Apr 21, 2021|General
Printed-circuit Boards (PCBs) are at the heart of the modern world, allowing electricity to flow through our devices in the way it needs to make them work. There are many of them in every computer, every radio, and even every digital clock.
There are many different types of PCBs, including single-sided PCBs, double-sided PCBs, rigid PCBs, flexible PCBs, and aluminum-backed PCBs. This article will focus on high-frequency PCBs, which are most often used in communications devices, microwaves, and similar devices. High-frequency PCBs are so-called because they operate in a frequency range between 500 MHz and 2 GHz.
In PCB manufacturing, there are many different options for the material that makes up the circuit board, each of which has advantages and drawbacks.
Why Do Materials Matter?
Table of Contents
- 1 Why Do Materials Matter?
- 2 The Importance of Correctly Selecting PCB Materials for High-Frequency Applications
- 3 Benefits of High-Speed Board Materials
- 4 Other Considerations When Using High-Speed Board Materials
- 5 Conclusions
The different materials on the market have varied mechanical, electrical, and thermal properties, making them appropriate for different high-frequency applications. This means that engineers and designers have to weigh many various factors when evaluating which high-frequency PCB material to use.
One has two major decisions to make when selecting the materials for a high-frequency PCB:
- The material of the substrate
- The material of the foil
The substrate is the material that encases the electrical components of a PCB. There are many different options for what the circuit’s base can be made out of, including epoxy resins, woven glass, and ceramics. When choosing the substrate material, high-frequency PCB design guidelines will tell you that it’s best to minimize the dielectric constant (Dk) of the material.
The second significant variable in material selection is the foil through which electrical currents run. The classic and most popular choice is copper foil, but this comes in many different forms nowadays. The options you’ll have include:
- Drum Side Treated Foil (DSTF)
- Reverse Treated Foil (RTF)
- Low Profile Foil (LP)
- Very Low Profile Foil (VLP)
- Standard Shiny Copper
These options come with different features and electrical properties that are taken into account during material selection. It’s also important to think about these components’ prices because some are significantly more expensive than others.
The Importance of Correctly Selecting PCB Materials for High-Frequency Applications
High-frequency PCB material directly impacts the signal integrity that flows through the circuit. An unsuitable or lower-quality material can therefore affect the reliability of a product, potentially leading to mixed signals and decreasing the shelf-life of a product.
There are specific high-frequency PCB design guidelines for ensuring that a circuit will work in an intended way. Guidelines are so important in this space because high-frequency PCBs can’t easily be tested in a breadboard system, which means manufacturers must produce more expensive prototypes before a circuit can be tested. This relative unpredictability makes the materials even more important in high-frequency applications.
Furthermore, one needs to consider that signal integrity requirements are higher in the domain of high-frequency PCB circuits. This makes the circuit board’s dielectric properties much more influential in whether or not the device meets the required level of signal integrity.
Since the standards for acceptable signal quality are much higher for high-frequency than other types of PCBs, every little thing needs to be considered (and the materials are not a little thing!).
Benefits of High-Speed Board Materials
Manufacturers have developed high-speed materials that are a great fit for high-frequency PCB applications. These materials avoid some of the pitfalls of the standard Flame Retardant Type 5 woven glass circuit substrates (FR-4). They don’t have the same voltage, heat, and power limitations, nor will there be an increase in the signal loss at high circuitry speeds.
Read about some of the positive benefits of high-speed board materials.
Materials like enhanced epoxy or PTFE (Polytetrafluoroethylene) don’t lose their Dk tolerance as easily as other PCB substrate materials. They usually perform to a standard of at least plus or minus 2% and sometimes do even better. If the controlled impedance is important to your application, one of the best choices you can make is choosing a high Dk tolerance material.
As we’ve discussed a couple of times in this article, it can be difficult to achieve stable signals in high-frequency PCB applications. Enhanced epoxy and PTFE have a much lower dissipation factor than standard FR-4. Without as much dissipation, the risk of signal loss will be much lower.
Engineers can sometimes think so abstractly that they forget about the physical limitations of their circuit boards. However, in high-frequency PCB applications, it’s crucial to remember that maintaining physical tolerance can be difficult. If you foresee mechanical stability being a problem, it’s worth considering thermoset hydrocarbon laminate because of its ability to withstand high temperatures.
One of the main obstacles to the longevity of PCBs is moisture buildup, so one should always consider how any material will impact it. Even a small amount of liquid has a huge impact on a high-frequency circuit’s electric performance. While a standard FR-4 will absorb around 50% of the surrounding moisture, the absorption rate of a material like PTFE can be as low as 2%.
Simply put, high-frequency operations mean that the PCB needs to contend with more heat. Materials like PTFE and other polyimides have excellent thermal management properties.
Other Considerations When Using High-Speed Board Materials
In engineering, it’s impossible to ever focus on just one thing. In this context, that means that more than just the PCB needs to be considered. A good designer will think about the many aspects of the entire product that the circuit will control.
For example, you might be able to use a high-frequency PCB material that conducts a lot of heat in some devices but needs to avoid such high temperatures in other devices for reasons of safety or consumer satisfaction.
This guide can only have scratched the surface of the incredibly complex decision of what materials are best for a high-frequency PCB application. However, the above high-frequency PCB design guidelines are a great first step to evaluating your options.