Effect of Conductor Surface Roughness upon Measured Loss and Extracted Values of PCB Laminate Material Dissipation Factor

Prediction of accurate values for insertion loss (S21) on printed circuit boards has become ever more critical to SI modeling as signal speeds required for next-generation networking equipment move into the 10+ GHz range. Existing industry-standard insertion loss estimation techniques assume that the copper conductors (PCB traces) are smooth, when in fact they are not. The error thus induced is less significant at lower speeds, but cannot be ignored at frequencies above a few GHz. Accurate estimation of PCB laminate dissipation factor (Df) is another goal integral to SI modeling. Industry-standard methods again assume the smooth copper case, with consequent frequency-dependent error introduced into extracted values of Df. This paper describes a set of stripline PCB test vehicles used to correlate copper trace surface roughness to insertion loss. The main errors induced in extracted values of Df resulting from increased conductor losses due to surface roughness have been analyzed.