The surface of the conductor will not be perfectly planar. Since the current flows in a thin sheet near the surface (within a skin depth or so), the roughness of the surface causes the current to travel further through the (lossy) conductor. This increases conduction losses. Surface roughness is included in the conduction loss portion of the loss calculations.
Surface roughness is measured as the rms deviation of the surface from a plane. A typical number for a printed circuit board may be 0.05 mils rms.
You should contact your vendor or processing engineers for surface roughness values.
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