Design007 Magazine

52 The PCB Design Magazine • October 2015 column by John Coonrod rOgerS COrPOrATiOn LIGHTNING SPEED LAMINATES Impact of Final Plated Finish on PCB Loss A variety of plated finishes are used in the PCB industry. Depending on the circuit con- struction and other variables, the plated finish can cause an increase in PCB insertion loss. The plated finish used on the outer ground planes of a stripline circuit have minimal or no im- pact on insertion loss. However, microstrip or grounded coplanar waveguide circuits, which are common on the outer layers of multilayer high-frequency PCBs, can be impacted by the plated finish for increasing the insertion loss. In multiple experiments, comparisons were done between circuits with bare copper and circuits with different plated finishes. The bare copper circuits were used for reference only and the circuit structure was a microstrip transmis- sion line circuit using substrates of different thickness. The reason that most plated finishes cause increase insertion loss as compared to bare cop- per is that most plated finish are less conduc- tive than copper. Electroless nickel/immersion gold (ENIG) is a very good finish. However, the simple fact is that nickel is about one-third the conductivity of copper, and a circuit with ENIG will have more insertion loss than the same cir- cuit with bare copper. There are several variables concerning how much difference in insertion loss to expect and one of them is the substrate thickness. A substrate that is relatively thin will be more influenced by the conductor effects re- garding insertion loss and the added plated fin- ish will add to the conductor losses more for thin circuits than thick circuits. Conductor loss is one component of insertion loss. A microstrip transmission line circuit is a simple structure with a signal conductor on the top layer and a ground plane beneath that layer. The microstrip transmission line circuit primar- ily has electric fields between the signal plane and ground plane, but there is a concentration of fields at the edges of the signal conductor. It

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