Extraction of Transmission Line Surface Roughness Using S-Parameter Measurements and Cross-Sectional Information

The intentional roughness created on conductor surfaces during the printed circuit board (PCB) manufacturing process leads to a substantial increase of conductor loss at frequencies in the order of tens of gigahertz. It is essential to know the roughness of PCB conductors to create adequate models of the high-speed channels. This article presents a novel method for extracting the roughness level of conductor foils using only measured
S
-parameters and cross-sectional information. The proposed technique is relatively easy to perform, cost-effective, and does not require the destruction of test boards, making it a promising alternative to existing methods that rely on optical or scanning electron microscope imaging. Besides, the proposed method can handle boards with nonequal roughness on different conductor surfaces, which is common in PCBs. The method is validated through both simulation and measurement, and a good correlation is achieved between the extracted roughness level and the values obtained by microscopic imaging.