What is in this article?:
Printed-circuit-board (PCB) materials with high dielectric constants can help miniaturize microwave circuits, but engineers using these materials must be aware of the impact of loss and dispersion on their designs.
The Economical Approach
In some cases, a hybrid multilayer PCB configuration may help to shave costs. For example, when only a few layers of a multilayer circuit construction are critical to microwave performance, lower-cost materials such as FR-4 may form some of the layers along with the microwave materials needed for the higher-performance circuit layers. Hybrid multilayer PCB constructions may also incorporate different circuit layers to help adjust the overall coefficient of thermal expansion (CTE) of the assembly to improve the thermal reliability.
Polytetrafluoroethylene (PTFE) circuit materials offer excellent microwave performance, but suffer from high CTE properties. These materials can benefit when combined with circuit materials having lower CTE values, allowing the overall CTE behavior of the multilayer circuit assembly to be adjusted to yield a circuit that is thermally robust and reliable.
This hybrid PCB material approach might also serve to miniaturize the patch antenna while also achieving low loss with its feed lines. Figure 1 shows a cross-sectional view of a hybrid multilayer PCB that could be used to fabricate a miniature patch antenna radiating element with low feed line insertion loss. Admittedly, this is a crude illustration of this example. It merely serves to show that low-loss, low-Dk circuit material is used for the standard stripline transmission line serving as the antenna element feed line. The low Dk material allows a wider conductor for the feed line, with lower insertion loss.
Of course, there are always tradeoffs with any design approach. A hybrid circuit construction can be more expensive than a homogenous PCB in some cases (and less expensive in others) with regard to fabrication costs. For example, if a ceramic-filled PTFE material is used as the high-Dk layer and the rest of the circuit construction is low-Dk, low-loss hydrocarbon-based microwave materials, the combination can result in some fabrication cost issues. The materials are compatible, but to ensure good plated-through-hole (PTH) reliability, viaholes must be specially treated for this combination of materials. It does not add a significant cost penalty, and the desired performance results in this case can be achieved with a homogeneous material construction.
Based on commercial materials, RO4000™ circuit materials from Rogers Corp. include RO4350B™ laminate and RO4450F™ prepreg materials that could be used for the stripline feed line, while RO4360G2™ laminate can serve as the high Dk material. All of these materials have the same base substrate system, are very compatible, and use the same circuit processing approaches, so that they usually do not incur added fabrication costs. The RO4360G2 material has a Dk of 6.4; the RO4350B laminate and RO4450F prepreg have Dk values of 3.66 and 3.52, respectively.
For a further reduction in antenna element area, a circuit material with higher Dk value can be used, such as RO3210 material with a Dk of 10.8. It is a ceramic-filled PTFE material and will require different PTH preparation than the RO4000 materials. However, a simple plasma process can be used to avoid higher fabrication costs.
In addition to antennas, many high-frequency circuit applications can benefit from the use of higher Dk circuit materials. Microwave power amplifiers have traditionally used materials with a Dk range of about 3 to 4, and maintaining low insertion loss has always been one of the important reasons behind this. A few years ago, a large-volume microwave power amplifier was designed and produced with ceramic-filled PTFE material having a Dk of 6.5. Size reduction was an important initiative for this amplifier, but low insertion loss was also achieved.
Although the conductor features were reduced in width according to the Dk of 6.5, which can result in higher conductor loss, the ceramic-filled substrate material is a low-loss dielectric material which contributed to low overall insertion loss. A 25-mil-thick (0.635-mm-thick) substrate was used, and conductors were still relatively wide even at the target 50-Ω impedance. Because such a thick laminate will not be dominated by conductor loss in the manner of a thinner circuit material, it was possible to achieve good insertion-loss characteristics with this amplifier even while also achieving the size reduction.