RF or Radio Frequency Printed Circuit Board manufacturing is a fast-growing sector within the PCB manufacturing industry. However, this sector covers RF PCB manufacturing for applications of regular radio frequency to Microwave frequencies. In broad terms, the RF PCB manufacturing industry considers RF boards to be any high-frequency PCB operating above 100 MHz. Again, within the radio frequency class, the RF PCB manufacturing industry considers any board operating at above 2 GHz as a Microwave PCB. The steps involved in an RF PCB manufacturing process include designing the boards for proper functionality, selecting materials for the most optimum performance, and following necessary processes during the actual fabrication of the board.
Difference between Traditional Boards and RF PCB Boards
Most RF PCB boards, including Microwave boards, are notoriously difficult to design when compared to traditional low-frequency PCBs. The major hurdle comes when processing radio signals during transmitting or receiving. Basic necessity is achieving tight impedance tolerances and high signal-to-noise ratio.
Depending on the application of the RF PCB, the designer must select materials that offer dependable chemical, thermal, and electrical characteristics for the high-frequencies involved. In addition, the RF PCB material must also be capable of withstanding environmental vagaries. Traditional boards made of glass epoxy FR-4 material exhibit high losses, and are not suitable for handling high frequencies.
Designing RF PCBs for Proper Functionality
Most designers consider RF board design as extremely difficult. However, using some basic guidelines makes the work easier. For instance, considering some key factors during the design is necessary when approaching an RF PCB manufacturing project:
Material constants, such as dielectric constants, are essentially single-value for low-frequency boards. However, for the range covered by RF boards, these constants no longer remain so, and usually exhibit a more dynamic behavior.
The soldering process during PCB assembly generates considerable heat leading to extensive thermal stress within the board. The thermal properties of the material play a critical role in maintaining the registration alignment of multilayer stacks during assembly.
RF circuits are prone to EMI/EMC issues. They interact with neighboring circuits and features in many unexpected ways. The designer must be aware of and implement proper spacing and layout to combat these issues.
Selecting the proper materials for the RF PCB manufacturing is an important basic requirement for the designer to consider. The final performance of the board for an intended target depends on the optimum selection of the materials.
Selecting PCB Materials for Optimum Performance
For a designer, the most critical decision for the RF PCB manufacturing process is selecting the right material. This decision is the key factor affecting the performance, manufacturability, and cost of the RF PCB. Designers consider several parameters when selecting materials for RF boards:
Dielectric Constant (Dk)
Although for low-frequencies, the dielectric constant is really a constant number, it is a dynamic variable when considering high frequency operations. Designers must evaluate the specifications of the material published by the manufacturer, and understand the testing methods they have used. They must look for values published by the manufacturer for the frequency range and operating conditions that closely match their target application.
Coefficient of Thermal Expansion (CTE)
CTE values tell a designer how the material size will change with differences in temperature. This is an important factor for the designer to consider when designing multi-layered boards. Alignment and registration between different layers in a board can be a problem if each layer grows or shrinks at a different rate than the layer above or below it. To mitigate such problems, mostly during assembly, designers prefer using materials with low CTE.
Materials tend to absorb more energy as the signal frequency goes up. While the material heats up because it is absorbing energy from the signal, the signal loses its amplitude and its waveform distorts, leading to a significant loss of signal integrity. The loss tangent factor of a material indicates the amount of energy the material will absorb at the operational range of frequencies of the application.
Depending on the environment the board will operate in, the designer may have to consider the moisture absorption quality of the material. Moisture ingress often leads to poor performance at high frequencies.
At low-frequencies, the entire cross-section of a conductor or trace carries the current. At higher frequencies, eddy current effects force the current to occupy only the outer periphery of the conductor’s cross-section. This leads to an increase in the resistance and inductance in the path of a high-frequency signal. The designer must be aware of such skin effect and compensate for it.
Commonly Used RF PCB Materials
Most materials making up the RF PCB boards are usually some combination of hydrocarbons, ceramic, PTFE, and different forms of glass.
Where quality of the RF PCB board is more important than the price of the material, designers use PTFE with woven glass or micro-glass fiber. However, this material has high CTE and is difficult to work with. If high quality is the demand, but there is a budget restriction, ceramic-filled PTFE is the answer. RF PCB manufacturing is easier with ceramic-filled PTFE, which decreases its cost.
As soldering during assembly stresses the RF PCB and components, thermal robustness of the PCB material is also an important factor besides price and electrical performance. Thermal robustness is also necessary for RF multi-layered boards that must undergo high amounts of drilling or meant for deployment in thermally demanding environment, such as in aerospace.
With excellent electrical properties, PTFE with woven glass or fiber has a high CTE. On the other hand, ceramic-filled PTFE has comparable electrical characteristics but low CTE. Thermally, this makes the latter a better choice. If slightly lower electrical characteristics are acceptable, one can also use ceramic-filled hydrocarbon, as this material also has a low CTE.
The introduction of woven glass with PTFE allows higher moisture absorption, as compared to ceramic-filled PTFE. Adding hydrocarbons to ceramic-filled PTFE causes the moisture absorption to rise to a small extent, but it makes a good choice for balancing cost and resistance to moisture.
Some designers use FR4 as a suitable material for RF PCB manufacturing, but this material is suitable for PCBs operating at the lower end of the RF spectrum.
With RF PCB boards and microwave PCB applications being multi-layered, designers can fine-tune their properties by matching and mixing various materials to augment the balance of thermal, electrical, mechanical properties and the cost.