Finish Makes a Difference In Broadband PCB Loss
Plated finishes are necessary additions to printed-circuit boards (PCBs). Not only do PCB finishes provide smooth, solderable surfaces for attaching components, they also provide protection for a PCB’s copper conductors. Without such protection, a PCB’s conductive copper would quickly oxidize and deteriorate when exposed to the environment, resulting in degraded circuit performance. The added protection provided by a PCB’s plated finish means added loss, however. The choice of plated finish can make a real difference in a PCB’s conductive loss, especially for broadband, high-frequency circuits. To better understand the loss performance of different plated finishes, various transmission lines were fabricated on different circuit laminates and different plated finishes applied.
PCBs, with or without a plated finish, suffer losses that typically increase with increasing frequency. The losses that circuit designers measure on a microwave transmission line, such as microstrip, stripline, or grounded coplanar waveguide (GCPW), stem from a combination of signal losses from the PCB, including conductor loss, dielectric loss, radiation loss, and leakage loss adding up to form insertion loss. Circuit design also contributes to the loss performance: Achieving good impedance matching along transmission lines, at circuit junctions, and at component mounting points helps to minimize signal reflections and losses from those reflections, often measured in a circuit’s transmission lines as return loss.
Copper is an excellent conductor, with low insertion loss for transmission lines and cables formed from copper. But the copper on the dielectric materials of PCBs does not always offer the smoothest, most level surface for mounting miniature circuit components, such as those in
ball-grid-array (BGA) housings or tiny surface-mount-technology (SMT) packages. A plated finish can provide that smooth mounting surface for miniature components, and it can deliver long-term protection against copper deterioration. Some finishes also protect the plated through holes (PTHs) that serve as the electrical connections between different circuit layers in multilayer PCBs. Unfortunately, most PCB finishes come with a price, since most increase insertion loss to some degree, depending upon frequency and other factors, including the thickness of the substrate, the choice of transmission-line technology, and the layout of the circuit and how it is affected by the finish.
Most plated PCB finishes are less conductive than the copper conductors formed on the PCB’s dielectric material, and will suffer more loss than copper, especially at higher frequencies. The exception is silver, an excellent conductor, which is also expensive and usually applied in a very thin layer as a finish. PCB conductor losses are frequency dependent mostly due to the manner in which RF current uses a conductor. At lower frequencies, the RF current will use more of the conductor. At higher frequencies, the RF current tends to flow along the surface of the conductor, using only the outside skin of the conductor. Conductor loss rises as the RF current uses less of the conductor, and because of these skin effects at higher frequencies, plated finishes can have greater impact on PCB insertion loss at higher frequencies.
The impact of a plated finish on PCB insertion loss can also depend on the transmission-line technology. For example, for microstrip, with high current density along the edges of the conductor, the plated finish can have significant impact on conductor loss. For GCPW with current density distributed along the four edges of the ground-signal-ground conductor, the plated finish will have more impact on conductor loss.
Finding a Finish
A number of different plated finishes are available for high-frequency circuit boards, including electroless-nickel-immersion-gold (ENIG) finish, organic surface protectant (OSP), electroless nickel, electroless palladium, immersion gold (ENIPIG) finish, and soldermask finish. For example, for an ENIG finish, nickel is plated onto a PCB’s conductive copper, serving as a barrier between the copper and a thin layer of gold that is applied thereafter. The thin layer of gold, an excellent conductor, typically disappears and is absorbed into soldered connections as components are soldered onto the PCB’s transmission lines and conductive traces. As might be apparent from the materials used in this finish, it is expensive, but it is RoHS compliant and provides excellent protection for PTHs in multilayer circuit assemblies.
PCB finishes using OSP are popular as environmentally sound, “green” PCB treatments that are lead free and provide extremely flat mounting surfaces for components. This low-cost finish is applied by means of a chemical bath process and it is a very low-cost finish, but it is not well suited for PTH protection and there is no way to measure the thickness of the finish when trying to evaluate the reliability of the finishing process. Additionally, OSP is typically considered a temporary finish and not a permanent, final finish, although in an optimum environment it may have extended life. Soldermask is a polymer material that provides a protective coating for copper traces and prevents solder from making unwanted connections and short circuits.
How do the different plated finishes compare in terms of PCB conductor loss and insertion loss? By fabricating some circuits with different types of transmission lines on some standard PCB laminates and using different plated finishes, it was possible to compare the impacts of different finishes on insertion loss by means of measurements and computer simulations. For example, with microstrip and GCPW transmission lines on RO4003C™laminates from Rogers Corp. (www.rogerscorp.com), measurements revealed significantly less loss for microstrip with bare copper than for microstrip with an ENIG finish. However, measurements also revealed that more difference in loss existed for GCPW with bare copper than for GCPW with an ENIG finish.
When circuits were fabricated on different thicknesses (6.6, 10.0, and 30.0 mil thick) of RO4350B™laminates from Rogers Corp., the total insertion loss tended to be less for the thicker materials. Thinner circuits are dominated more by conductor losses than other losses and, for each plated finish evaluated, it added to the PCB’s conductor losses.
For yet another circuit material evaluated during these plated finish tests and simulations, 5-mil-thick RT/duroid®6002 circuit laminates using rolled copper from Rogers Corp., significantly higher conductor losses were found for microstrip circuits with ENIG plated copper conductors than for microstrip circuits with bare copper conductors, when tested at frequencies through 40 GHz. However, when the copper conductors for the same material were plated with immersion silver, little difference in conductor loss was found between microstrip circuits with bare copper and those with immersion silver plating, even for frequencies through 100 GHz (using a differential measurement method). For the same circuit material, little difference was found for microstrip circuits with bare copper conductors and with OSP copper conductors, even through 100 GHz. When soldermask was evaluated for this circuit material, microstrip circuits with bare copper conductors exhibited considerably less loss than copper conductors with soldermask.
In short, the lowest conductor losses, with microstrip and GCPW, are achieved using bare copper conductors. But it is not realistic to fabricate reliable PCBs with bare copper conductors, and plated PCB finishes provide much-needed long-term protection. As was discovered from measurements and simulations, all PCB plated finishes are not the same, with some suffering less loss than others. For measurements on high-frequency PCB materials through 110 GHz, circuits with bare copper conductors have the least conductor loss, followed by circuits with immersion tin, ENIPIG finish, and then ENIG finish.
Author’s Note: This ROG blog is based on a presentation at PCB West 2015 Conference & Exhibition (www.pcbwest.com), “Wideband Insertion Loss Testing of Multiple PCB Final Plated Finishes.” In that presentation, considerable broadband, high-frequency test data and computer simulations were compared for different finishes on several commercial circuit laminates from Rogers Corp.
Do you have a design or fabrication question? Rogers Corporation’s experts are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.