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84 DESIGN007 MAGAZINE I SEPTEMBER 2019 the appropriate electric field and current den- sity configurations for a typical GCPW circuit. As was the case for the microstrip circuit, the final plated finish cannot impact the copper- substrate interface; however, the coplanar side- walls will be plated with the finish. In the case of a GCPW circuit, there are four sidewalls where the plated finish will be applied, and significant current density occurs in those ar- eas. A lossy plated finish will cause a more sig- nificant increase in conductor loss for a GCPW circuit as compared to a circuit based on mi- crostrip transmission lines. The GCPW circuit in Figure 4 is considered tightly coupled. This means that the space be- tween the ground-signal-ground (GSG) plane on the coplanar layer is relatively small com- pared to the substrate thickness. If a loosely coupled GCPW (with large GSG coplanar spac- ing) was drawn, there would be much less cur- rent density along the sidewalls. The impact of final plated finish on this circuit configuration would be considerably less than on a tightly coupled GCPW circuit configuration. In gen- eral, the loss of a microstrip transmission-line circuit will be less impacted by the final plated finish than a GCPW circuit, as illustrated by Figure 5. As can be seen in Figure 5, the increase in insertion loss due to a lossy (ENIG) plated fin- Figure 4: This cross-sectional view of a GCPW transmission-line circuit shows its electric field lines and current density. Figure 5: For the same circuit laminate, the differences in loss can be seen for bare copper conductors and conductors with ENIG plated finish for (a) microstrip and (b) GCPW circuits.