Meaning
Electromagnetic parameter defining the distance into a conductor that a high-frequency current penetrates before its density drops to a fraction of the surface value. Skin depth 28 GHz determines the effective cross-sectional area of a trace at millimeter-wave frequencies. This measurement applies to metallic conductors in high-speed transmission lines.
Surface Resistance
Conduction occurs primarily in the top few hundred nanometers of the copper foil. The skin depth 28 GHz makes the surface roughness of the printed circuit board a critical factor in signal loss. If the roughness is comparable to the depth of the current, the electrons must travel a longer path, which significantly increases the attenuation of the signal.
Plating Influence
Finishing metals like nickel or gold have a different conductivity than the underlying copper. Because of the skin depth 28 GHz, the signal may travel entirely within the plating layer rather than the copper. Designers must choose low-loss finishes like immersion silver or organic solderability preservatives to maintain the efficiency of the transmission line.
Geometric Design
Thinning the dielectric layers does not reduce the resistance when the current is concentrated at the surface. Skin depth 28 GHz calculations are used to determine the optimal thickness of the copper for high-frequency circuits. Any metal thicker than three times this depth provides no additional benefit for signal conduction and only adds to the cost and weight.
