Copper is an excellent conductor for high-frequency PCBs, with typically low-loss performance at RF and microwave frequencies. But it tends to oxidize if left unfinished, making solder connections unreliable. Over the years, PCB manufacturers have developed dependable processes for adding finishes to copper, plating it with such materials as nickel, gold, tin, and silver. While these materials provide protection from the effects of oxidation, most have lower conductivity than copper—except silver. As a result, it is the one plated finish that will not decrease the conductivity of the conductor and, in the process, increase the insertion loss of circuits formed from that plated copper.
Gold is highly touted as a plating material for conductors and electrical contacts. It exhibits high conductivity, at 4.52 × 107 S/m, although this is less than the conductivity of copper, at 5.817 × 107 S/m. Silver actually has higher conductivity than copper, at 6.301 × 107 S/m. Plated finishes applied with silver tend to be extremely thin, so the benefits of such high conductivity typically won’t be seen at lower frequencies. However, at higher frequencies, with very thin skin depth, the effects of having a plated finish with conductivity that is higher than copper may be more noticeable.
The impact of final surface finish on circuit loss will depend not only on the type of surface finish but on the thickness of the substrate material and the type of transmission-line technology, such as microstrip or grounded coplanar waveguide (GCPW). In a microstrip circuit, with its configuration of dielectric material between a bottom layer ground plane and top-layer signal conductor, the electric (E) and magnetic (H) fields form high current density at the edges of the microstrip transmission line. For a circuit that is plated with a material having less conductivity than copper, such as electroless nickel immersion gold (ENIG), the high current density at the edges of the transmission lines will use the plated finish more than the copper beneath, resulting in higher conductive loss than when using pure copper.
In the case of copper plated with ENIG, the losses are frequency dependent, increasing at higher frequencies. The frequency dependence is caused by the way that current uses a conductor differently at different frequencies—the so-called “skin effect.” At lower frequencies, the current uses more of the full cross section of the conductor. At higher frequencies, the current flows only along the outside or the skin of the conductor. As the frequency of a microstrip transmission line increases, the current flows through less of the copper conductor and more of whatever plated finish is on that conductor.
For a microstrip line with ENIG finish, the current will use different combinations of the three conductive metals, depending upon frequency. At lower RF, all three of the conductive metals contribute to current flow. At higher RF or lower microwave frequencies, the current flow takes place more through the nickel and gold. And at higher microwave frequencies or lower millimeter-wave frequencies, where the current density is more concentrated around the edges of a microstrip line, current uses more of the gold as a conductor, with the conductivity characteristics of the gold contributing more to the final conductor-loss performance than at lower frequencies.
The type of circuit layout and transmission-line technology can also play a hand in how the choice of a plated finish translates into the conductor loss of a high-frequency circuit. For microstrip transmission lines, where the current density is high around the edges of the conductor, some loss will occur because of the skin effects at higher frequencies. But in a circuit configuration like GCPW, in which the two waveguide-like transmission lines each have two edges, for a total of four edges, the current density will be high at those four edges of the circuit. As a result, for a GCPW circuit with its four conductor edges, a plated finish will have a greater impact on high-frequency loss than for a microstrip circuit with its two edges.
As a way of demonstrating these differences in surface-finish conductivity for different transmission-line circuits, broadband measurements were performed on equivalent-length circuits with microstrip and GCPW, each fabricated on two versions of 8-mil-thick RO4003C™ circuit laminates from Rogers Corp., one version with bare copper conductor and the other with ENIG finish. Broadband measurements were made on all four circuits from 10 MHz to 50 GHz with a commercial vector network analyzer (VNA). Significant differences were found between the ENIG finished and bare-copper circuits for both circuit types, although with greater losses exhibited for the GCPW circuit with ENIG finish.
For example, for the microstrip circuits, very little difference in conductor loss was apparent between the unplated (bare-copper) and ENIG circuits though about 2 GHz, although the higher losses of the ENIG circuit became more noticeable above that frequency. Compared to the bare copper circuit, the microstrip with ENIG measured about 0.25 dB higher in loss at 10 GHz, increasing to more than 0.5 dB higher at 40 GHz. The loss difference between GCPW circuits using bare copper conductors and using ENIG finish are even more dramatic, with about 0.5 dB more loss at 10 GHz and greater than 1 dB more loss at 40 GHz. The measurements clearly show the frequency dependence at higher frequencies but also how a particular circuit type can also contribute to the impact that a plated finish can have on a circuit.
This blog is based on a presentation by John Coonrod during the MicroApps sessions at the 2017 IEEE International Microwave Symposium (IMS). His presentation, “The Impact of Final Plated Finishes on High Frequency Performance of PCBs,” provides details on the types of circuit finish offered by Rogers Cop. with their high-performance circuit materials. More information on these circuit finishes is available by visiting the Rogers Corp. website at www.rogerscorp.com.
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