摘要：
Copper corrodes quickly when exposed to oxygen. You must protect copper traces on outer layers from corrosion by passivation or by coating them with an inert material. Passivation refers to any treatment of the metal surface that renders it less reactive. For example, plating traces with tin or nickel produces a surface less susceptible to oxidation, but the extra thickness lowers the characteristic impedance of the trace. Pc-board fabricators normally compensate for the decrease in impedance by reducing the width of the etched trace before plating. As a result, for the same impedance, a composite plated structure never achieves as low a value of dc resistance as an unplated, bare copper trace.
These factors can also adversely affect the ac resistance of the trace, particularly if the plating material you use is a magnetic nickel alloy. Such materials suffer from a shallow skin depth and correspondingly large sheet resistivity at high frequencies, exaggerating resistive-trace losses. Commonly available 2-D electromagnetic-field solvers do not predict the amount of increase. Those tools assume you have enough sense to avoid nickel plating on long, high-speed traces.
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