December 20, 2010

John Coonrod is a Market Development Engineer for Rogers Corporation, Advanced Circuit Materials Division. John has 23 years of experience in the Printed Circuit Board industry. About half of this time was spent in the Flexible Printed Circuit Board industry doing circuit design, applications, processing and materials engineering. The past ten years have been spent supporting circuit fabrication, providing application support and conducting electrical characterization studies of High Frequency Rigid Printed Circuit Board materials made by Rogers. John has a Bachelor of Science, Electrical Engineering degree from Arizona State University.

Microstrip or stripline? That choice has been faced by high frequency designers for decades. Both transmission-line technologies are widely used in both active and passive microwave circuits, with excellent results. Is one approach better than the other? Before tackling such a question, it might help to know how each transmission-line technology works and what kind of demands each place on a printed circuit board (PCB) material.

Microstrip is a transmission-line format in which the conductor is fabricated on a dielectric substrate which itself has a bottom ground-plane layer. Conductors are usually formed by etching away unwanted metal from a conductor layer, such as copper.

 

Stripline is often compared to a flattened coaxial cable in that, like the cable, it consists of an inner conductor completely surrounded by dielectric material which is itself surrounded by a ground braid or foil. Of course, stripline circuits are planar, so that they appear as a sandwich of conductors in the middle, surrounded by dielectric layers, which in turn have parallel ground planes on the top and bottom.

 

Stripline circuits are usually fabricated by adhesively bonding a top-layer dielectric substrate/ground plane or prepreg with a single metal layer to a PCB laminate material on which circuitry has been photo-etched. To avoid unwanted propagation modes in stripline, the two ground planes must be shorted, often by means of shorting screws, or in the case of PCB technology, plated through hole via’s.

Why choose one transmission-line format over the other? Both provide excellent electrical performance through millimeter-wave frequencies, depending upon the choice of PCB materials. Microstrip circuits are easier (and less expensive) to fabricate than stripline, with less processing steps and easier placement of circuit components. The stripline format affords more isolation between adjacent circuit traces, supporting more densely integrated circuits than with microstrip. Stripline circuits are also well suited for fabricating multilayer circuits, with good isolation between layers. The layers are interconnected by means of plated through holes (PTHs).

In both microstrip and stripline, the electrical behavior of the conductors are affected by the relative dielectric constants of the insulator materials as well as the proximity of the ground planes. In microstrip there is one ground plane, while in stripline, there are two. In microstrip the effective dielectric constant impacting the impedance of a conductor is a combination of the relative dielectric constant of the insulator material as well as that of the air above the circuit (which is equal to 1). In stripline the effective dielectric constant is a combination of the relative dielectric constants of the substrate layers above and below the conductors.

For any high frequency circuit, maintaining controlled impedance across a PCB panel is critical to achieving consistent electrical performance in terms of amplitude and phase responses. The impedance of a conductor in either transmission-line format is a function of the width of the conductor, the thickness of the conductor, the thickness of the dielectric substrate, and the relative permittivity or dielectric constant of the substrate, among other things. In stripline it is not critical that a center conductor be equally spaced between the two ground planes, or that the insulators above and below the conductor have the same dielectric constant (as is the case with microstrip).

Because of the second ground plane, the width of a 50-Ω (or any given impedance) line in stripline will be narrower than for a conductor with the same impedance in microstrip. While the inherently thinner lines support greater circuit densities, they also require tighter fabrication tolerances as well as substrate materials with extremely consistent dielectric constant across a board. For a single-ended (unbalanced) transmission line in microstrip, dielectric losses (defined by a substrate’s dissipation factor) will be less than for stripline, since some of the field lines in microstrip are in air where the dissipation factor is negligible.

Of course, the performance available from either of these transmission-line formats is only as good as the performance of the dielectric substrates upon which they are fabricated. Just as the use of a PCB material such as FR-4 will cut costs but limit performance, choosing materials optimized for different microstrip and stripline applications can better take advantage of the benefits of each transmission-line format.

For example, microstrip circuit designs can benefit from a PCB material with a combination of low dielectric constant and low loss, such as RT/duroid® 5880 from Rogers Corporation. It is based on polytetrafluoroethylene (PTFE) dielectric for minimal loss (a dissipation factor of only 0.0004 at 10 GHz), but reinforced with glass microfibers for enhanced mechanical stability compared to pure PTFE. The dielectric constant is low and tightly controlled, 2.20 ± 0.02 at 10 GHz, in order to maintain the impedance of conductive traces fabricated in microstrip circuits. Of course, premium performance is not inexpensive, and the material requires tightly controlled processing steps to achieve consistent yields.

For microstrip circuits requiring an even lower dielectric constant, but where dielectric loss is less critical, Rogers developed RT/duroid 5880LZ laminate, consisting of PTFE with a unique filler material. It exhibits a dielectric constant of 1.96 ± 0.04 at 10 GHz with typical dissipation factor of 0.0019 at 10 GHz. Rogers RO3003™ dielectric materials use ceramic loading for stability, and are easier to fabricate for microstrip circuits than more costly RT/duroid 5880 materials. The tradeoff is in higher dielectric constant, 3.00±0.04 at 10 GHz, and increased dielectric loss, 0.0013 at 10 GHz, although they are nonetheless usable at frequencies through 30 to 40 GHz.

With the “layered” nature of stripline circuits, a dielectric “system” rather than a single material is usually selected for a particular application, such as the combination of RO4350B™ laminate and RO4450F™ prepreg from Rogers. Circuits are etched on the RO4350B dielectric material, which is metalized on both sides. The RO4450F prepreg is used as a bonding layer, which is metalized on only one side, forming the top dielectric layer and top ground plane. The RO4350B material is similar to FR-4 materials in terms of cost and ease of processing, although with higher dielectric constant, 3.48 ±0.05 at 10 GHz, and dielectric loss, 0.0037 at 10 GHz, than the materials mentioned above for microstrip circuits.

Fabricated from glass-reinforced hydrocarbon and ceramic laminate materials (not PTFE), RO4350B laminate provides excellent dimensional stability. Its companion, RO4450F prepreg material, has a dielectric constant of 3.52±0.05 at 10 GHz and dissipation factor 0.004 at 10 GHz, both parameters closely matched to those of RO4350B laminate. Two or more plies of prepreg are typically used in stripline circuits, especially in applications requiring high isolation between circuit layers. As mentioned earlier, the dielectric constants and thicknesses of stripline layers can differ, although these factors should be accounted for during any design review process.

As with many engineering decisions, a choice between microstrip and stripline involves considering a number of tradeoffs. The high circuit density of stripline, for example, requires more material layers, processing time and expense, and attention to detail than for microstrip circuits operating at the same frequencies. In either case, a variety of PCB materials are available to help a designer achieve the best performance for a given design, whether it is based on microstrip or stripline technology.