It is becoming increasingly difficult to define a frequency at which it becomes necessary to transition from FR-4 types of material to high-frequency materials for circuits. This is because different technologies now accept various performances from a laminate. For instance, with enhanced signal processing one can obtain performance at higher speeds and frequencies from FR-4 material that was earlier thought possible. As a rule of thumb, high-speed digital applications at 10 Gbps will certainly need to use a high-frequency laminate.
In exceptional cases, some applications may demand a high-frequency laminate even if working at a lower data rate, as they require a very well controlled dielectric constant and low losses. As a general rule, insertion loss concerns preclude the use of FR-4 materials above 3 GHz in RF applications, especially where dielectric constant control is a critical concern, equally as important as controlling the substrate thickness.
Mixing High-Frequency Materials with FR-4
Contrary to popular belief, the microwave industry often mixes high-frequency circuit material such as PTFE (Teflon) with FR-4 material, as they have fewer compatibility issues. Such hybrid builds result in lower cost as the more expensive laminates are used in only those layers that need better electrical performance.
In addition, hybrid materials often improve the reliability of a circuit. With circuits built with several layers of PTFE, the high CTE or coefficient of thermal expansion of PTFE can be a matter of concern. The thermally stable FR-4 has a low CTE, and when used on non-critical layers, helps to offset the higher CTE of the PTFE layers. On the other hand, there are PTFE laminates with ceramic fillers that exhibit extremely good stability and low CTE values.
Fabricating High-frequency Circuit Materials
Fabricating with high-frequency circuit materials such as pure PTFE is indeed difficult, as the material is soft and does not readily accept bonding with copper. However, PTFE materials such as those with ceramic and hydrocarbon fillers make it easier for fabricators to handle these alternative high-frequency circuit materials. PTFE laminates are also available reinforced with woven glass. This negates several issues such as softness, dimensional stability, and handling.
In addition, the filled PTFE substrates show much better CTE, and fabricators find the PTH preparation process more forgiving. The most fabrication-friendly and recommended material for the microwave industry is the woven glass-reinforced ceramic-filled PTFE laminate. Laminates made of this material offer extremely good electrical performance as well.
Option of Different Dielectric Constants
RF applications need a larger variety of dielectric constants than digital applications do. Since the microwave industry works with frequencies varying from 300 MHz to 30 GHz, they often use the PCB circuit patterns as the microwave circuit component. For instance, the conductor pattern on the PCB may as well perform as a band pass filter rather than an extra component soldered onto the PCB.
The physical size of such simulated components depends on the wavelength at the frequency of interest. If the designer selects the dielectric constant of the circuit material to be very high, the size of the circuit would shrink to maintain the same properties at that wavelength. Since wavelength is related to dielectric constant, a higher dielectric constant is necessary when working with shorter wavelengths.
We at PCB Global can assist with any of your requirements, including the application of Teflon PCB’s. For more information on the application of Teflon PCB’s or to arrange a quote, please email email@example.com