Conductor Loss
High-frequency alternating currents flowing in conductive trace paths tend to concentrate near the outer surface of the conductor, reducing the effective cross-sectional area available for electrical transport. Skin depth loss represents the resistive attenuation caused by this non-uniform current density distribution inside printed circuit board traces. The skin depth decreases inversely with the square root of signal frequency, dropping to less than one micrometer in copper at frequencies above 10 GHz.
As current is squeezed into this thin outer layer, trace resistance increases far beyond its direct-current baseline, generating higher signal insertion loss.
Surface Roughness
Conductor surface roughness severely amplifies skin depth loss when the microscopic peak-to-valley profile depth of the copper foil approaches or exceeds the skin depth dimension. At high frequencies, electromagnetic current paths follow the physical microscopic contours of the copper-dielectric interface, effectively lengthening the travel path and increasing local resistive losses. Substrate fabricators specify low-profile or ultra-low-profile copper foils for high-speed digital boards to present a smooth conductor surface.
Advanced numerical models incorporate surface roughness parameters, such as the Hammerstad or Huray models, to accurately predict conductor attenuation during circuit simulation.
Current Redistribution
Nickel-gold surface finishes introduce extra conductor loss because nickel possesses magnetic permeability and higher resistivity than copper. Electro-less nickel immersion gold finishes pull high-frequency current into the resistive nickel layer, increasing signal attenuation on outer-layer traces.
