Effective Cross Section
Concentration of alternating current near the surface of a conductor limits the effective area available for high frequency signals. An increase in frequency causes skin depth restriction to become a major factor in trace impedance. The current flows primarily within a thin outer layer rather than throughout the entire copper volume.
Resistance Calculation
Formulaic determination of the penetration depth involves the resistivity of the metal and the frequency of the signal. In copper traces at ten gigahertz, skin depth restriction forces current into a layer thinner than one micron. The concentration raises the effective resistance of the conductor to a measurable degree.
Surface roughness on the copper foil further increases the path length of the signal. Energy losses occur as heat within the dielectric and the metal interface. Designers must account for this loss when calculating link budgets for high speed differential pairs.
Modeling software accounts for the skin effect to predict the attenuation of microwave signals.
Plating Thickness Selection
Finishing processes ensure that the outer metal layer is thick enough to contain the signal. A failure to manage skin depth restriction results in excessive attenuation. Electroless nickel immersion gold (ENIG) often exhibits higher losses because the nickel layer is magnetic.
