Characterization of surface roughness of printed circuit board (PCB) conductors is an important task as a part of signal-integrity analysis on high-speed multi-GHz designs. However, there are no methods to adequately quantify roughness of a signal trace or a power/reference plane layer within finished PCBs. Foil roughness characterization techniques currently available can only be applied to the base foil, prior to its incorporation into a finished board. In a finished PCB, a foil surface is not directly accessible, as it is embedded in the dielectric of the board, and attempting to expose the surface will damage the board and the surface of interest. In this paper, a method of surface roughness quantification from microsectioned samples of PCBs is presented. A small, non-functional area, e.g., a corner of the PCB, can be removed, and the surface roughness of the circuit layers can be assessed without impairing the function of the PCB. In the proposed method, a conductor (a trace or a plane) in the microsectioned sample is first digitally photographed at high magnification. The digital photo obtained is then used as an input to a signal-and image-processing algorithm within a graphical user interface. The GUI-based tool automatically computes and returns the surface roughness values of the layer photographed. The tool enables the user to examine the surface textures of the two sides of the conductor independently. In the case of a trace, the composite value of roughness, based on the entire perimeter of the trace crosssection, can be calculated.
Abstract-Thin absorbing layers containing magnetic alloy or ferrite inclusions can be effectively used for attenuating common-mode currents on extended structures, such as power cords, cables, or edgecoupled microstrip lines. An analytical model to evaluate attenuation on the coaxial line with the central conductor coated with a magnetodielectric layer is proposed and validated by the experiments and numerical modeling. The analytical model is validated using available magneto-dielectric samples of different thicknesses. This model can serve for comparing and predicting the absorptive properties of different samples of magneto-dielectric materials, whose compositions may be unknown, but dielectric and magnetic properties can be determined by independent measurements over the specified frequency ranges. From modeling the absorption in a coaxial line with a wrapped central conductor, it could be concluded whether it is reasonable to use this particular material in such applications as a shield on an Ethernet or other cable, for reducing potential common-mode currents and unwanted radiation in the frequency range of interest.
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