Thin can be a good thing for high-frequency circuit laminate materials. As this ROG Blog detailed some years ago (see “Thinner Materials Help Target Higher Frequencies,” http://mwexpert.typepad.com/rog_blog/2010/11/thinner-materials-help-target-higher-frequencies.html), thinner printed-circuit-board (PCB) laminates offer many electrical benefits as well as mechanical advantages compared to thicker circuit materials, especially at higher frequencies reaching into millimeter-wave bands. For applications where weight and size are critical, such as circuits for portable and mobile products, thinner circuit laminates are important starting points that can lead to miniature, lightweight solutions. In terms of electrical performance, thinner laminates offer many benefits over thicker circuit materials, in particular for microstrip circuits operating at millimeter-wave frequencies.
Microstrip is one of the most widely used transmission-line technologies for RF/microwave circuits. As the ROG Blog has noted many times, microstrip circuits are highly dependent upon the choice of circuit laminate for optimum performance. The best microwave performance results from the right mix of circuit material parameters, such as consistent dielectric constant (Dk) and high-quality copper conductor layers, right down to the thickness of the laminate. Ideally, microstrip circuits fabricated on the right laminate composition and thickness will achieve excellent electrical performance with low loss and minimal unwanted resonances or spurious signals.
However, thicker circuit laminates can pose problems for microstrip circuits, with the thickness measured as substrate only and without the thickness of the copper included. When microstrip circuits are fabricated on thicker laminates, unwanted resonances can occur. These resonances arise between a laminate’s metal layers and can disrupt the desired signal propagation of the quasi-transverse-electromagnetic (quasi-TEM) waves through the microstrip transmission lines.
Excessive conductor width can also be a concern when attempting to minimize spurious generation in microstrip circuits. If microstrip signal conductors are wider than one-eighth wavelength at the design frequency, resonances can occur between the edges of the conductors. These spurious-mode resonances can interfere with the desired signal propagation through the microstrip conductors. Since wavelengths shrink with increasing frequencies, attention must be paid to circuit structures and circuit material dimensions to avoid such opportunities for spurious generation. At millimeter-wave frequencies (above about 30 GHz), in particular, where the wavelengths become extremely diminutive, careful balance is critical between circuit laminate thickness and circuit dimensions for optimum circuit performance.
Those smaller wavelengths call for thinner laminates to minimize any opportunities for spurious signal generation. At the same time, narrow circuit conductors can help prevent any generation of edge-to-edge conductor resonances. At higher frequencies, microstrip conductors are typically designed and fabricated for a controlled impedance, such as 50 Ω, to achieve signal transference with low losses and minimal reflections. The consistently narrow conductor widths required to achieve a controlled impedance across a PCB also provide the circuit physical conditions needed to minimize edge-to-edge conductor resonances.
As noted in the earlier ROG Blog, the Dk of the circuit laminate also plays a role in determining the circuit dimensions required for a particular design impedance, including the conductor widths. For a given laminate thickness, design frequency, and microstrip impedance, the circuit dimensions will shrink with increasing value of Dk. As a result, circuit miniaturization can be achieved by designing and fabricating microstrip and other transmission-line technologies on circuit laminates with higher Dk values.
Thinner circuits offer benefits in terms of controlling electromagnetic interference (EMI). As microstrip circuits increase in frequency, they also tend to radiate more EM energy. When the level of radiated EM energy becomes excessive, it can interfere with the proper operation of the circuit from which it originates as well as any circuits nearby. When compared at the same high operating frequency, thinner microstrip circuits will radiate less EM energy than thicker circuits, so that thinner circuits have the potential for less EMI problems. Less radiation loss also equates to less signal loss for a microwave circuit.
Microstrip is a practical and straightforward transmission-line approach for many high-frequency circuit designs, but it may not always be the best choice for all designs, especially those sensitive to the effects of spurious signals and radiation. Grounded coplanar waveguide (GCPW) is an alternative transmission-line technique that has proven effective for minimizing spurious modes and EM radiation. It can be used with thicker circuit laminates, although better results can be achieved with thinner circuit materials. When comparing microstrip and GCPW for the same circuit material and material thickness, GCPW circuitry has much less spurious generation and suffers much less EM radiation than microstrip circuitry for the same operating frequency.
The choice of transmission-line technology and circuit laminate thickness at higher frequencies can also be influenced by whether or not dispersion is a concern. Dispersion is a characteristic of transmission lines and circuit substrate materials in which different transmission lines may exhibit different group velocity or group delay with frequency, essentially with the smaller waves of higher frequencies slowing down as a result of the transmission lines. For narrowband circuits, dispersion is not a problem. But it can be problematic for broadband circuits, for longer circuits (with longer delays), and for pulsed waveforms, since the time for a high-speed pulse to travel through one type of transmission line will not be the same as for a transmission line with longer group delay. Transmission lines differ in their dispersion characteristics: microstrip and some types of waveguide suffer longer group delays compared to nondispersive transmission-line formats like stripline and GCPW.
For higher-frequency circuits, GCPW can minimize dispersion compared to microstrip, but it can also be more challenging to manufacture at higher frequencies, especially with the fine dimensions and circuit features required for millimeter-wave frequency operation. GCPW is more sensitive to the copper plating thickness variation due to the PCB fabrication process than microstrip, and can suffer circuit-to-circuit performance variations in insertion loss and phase response as a result of variations in laminate copper plating thickness. The inherent advantages of GCPW over microstrip in terms of dispersion characteristics can be nullified unless a circuit with tight tolerance in copper plating thickness is specified, along with tightly controlled Dk and overall laminate thickness. Thinner circuit materials can provide many benefits, provided that the tolerances of those circuit laminates are tightly controlled.