| 摘要 |
It is an object of the invention to provide a two-layer flexible substrate that excels in folding endurance and free from occurrence of Kirkendall voids or the like even when lead portions of COF are plated with tin and heat treatment is performed. The present invention is directed to a two-layer flexible substrate in which a copper layer is provided on one or both faces of an insulating film by using a copper electrolytic solution, wherein an average size of copper crystal grains constituting the copper layer is equal to or greater than 1μm and equal to or less than a thickness of the copper layer, and a ratio of peak intensity of (200) to a sum total of intensities of six principal peaks {[peak intensity of (200)]/[sum total of peak intensities of (111), (200), (220), (311), (400), (331)]} in the X-ray diffraction of the copper layer is equal to or greater than 0.4. The above copper electrolytic solution for forming the copper layer contains a chloride ion and one or more of thiourea, thiourea derivatives, and thiosulfuric acid as additives.
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