A Method of Extracting the Effective Copper Surface Roughness of a PCB Laminate In Situ

A method for extracting the effective copper surface roughness of a laminate in situ is introduced. The rough copper attenuation as a function of surface roughness is removed from the total attenuation. The residual loss is fit to coefficients, and an error is calculated based on the remaining apparent surface roughness contribution. When the error is minimized or when the apparent surface roughness contribution is nearest zero, the effective surface roughness is found, and the dielectric attenuation is isolated by subtracting the rough copper attenuation from the total attenuation. Using this value, accurate broadband permittivity values can be extracted by simply measuring the S-parameters of two stripline transmission lines. Further, the extracted dielectric electrical properties are calculated with a higher accuracy than the existing methods that neglect copper surface roughness. A supplementing method to unwrap the phase constant is also included in this work to avoid errors do to the length of the devices. Previous methods of extracting laminate material properties that consider copper surface roughness rely on cutting the sample to make a cross-section and observe the surface roughness directly. In this work, the effective surface roughness is gathered without destroying the sample.