Conductor Attenuation
Increased electrical resistance that occurs when alternating current concentrated at high frequencies travels along the outer boundary of a conductor degrades signal power. This signal attenuation, known as skin effect loss, is a major factor in the design of high-speed transmission lines on printed circuit boards. As the operating frequency increases, the current is forced out of the conductor’s center and flows within a thin surface layer.
This restriction effectively reduces the cross-sectional area available for current flow, driving up the conductor’s resistance and attenuating the signal.
Roughness Impact
Surface micro-roughness of the copper foil increases the path length of the current, which worsens the attenuation. At high frequencies, the skin depth becomes comparable to or smaller than the copper surface peaks and valleys. The current must travel along these complex surface contours, which increases the total resistance.
Utilizing low-profile or ultra-low-profile copper foils is the primary way to minimize this additional loss in high-frequency laminates. This choice ensures that the current path remains as straight as possible, reducing thermal dissipation and signal degradation.
Frequency Dependency
Skin depth shrinks in inverse proportion to the square root of the frequency. At gigahertz speeds, the current flows in a skin depth of less than one micron. Consequently, even minor surface defects or oxidation layers on the copper can cause a significant drop in signal strength.
