Stripline is one of the transmission-line options facing high-frequency circuit designers, especially for circuits where minimal electromagnetic (EM) radiation is important. Stripline can be thought of as a flat conductor suspended between two ground planes, with dielectric material separating the conductor from the ground planes. The configuration results in less EM radiation than circuits with microstrip transmission lines, with greater isolation between adjacent circuit traces compared to microstrip, but in a buried configuration that is more difficult to fabricate and service than microstrip. In spite of the challenges, the high performance possible has encouraged designers to implement high-frequency multilayer stripline constructions in extremely compact configurations. The choice of printed-circuit-board (PCB) material can contribute a great deal to the success of a single-layer or multilayer stripline circuit assembly.
Stripline circuits can be assembled in numerous forms, including as balanced, offset, and suspended stripline versions. The characteristics of the circuit-board material, such as its dielectric constant (Dk), can contribute much to the performance and behavior of these different stripline versions. Stripline circuits have even been fabricated with different dielectric materials (with their different Dk values) on either side of the conductor, for added design flexibility.
Ground and signal paths in stripline are typically created by forming plated viaholes through the conductive and dielectric materials, and the use of plated viaholes also lends itself to forming signal paths in multilayer circuit constructions. The EM fields within a stripline circuit assembly are strongly contained near the center conductor and the top and bottom ground planes of each layer, and it is important to closely match the top and bottom ground planes to the same potential to prevent propagation of any unwanted parallel-plate modes between the two ground planes. Ideally, high-frequency EM signals are contained entirely within a stripline PCB, with no leakage or emissions and with excellent shielding against spurious signals.
Stripline is considered a transverse-electromagnetic (TEM) medium in contrast to microstrip, which is a quasi-TEM medium. Most stripline designs aim for a characteristic impedance of 50 ?, which is determined by the width of the conductive strip, the thickness of the circuit substrate material, and the dielectric constant of the substrate material. For a 50-? characteristic impedance, and a given thickness of circuit dielectric material, a stripline circuit will employ a narrower conductive strip than a microstrip circuit, and it will suffer greater loss through the dielectric material than microstrip, which uses the lossless air above the circuit in part for its signal propagation. For a comparison of stripline and microstrip, please visit the December 20, 2010 ROG Blog: “Microstrip Versus Stripline: How To Make The Choice” (http://mwexpert.typepad.com/rog_blog/2010/12/microstrip-versus-stripline-how-to-make-the-choice.html).
In terms of selecting circuit materials for stripline, a smooth copper conductor surface is important for minimizing loss and the condition of the copper conductor is often overlooked when assessing the loss characteristics of a PCB. A copper conductor with rough surface will exhibit more loss than a copper conductor with smooth surface, so circuit materials intended for low-loss circuits should include smooth copper conductor surfaces. Since stripline has four copper-substrate interfaces, in addition, there is the opportunity for the roughness of the copper surface to be inconsistent across the different interfaces, resulting in differences in the conductor loss characteristics across the circuit board. Particularly in thin stripline circuits (where the ground-to-ground plane separation is 20 mils or less), copper conductor surface roughness can be a major contributor to the insertion loss of the circuit.
Stripline circuit fabricators may pay great attention to using a substrate with smooth conductive copper for the circuitry but may use a copper foil with much rougher surface for the other conductive layer. The rougher surface helps achieve increased bond strength when bonding the different circuit layers, resulting in a reliable multilayer circuit assembly. Unfortunately, it sacrifices some of the electrical performance possible through the use of the smoothest possible surfaces for all copper surfaces.
This difference in the surface roughness of the copper layers can impact the performance of offset stripline circuit assemblies, where the signal is not in the geometric center of the cross-sectional view of a circuit board. In such an assembly, the surface roughness of the copper plane that is closest to the signal plane will have the greatest effect on the circuit’s electrical performance. If this copper layer has the rougher surface, it can have an impact on the loss of the circuit. Even if the circuitry is fabricated on a copper layer with extremely smooth surface, the presence of another copper layer with rougher copper surface that is closer to the signal plane can override any benefits from the smooth copper surface used for the circuitry.
Stripline circuits are generally thought to be nondispersive in nature, but this may not always be the case. Circuits that incorporate a bonding layer with Dk that is different than the rest of the stripline structure may exhibit dispersive characteristics. To achieve a nondispersive circuit structure, the Dk should be fairly close throughout the stripline circuit. As examples, the RT/duroid® 6002 or RO3003™ circuit laminates are closely matched in Dk with the 2929 bondply materials, all from Rogers Corp. (www.rogerscorp.com). This consistency in Dk throughout a stripline circuit structure based on these materials will minimize dispersion.
The variations in stripline circuits include offset stripline and suspended stripline. Offset stripline can be formed by gluing together two stripline substrates of unequal heights. Suspended stripline makes use of air as part of the dielectric material, supporting pure TEM mode propagation. It is usually a circuit suspended within a metallic structure, with the entire structure enclosed. For low loss, suspended stripline technology has been used with the same conductive circuit pattern on both sides of the circuit assembly, electrically connected by plated viaholes. The entire assembly is then protected within a metallic enclosure with air cavities top and bottom serving as air substrates, where the lids are the ground planes. Suspended stripline circuits are capable of wide bandwidths with low loss and minimal spurious radiation, but assembly can also be complex and expensive.
In terms of propagation speed, stripline in its various forms will be slower than microstrip. That is, the propagation time through a microstrip circuit will be a fraction of the propagation time required for a similar stripline circuit on the same substrate material. Microstrip benefits from the use of air as a dielectric while stripline’s propagation characteristics are based solely on the dielectric material surrounding the signal trace. Propagation delays will increase in both cases with increased value of Dk for the dielectric material.
Stripline circuits in their various forms represent numerous design tradeoffs, including the difficulty of assembling, accessing, and testing stripline’s buried circuit traces versus the benefits of minimal signal leakage and only minor effects from external interference. Stripline may suffer somewhat higher losses than a microstrip circuit fabricated on a similar circuit material, but the stripline circuits will also be less affected by external interference signals and will exhibit less radiation of its own.
Do you have a design or fabrication question? John Coonrod and Joe Davis are available to help. Log in to the Rogers Technology Support Hub and “Ask an Engineer” today.