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SEPTEMBER 2019 I DESIGN007 MAGAZINE 83 cuit's conductor loss. Thinner circuits will be more impacted by lossy plated finishes than thicker circuits (Figure 2). Figure 2 compares 50-Ω microstrip transmis- sion-line circuits, using the same substrate at different thicknesses and comparing insertion loss of a circuit with bare copper conductor and the same circuit with ENIG finish. As the plots of loss versus frequency show, the differ- ence in insertion loss between bare copper and ENIG is more dramatic for a thin circuit (an 8-mil substrate) than for a thick circuit (20-mil substrate). The ENIG finish results in addition- al conductor loss and a thinner circuit is more affected by differences in conductor loss than a thicker circuit. Loss Mechanisms for Plated Finishes Considering a cross-sectional view of a mi- crostrip transmission-line circuit, the concen- tration of electric fields and the current densi- ty between the bottom edge of the signal con- ductor and the top edge of the ground plane can be readily visualized (Figure 3). The op- posing metal faces of the top copper (signal) plane and the bottom copper (ground) plane have a concentration of electric fields. Howev- er, there is also a high concentration of electric fields and current density to be found at the edges of the signal conductor as can be seen in Figure 3. The depiction of a microstrip transmission- line conductor in Figure 3 is not meant to be overly rigorous, although great care was taken to show the appropriate field lines and current density for a typical microstrip transmission line circuit. It is a general representation of the electric fields and current density for a mi- crostrip circuit; the left sidewall and right side- wall of the signal conductor will have higher current density near the base of the conduc- tor. Final plated finishes cannot penetrate the signal conductor and are typically applied to the three edges of the conductor other than the copper-substrate interface. The additional con- ductor loss from a lossy plated finish is typi- cally an edge effect, coming from the left and right conductor sidewalls with the plated fin- ish. Conductor losses will increase due to the finish having lower conductivity than copper. This lossy edge effect is accumulative: a circuit with short length will suffer minimal addition- al loss due to the plated finish while a circuit with long length will exhibit significantly high- er loss as a function of length. The increased loss due to final plated fin- ish is also dependent on circuit design, and a grounded coplanar waveguide (GCPW) trans- mission-line circuit will suffer more loss due to a plated finish than a microstrip circuit with a plated finish. The GCPW configuration results in more of the lossy finish being part of the sig- nal path than in a microstrip circuit (Figure 4). As with the image in Figure 3 for microstrip, the drawing of GCPW in Figure 4 may not be exact, but much diligence was applied to show Figure 3: This cross-sectional view of a microstrip transmission line circuit shows the electric fields and current density between the two metal faces of the PCB.