A surface-treated steel sheet including a steel sheet, a zinc-based plated layer, and a chemical conversion treatment layer that contains Si, C, O and P, and has a C concentration of 20.0 mass % or more, an O concentration of 15.0 mass % or more, a Si concentration of 10.0 mass % or more, and a P concentration of 0.10 mass % or more. When t is a thickness of the chemical conversion treatment layer, an area from a surface to a position of t/10 from the surface is a surface layer region, an area from the position of 9t/10 from the surface to an interface between the treatment layer and the plated layer is an interface side region, and a region sandwiched therebetween is an intermediate region. A maximum value of a P concentration of the surface layer region is 1.5 times to 5.0 times the average P concentration of an intermediate region.

Disclosed are surface-treated copper foils having at least one treated surface that exhibit high conductivity and a set of surface properties. Also provided are flexible copper-clad laminates and printed circuits made therefrom. The present printed circuits exhibit low insertion loss and are suitable for use in high speed/high frequency applications.

The invention relates to metallic substrates surface coated with a coating layer comprising a metal and an additive.

Disclosed are surface-treated copper foils having at least one treated surface that exhibit high conductivity and a set of surface properties. Also provided are flexible copper-clad laminates and printed circuits made therefrom. The present printed circuits exhibit low insertion loss and are suitable for use in high speed/high frequency applications.