Another benefit in using the higher-Dk ceramic-filled PTFE material for the power amplifier was the high thermal conductivity of the material. Thermal conductivities for PCB laminates are typically in the range 0.2 to 0.3 W/m/K, although some of the high Dk materials have thermal conductivities above 0.5 W/m/K, which is considered good. The ceramic-filled PTFE material with Dk of 6.5 used for the power amplifier had a thermal conductivity value of 0.72 W/m/K. Combined with a well-designed heat-sink configuration, this allowed the circuit laminates to effectively channel heat from the power amplifier’s active devices to the heat sink.

To better understand the thermal conductivity properties of microwave PCB materials, a study was conducted using several microwave laminates with a surface-mount resistor soldered to each. A DC current was used to heat the resistor and thermal imaging was performed on the different laminates. The same circuit structure was used in all cases: a 20-mil-thick microstrip circuit on 1-oz. copper, attached to a heat sink, with the only difference in each case being the substrate material. Materials with significantly different thermal conductivities were fed the same DC power levels, and Fig. 2 shows the thermal images for three of these circuits.

 Benefit From High-Dk Microwave Circuit Materials, Fig. 2

In all three cases, the temperatures of the resistors rose above the ambient temperature, which was about +75°F. The PTFE woven-glass (WG) circuit material at left, with thermal conductivity of 0.30 W/m/K, had the highest rise in temperature, compared to the RO3006 laminate in the middle with thermal conductivity of 0.72 W/m/K and the RO4360G2 circuit material on the right with a thermal conductivity of 0.80 W/m/K. There are many considerations when dealing with thermal management issues, and some of these considerations are covered in a prior report.1

Along with couplers and other passive components using PCB technology, there are many microwave filter applications that can benefit from high-Dk materials. Microwave filters can be reduced in size with high Dk materials but can also gain significant performance benefits. A good reference which suggests the use of high-Dk materials for filters2 shows how these materials are used to improve coupling and filter performance for mostly bandpass filters, even though lowpass and highpass filters are covered as well.

It is not unusual for a microwave circuit structure to have multiple passive components as part of its PCB design. It might include filters, couplers, impedance tuning structures, and transmission lines to connect the different circuit functions. As an example of one of these components, edge-coupled filters with half-wavelength resonators are often designed using microstrip circuit technology. Such filters can be large with lower-Dk circuit materials, but the size can be reduced through the use of a high-Dk material. The higher-Dk materials can also provide better filter performance due to the materials’ concentrated electric fields which help improve coupling between resonators.

To evaluate the impact of circuit material Dk on such filters, two microstrip edge-coupled bandpass filters were designed with the same criteria, with one implemented on circuit material having Dk value of about 3, and the other on circuit material with Dk value of approximately 10.8. Both were 25-mil-thick (0.635-mm-thick) materials with 0.5-oz. copper. The design criteria included a center frequency of 2.5 GHz, a bandwidth of 150 MHz using a Chebyshev transfer function, and passband ripple of 0.1 dB. Sonnet Software V13.54 was used to perform the EM simulations.