The purpose of this study is to analyze the performance of the flexible conductive substrate by interfacial shear stress. This study performed automatic liquid-crystal modulating common-path interferometry (LMCI) optical inspections on 125-µm polyethylene terephthalate (PET) substrates having indium tin oxide (ITO) coating thicknesses of 80 nm, 160 nm, and 230 nm. The nonlinear characteristic of the stress-optical coefficient of the ITO material is obtained using LMCI. To validate the performance of the flexible conductive substrate, this study has utilized an automatic sliding-folding testing platform (ASTP) for a whole-folding test. Eventually, this study successfully has utilized interface shear stresses to validate the performance of the flexible conductive substrate depended on the different thicknesses of the conductive layers for whole-folding test by using LMCI and ASTP. According to the measurement results, as the folding radii decrease and the ITO thicknesses of ITO-coated PET substrates increase, the changes of interfacial shear stresses increase in compressive direction and the change-rates of electrical resistance of ITO-coated PET substrate also increase. Therefore, the interfacial shear stress measurement and analysis results depicted on flexible conductive substrates provide a validation for improving the manufacturing process and for reducing process residual stresses.
For flexible displays in manufacture, the full-field residual stress inspection for films deposited on flexible substrates is an important issue. According to the literature, the material phase retardation is related to its residual stress. This study proposes a novel method of the fast full-field phase-retardation distribution inspection with a flat large-area twisted nematic liquid crystal modulator based on a white light source by using a common-path interferometry. To fast inspect two-dimension (2D) phase-retardation distribution, a tensile testing machine is utilized to provide flexible substrates with tensional forces to enhance the residual stresses for measuring phase retardations in the tensional force direction. Accordingly, a 2D phase-retardation distribution concerning a 125-dpi line-pattern poly(ethylene terephthalate)/indium tin oxide substrate is inspected and analyzed by using a full-field flat liquid-crystal modulating common-path interferometry. 2D phase-retardation distribution inspection results depicted on flexible displays help a designer or maker of flexible displays design useful and comfortable products.
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