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PCBD-Oct2017

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56 The PCB Design Magazine • October 2017 physical length, the additional loss due to the final plated finish effect at the corners of the conductor can be minimal. However, this cor- ner effect is accumulative and the same circuit design with increased length will have much more insertion loss due to the final finish. Another consideration is the thickness of the circuit and again a reference will be given to the simple microstrip circuit as example. A thinner circuit is more dominated by conductor loss than a thicker circuit. The final plated fin- ish is affecting the conductor loss of the circuit, and for a thick circuit where conductor losses are minimal, a change to conductor loss due to the final plated finish will be less significant. However, a thinner circuit which is much more sensitive to differences in conductor effects, will certainly be more impacted by the effect of final plated finish on insertion loss. There are also circuit design related issues. A stripline circuit, where most of the fields are contained within the body of a multilayer PCB are not significantly impacted by the final plat- ed finish which is applied to the outer layers of the PCB. A microstrip circuit will have inser- tion loss affected by the final plated finish as previously discussed, however, a grounded co- planar waveguide (GCPW) will typically have more insertion loss due to the plated finish. The GCPW has coupled fields between the ground- signal-ground configuration, which is on the top copper layer or signal layer of the circuit. These coupled fields are basically using four lay- ers of the final plated finish, which is between the side walls of these coupled conductors. Ad- ditionally, if the GCPW is tightly coupled (with a small space between ground-signal-ground), the increased loss due to the final plated finish will be more than if it were a loosely coupled GCPW circuit. There is also a frequency dependency for the effects of final plated finish on insertion loss, due mostly to skin depth. The skin depth of an RF circuit is the amount of the conductor being used by the RF current. At higher frequencies, the current will use less of the cross-sectional area of the conductor and at very high frequen- cies only the "skin" of the conductor is used by the current. As a basic example, using the microstrip circuit again, at low frequencies, the composite conductivity at the corners of the signal conductor is made up of the conductivi- ties of copper, nickel and gold for a circuit using ENIG plated finish. As the frequency increases, the skin depth will decrease and at some point, only the nickel and gold will be used by the RF current. At this point, the composite conductiv- ity has become worse because the good conduc- tivity of copper is no longer being used by the RF current. When the frequency increases even more, the skin depth is causing the RF current to mostly use the layer of gold, which is a pretty good conductor. However, at these very high frequencies, there are other circuit-RF properties which change and cause more loss not related to the final plated finish. The difference of insertion loss due to fi- nal plated finishes is dependent on the circuit thickness, the design and frequency. Basically, a thinner circuit is more sensitive to differ- ences related to conductor effects and a change in insertion loss will be more significant with a thin circuit as compared to a thick circuit of the same design. Additionally, GCPW will have more insertion loss change due to final plated finish than a microstrip circuit. And finally, as the frequency increases, the losses related to the finish can become more significant as the skin depth becomes thinner and uses less of the conductor. PCBDESIGN John Coonrod is the technical marketing manager at Rogers Corporation. To contact Coonrod or read past columns, click here. " A thinner circuit is more dominated by conductor loss than a thicker circuit. " THE IMPACT OF FINAL PLATED FINISHES ON RF PCB PERFORMANCE

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