Quantification of Conductor Surface Roughness Profiles in Printed Circuit Boards

Conductor (copper) foil surface roughness in printed circuit boards (PCBs) is inevitable due to adhesion with laminate dielectrics. Surface roughness limits data rates and frequency range of application of copper interconnects and affects signal integrity (SI) in high-speed electronic designs. In measurements of dielectric properties of laminate dielectrics using traveling-wave techniques, conductor surface roughness may significantly affect accuracy of measuring dielectric constant (DK) and dissipation factor (DF), especially at frequencies above a few gigahertz, when copper roughness is comparable to skin depth of copper. This paper proposes an algorithm for characterization of copper foil surface roughness. This is done by analyzing the microsection images of copper foils obtained using optical or scanning electron microscopes. The statistics obtained from numerous copper foil roughness profiles allows for introducing a new metric for roughness characterization of PCB interconnects and developing "design curves," which could be used by SI engineers in their designs.