Dielectric constant (Dk) is a key parameter to consider when choosing a microwave printed-circuit board (PCB). But what microwave circuit designers may not always appreciate is the “choice within a choice” with some PCB materials, or when it might make sense to select a circuit material with a higher Dk value. High-Dk circuit materials can make it possible to miniaturize high-frequency circuits beyond what is possible with lower-Dk circuit materials. Understanding where high-Dk circuit materials fit within an RF/microwave designer’s toolkit can provide engineers with a great deal of flexibility when developing both active and passive high-frequency circuits.
Several recent blogs took a closer look at the meaning and importance of PCB Dk, without exploring the use of higher-Dk circuit materials. See “Getting the Low Down on Dielectric Constant,” and “Digging Deeper into Dielectric Constant for PCB Materials.” More and more, designers are using higher-Dk circuit materials for active circuits, such as power amplifiers, and passive circuits, such as antennas, for reduced size and to take advantage of some of the other characteristics of high-Dk circuit materials. The dimensions of a circuit to support a particular wavelength/frequency decrease as the Dk of the circuit-board material increases. But using a higher-Dk material in place of a low-Dk material is not just about going smaller, since the electromagnetic (EM) properties of the different materials must also be considered for a given circuit design.
“High” Dk is a relative term, of course, and what is defined as a “high-Dk” circuit material may vary across different circuit-material manufacturers. Since materials with relatively low Dk values are typically used for RF/microwave applications, a Dk value of 6.0 is often used as the lower threshold for high-Dk RF/microwave circuit materials.
Shrinking a circuit, whether active or passive, can help many applications. Using a high-Dk circuit material can also impact the EM wave properties of the circuits on that material by slowing the phase velocities of the EM waves, condensing the electric fields and the power handled by the circuits, improving coupling, and reducing higher-order modes and radiation losses. Of course, higher-Dk circuit materials can also lead to potential issues with dispersion and transmission lines with higher impedance values, which may not always be desired.
Simulations Compare Size Reductions for Different Dk Values
To get a handle on the impact of higher-Dk materials, a number of simulations on different types of circuits were performed with Sonnet Professional EM simulation software from Sonnet Software. The simulations were based on RF/microwave circuit materials of the same thickness, with Dk values of 3.0, 6.5, and 10.8, and they were run for patch antenna radiating elements based on 50-? impedance. The low-Dk material (Dk = 3.0) was used as the reference in terms of antenna radiating element area to achieve a given center frequency, such as 2.5 GHz. The simulation results projected a better than 40% savings in size for the patch antenna element with the Dk = 6.5 material, and a better than 60% reduction in area for the patch antenna element with the Dk = 10.8 circuit material.
As an example of a high-Dk material, RO3210™ laminate from Rogers Corp. is a ceramic-filled PTFE material with a Dk of 10.8. It should be noted that, as these patch antenna elements shrink, the feed lines needed to get signals to and from the antenna elements must also reduce in size (and can suffer higher loss when fabricated on higher-Dk materials), which may require design strategy if the feed lines are fabricated on a separate circuit material than the patch antenna elements.
Higher-Dk circuit materials can also help miniaturize RF/microwave filters, such as microstrip edge-coupled bandpass filters. To predict the size reductions, the Sonnet simulation software was again employed to simulate two Chebyshev bandpass filters designed with the same criteria, including a 150-MHz bandwidth centered at 2.5 GHz, except that one filter was built on circuit material with Dk of about 3 and the other on circuit material with Dk of around 10.8. The simulations were based on 25-mil-thick (0.635-mm-thick) circuit materials with 0.5-oz. copper. Little time was spent on optimizing the filters, although the simulations were performed with the goal of achieving the same electrical performance for the two filters. The center frequencies and bandwidths were within a few percent of each other. The main difference between the two was about a 37% reduction in PCB area for the filter designed on the Dk = 10.8 material.
Reduction in size can also be important for active circuits, such as power amplifiers, although design considerations when choosing a circuit material can be somewhat different than when sorting through circuit materials for passive components, such as antennas and filters. Microwave power amplifiers have commonly been fabricated on circuit materials with Dk values of 3 to 4, which are materials capable of low insertion loss, a parameter critical for achieving high performance in high-frequency amplifiers. Dramatic size reductions in amplifiers can be achieved by using a circuit material with a higher-Dk value, such as Dk = 6.5, although designers should also be aware of the importance of maintaining low insertion loss when choosing a higher-Dk circuit material for a power amplifier. Another key higher-Dk circuit material property to consider for a power amplifier is thermal conductivity, since any reduction in an amplifier’s size means a higher power density for the amplifier PCB and its associated components, such as heat sinks.
Hybrid Multilayer PCBs
Circuit designers may sometimes find that it is advantageous to fabricate hybrid multilayer PCBs formed with circuit materials of different Dk values. By “customizing” the Dk values of the circuit materials to different portions of a circuit design, such as amplifiers and filters, a certain level of control is achieved over the size of the components in the overall circuit or system. And by working with different circuit materials, each material can be specified for other performance characteristics, such as thermal conductivity or loss, as needed by that part of the circuit. A multilayer PCB can be “tailored” in terms of its choice of materials, with lower-Dk circuit materials used where appropriate and higher-Dk circuit materials serving to miniaturize active and passive circuit functions where needed.
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.