Most of the signal at RF frequencies is carried in the first few
1/100000ths of the conductive material. This is called the skin effect. It
is also why some RF connectors and waveguides are silver plated (silver
being a better conductor than copper). Tin or tin/lead are poor conductors
compared with copper. I prefer solder mask over bare copper for most high
speed designs so that the signals  propagate in relatively low resistance
copper. A digial signal is composed of many harmonics of its fundemantal
period or rise time. Some of the higher harmonics are carried exclusively
in the skin of the conductor. Keeping the skin of the conductor copper
ensures the all components of the signal are carried with relatively equal
resistance.

Ray Klein

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