Meaning
Microscopic surface irregularities on the conductive copper tracks of printed circuit boards establish the baseline conductor loss at gigahertz frequencies. This physical characteristic, known as copper foil roughness, increases the effective trace length for high-frequency currents that are forced to travel close to the conductor surface due to the skin effect. It sets the boundary where bulk conductivity formulas are no longer accurate.
Topographical Measurement
Industrial classification of copper foil roughness relies on mechanical or optical profilometry to determine root-mean-square surface profiles. Manufacturers designate foils by their profiles, such as standard electrodeposited, low profile, or very low profile, which vary from several micrometers down to sub-micrometer levels. These micro-scale structures are necessary to secure mechanical adhesion to the dielectric resin during lamination.
Attenuation Mechanism
High frequency signaling experiences increased resistance when the signal wavelength approaches the scale of the conductor surface irregularities. For example, at ten gigahertz, electromagnetic fields propagate primarily within a thin shell at the outer boundary of the trace, a phenomenon which causes the current to follow the convoluted contour of the irregular surface rather than a straight line. This extended travel path increases the resistive attenuation of the signal.
The additional attenuation reduces the eye height in digital transmissions.
Adhesion Budget
Chemical treatments applied to the copper foil create a dendritic surface that improves physical interlocking with the resin matrix. While this treatment prevents delamination during thermal cycling, it reduces the overall electrical performance of the transmission lines. Thus, circuit designers must optimize the balance between the mechanical peel strength of the board and the signal loss at high frequencies.
