and Petermann and Hornbogen (P-H) models yielded grain-boundary diffusion data. The activation energies for the base and Co alloys obtained using the T model were nearly identical and 125 and 124 kJ mol ¹1 , respectively. The values for both alloys determined from the P-H model were slightly larger than those obtained from the T model and about 130 kJ mol ¹1 . These results strongly suggested that the growth of DP cells in base and Co alloys was controlled by the boundary diffusion of Ni, and Ni or Co in Cu matrix, respectively. The smaller growth rate of DP cells in the Co alloy was ascribed to the higher number density of continuous ¤ precipitates than that of base one, which successively suppressed the boundary migration. [

Thickness effect on fatigue life of pure copper single-crystal foils with two different surface orientations, ð211Þ and ð5 11 1Þ, but with the same stress-axis orientation was investigated by means of S-N curve measurement and microstructural observation. Thicknesses of the foils were in range from 140 to 500 µm. Fatigue life of the ð21 1Þ specimen was much longer (by a factor of 1000) than that of the ð5 11 1Þ specimen within the given range of the foil thickness. The orientation dependence of fatigue life became more pronounced with decreasing the foil thickness. The size and crystal orientation dependence of the fatigue life can be explained reasonably by considering the reduction of the net area of the primary and secondary slip planes during deformation.

In order to consider the effect of cooling conditions on microstructure formation during casting of Cu-Ni-Si alloy (Corson alloy), the microstructures of unidirectional solidification of the alloy were evaluated. Ingots of Corson alloy with several cooling conditions were prepared by Mizuta method, and microstructural observations and analysis of the distribution of added elements were performed. The primary and secondary dendrite arm spacings (P-DAS and S-DAS) became smaller with increasing cooling rate, and it was found that P-DAS and S-DAS can be accurately approximated by multiplier of cooling rate times temperature gradient, and by that of cooling rate, respectively. Between secondary dendrites, Si was enriched from the initial solidification zone to the final solidification zone, and Ni was almost constant around the initial solidification zone and enriched slightly around the final solidification zone. The Si enrichment was larger under the condition of lower cooling rate, which is expected to be closer to the equilibrium state. In this study, it was found for Corson alloy that DAS became bigger and Si enrichment between secondary dendrites was larger under the condition of lower cooling rate, and Ni enrichment was not affected by cooling rate.