The PCB Design Magazine

PCBD-Oct2017

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54 The PCB Design Magazine • October 2017 Final plated finishes are used on PCBs for many different reasons. These include protect- ing the copper from tarnishing, improving long-term reliability, and enhancing assembly efficiency. Whatever the reason, the final plated finish can have an impact on a PCB's RF perfor- mance. Most notably, insertion loss is often in- creased due to the final plated finish. For some PCB configurations, the plated finish affects in- sertion loss substantially and other configura- tions are less affected. The final finish can have more or less impact on insertion loss due to several potential variations of the PCB construc- tion and/or the design of the circuit. Most final plated finishes have lower electri- cal conductivity than copper, with one excep- tion being silver. A common finish is electroless nickel immersion gold (ENIG), which can cause increased insertion loss because nickel has ap- proximately ¼ the conductivity of copper. Aside from the conductivity issue, nickel is also fer- romagnetic and has the potential to increase losses due to magnetic field interactions. Using a simple two copper-layer microstrip circuit as an example, most of the electric fields and higher current density areas are between the signal plane and the ground plane. Due to that configuration, many people may assume that adding metal to the copper will not impact the electrical performance because the interface between the signal and ground plane is unaf- fected by the added metal. However, if simple electromagnetic modeling is performed on a microstrip transmission line circuit, it will be seen that there is high current density at the edges of the signal conductor and that is also where more electric fringing fields reside. It is at these corners of the signal conductor and where it meets the substrate surface that the composite conductivity of the copper and the final plated finishes can cause differences in the conductor loss of the circuit. Conductor loss is a component of insertion loss (the total RF loss of the circuit) and an in- crease in conductor loss will cause the insertion loss to increase. When the circuit is short in by John Coonrod ROGERS CORPORATION FEATURE COLUMN: LIGHTNING SPEED LAMINATES

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