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The Rog Blog is contributed by John Coonrod and various other experts from Rogers Corporation, providing technical advice and information about RF/microwave materials.

CPW Can Minimize Conductor Profile Effects

July 31, 2012

Conductor surfaces on a printed-circuit board (PCB) can vary widely, from a “smooth-as-glass” finish to a more typically rough profile. As detailed in an earlier ROG Blog, a conductor surface profile can impact the performance of a sufficiently high frequency circuit, increasing insertion loss and dispersion and even causing propagation delays and changes in the effective dielectric constant of the circuit. Designers working with microstrip transmission lines are generally aware of these effects and often specify circuit substrate materials with smooth, low-profile copper conductors for optimum performance at high frequencies. But are all RF/microwave transmission-line technologies affected by conductor surface profiles in the same way? What about designs based on coplanar-waveguide (CPW) transmission lines, or stripline? Do they require circuit materials with low-profile conductors, or are they less affected than microstrip by conductor surface roughness?

Most engineering choices involve tradeoffs of some kind, and the same is true for a choice of high-frequency transmission-line technology. For double-sided and multilayer planar circuits, those choices typically include microstrip, stripline, CPW, and conductor-backed CPW (CBCPW). Stripline, for example, provides excellent performance, but is the most difficult to fabricate. It is essentially a signal conductor sandwiched between dielectric layers which are each topped by a ground plane. Factors such as a transmission-line’s width, the thickness of the substrate material, and the dielectric constant of the substrate materials will determine the characteristic impedance of a stripline transmission line. Fabricating a stripline circuit requires that the top and bottom ground planes be connected, typically by drilled via holes through the dielectric substrate materials. Because the signal trace is not exposed, component placement in a stripline circuit is more difficult.

In comparison, circuits based on microstrip and CBCPW transmission-line technologies are much easier to fabricate. Both have exposed signal layers, simplifying component attachment. Both microstrip and CBCPW start with a dielectric circuit board laminated on top and bottom with copper conductor layers. Microstrip circuits are formed by etching signal traces on one of the conductive layers, and using the other conductive layer as a ground plane. A CBCPW circuit has an even larger ground plane, etching signal patterns in a ground-signal-ground (GSG) configuration on one of the conductive layers, with the other conductive layer also serving as the ground plane. As with stripline, the ground planes are connected by drilled and conductively filled viaholes through the dielectric material. In a CBCPW circuit, the placement of the viaholes is a design element, and can impact the characteristic impedance of the circuit as well as its loss characteristics. But if the viaholes are properly positioned, a CBCPW circuit can operate at higher frequencies with a thicker circuit board than when using microstrip.

Microstrip may suffer less conductor loss than a CBCPW circuit across most of the microwave frequency range, but the conductor losses for CPW and CBCPW increase at a constant slope with increasing frequency. Microstrip exhibits a sharp loss transition at a frequency related to thickness of the circuit substrate and its relative dielectric constant and is associated with a rise in radiation loss. That transition frequency can be extended to a higher frequency when microstrip is paired with a CBCPW launch. In contrast, radiation losses for CBCPW can be minimized through proper line spacing and viahole placement.  

Much has been written about microstrip, and free calculators such as the MWI-2010 Microwave Impedance Calculator from Rogers Corp. (www.rogerscorp.com) are available to quickly determine the transmission-line dimensions required for different impedance values. Microstrip has been well modeled in circuit and electromagnetic (EM) software simulators, simplifying the design of microstrip circuits. But microstrip can also be victimized by the effects of conductor surface roughness. As frequencies increase, EM waves tend to propagate along the outer surface of a microstrip copper conductor, with current flowing at a “skin depth” that is dependent upon frequency, closer to the conductor surface as frequency increases. As the skin depth approaches the roughness of the conductor’s surface profile, the current essentially travels along a longer path, with a resulting increase in resistance and in signal attenuation. For designs where any additional loss is critical, microstrip circuit designers may specify a circuit laminate with low-profile copper conductor, such as RO4000® LoPro™ series circuit laminates from Rogers Corp.

With CPW or CBCPW circuits, the switch to a laminate with a low-profile copper conductor may not be necessary, or even reap the same rewards as with a microstrip circuit. Recent studies have shown that CBCPW is less susceptible than microstrip to the effects of conductor surface roughness at higher frequencies, for example, in terms of insertion-loss performance (read the article “Comparing Microstrip and CPW Performance,” in Microwave Journal, at http://www.microwavejournal.com/articles/17869-comparing-microstrip-and-cpw-performance for further details). One reason for this is that the current and EM field distribution within a CBCPW circuit tend to remain within its GSG layer, not along the surface of the copper conductor (where the roughness is present) as in microstrip. The result is less insertion loss for CBCPW attributed to conductor surface roughness than for circuits based on microstrip.

Where the impact of conductor surface roughness on propagation constant is a concern, CBCPW is also less affected than microstrip. CBCPW will also exhibit more predictable effective dielectric constant values with varying degrees of copper conductor surface roughness than microstrip, which can exhibit wide variations in the expected value of dielectric constant as conductor surface roughness varies.

CBCPW and CPW circuits are not perfect by any means. The performance of these circuits can be impacted by fabrication defects known as the conductor trapezoidal effect, where conductive traces are trapezoidal rather than rectangular as seen in a cross-sectional view. The trapezoidal conductor shape can cause elevated current density at the base of the conductor. Under this condition, a laminate with rough conductor profile can affect the loss and propagation constant of a CBCPW circuit compared to one with more ideal rectangular conductor shape and more even current distribution.

Designers committed to microstrip can gain performance advantages from using circuit materials with low-profile conductors. But for those willing to work with CPW or CBCPW circuits, the effects of rough conductor surfaces are less pronounced, and there is less to be gained by specifying a material with low-profile conductor. There are fewer impedance calculators for CPW and CBCPW than microstrip, but a growing number of EM simulators provide such capabilities for CPW and CBCPW circuits. For example, the three-dimensional (3D) planar EM tools available from Sonnet Software (www.sonnetsoftware.com) incorporate CPW and CBCPW impedance calculators as well as effective models for predicting the effects of conductor surface roughness on these circuits. The next blog will continue this examination of conductor surface roughness, but switch the focus to stripline and how it is affected by conductor surface roughness compared to microstrip and CBCPW circuits.

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.   

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