Boundary Interaction
Topographical irregularities at the interface between a copper conductor and a dielectric substrate increase the effective length of the signal path. The surface roughness effect becomes more pronounced as frequencies rise because the skin depth of the current becomes smaller than the height of the copper peaks. These peaks and valleys force the current to follow a convoluted path, which increases resistive losses.
Performance drops as the tooth profile of the foil becomes larger relative to the signal wavelength.
Signal Attenuation
Energy loss in a transmission line is exacerbated by the increased resistance caused by the uneven copper surface. Managing the surface roughness effect is necessary for high speed digital and microwave circuits where every decibel of loss matters. Designers often specify low profile or very low profile copper foils to minimize this impact.
Calculations of the total loss must include a correction factor that accounts for the specific roughness of the chosen foil.
Conductor Adhesion
Mechanical bond between the metal and the polymer relies on the physical interlocking of the copper teeth with the resin matrix. While a smooth surface reduces electrical loss, it also lowers the peel strength, making the board more susceptible to pad lifting. Finding the right balance for the surface roughness effect is a trade off between signal integrity and mechanical reliability.
Engineers use specialized chemical treatments to improve adhesion on smooth foils without significantly increasing the electrical resistance. This optimization is a requirement for advanced multilayer boards.
