On the Modeling, Characterization, and Analysis of the Current Distribution in PCB Transmission Lines With Surface Finishes

Due to manufacturing requirements, surface finishes have become a necessity in printed circuit board design. These finishes have significant effects on the RF performance of the transmission lines. In this paper, a filament modeling approach is used to model skin, proximity, and surface roughness effects in transmission lines with surface finishes up to 70 GHz. The approach shows a high accuracy compared with measurements. The model also gives an insight into how the current distributes itself by showing the frequency dependent proportion of the current that flows in each surface finish layer. In the case of NiP-Au or Ni-Au surface finishes, current migrates increasingly into gold at high frequencies and reaches a maximum in the Ni or NiP at around 3.5 GHz, and then declines. The distribution of the current in different materials can also be explained as the decay of an electromagnetic wave at the surface of the conductor. This approach shows that the evanescent wave in the cross section of the conductor can be analyzed as analog to a transmission- reflection problem, what we will call the surface finish effect. This effect brings into question the accuracy of the traditional skin-depth value, δ, and the models that depend on it, such as most surface roughness correction factors, for structures where different metals are layered in thicknesses that are not much larger than δ.