Considerations When Selecting PCB Materials for High-Frequency Applications

Some of my favorite music is what is written for movies. If done correctly, the music will highlight and enhance what is happening on the screen without drawing attention to itself. It’s not just the volume or intensity of the music, it is also the tempo, instrumentation, and the key that the composer selects which will accent the story.

If the star of the movie is doing something heroic, there will often be a bright rapid brass fanfare in a major key. To convey a more somber mood though you will likely hear low slow tones from the strings and horns in a minor key. The music in a movie creates an emotional foundation that supports the entire story, and most of the time we don’t even notice it.

In a similar way, the materials that go into the construction of a high frequency circuit board also play an important supportive role. Where the performance of the board is typically seen as a drama played out between the component placement and trace routing, it’s the circuit board materials that provide the foundation for the story.

And just like the music in a movie, if the board materials are doing their job correctly they will, for the most part, go unnoticed. But if the wrong materials are used, lookout! It could create a real sour note in the performance of your PCB. The key then is in selecting PCB materials for high-frequency applications that will do the job that you need them to; supporting the performance of your high-speed design.

The Need for Correctly Selecting PCB Materials for High-Frequency Applications

The circuit board itself is a composite of non-conductive dielectric substrate core layers, and dielectric laminate layers. The copper foil that will become the metal traces and power planes of the board use the laminate layers as their base. Both the core and the laminate layers are made out of a variety of materials, and they are designed to meet specific dielectric constants and other property requirements. With the tighter signal integrity requirements of high-frequency circuits, the dielectric properties of the circuit board materials become a much larger part of the overall signal integrity equation.

The standard circuit board materials used throughout the years has been FR-4, which is a Flame Retardant type 4 woven glass reinforced epoxy laminate. This material is very cost effective and easy to work with making it ideal for PCB fabricators. It also is a very good electrical insulator and a sturdy material for printed circuit boards.

The problem with FR-4 is that it does have power, voltage, and heat limitations, and when your circuitry exceeds those operating limitations the dielectric properties of FR-4 will begin to deteriorate. This will create a condition where the insulating properties of FR-4 will begin to break down and it will start allowing current through.

Another problem that FR-4 has is that at higher circuitry speeds the dielectric constant will vary and the signal loss will increase. All of these behaviors may cause problems in high-frequency designs, and if that is a concern for your design you should investigate the high-speed board materials that are available.

For years FR-4 has been the standard material used in the fabrication of PCBs

The Benefits that High-Speed Board Materials Offer

By using high-speed materials in the construction of your circuit board, you can realize a number of benefits:

• Controlled Impedance: High-speed board materials such as enhanced epoxy or PTFE (Polytetrafluoroethylene) will hold their dielectric constant (Dk) tolerances at +/- 2% or better. For circuits that demand tight controlled impedance routing, this becomes a very important consideration.

• Signal Performance: Another important consideration is in the signal performance across the board. Signal loss becomes more of a problem as transmission line frequencies increase. Enhanced epoxy or PTFE materials can offer a much lower dissipation factor which in turn helps to reduce signal loss.

• Dimensional Stability: High-frequency designs need to maintain tight physical tolerances as well in both their manufacturing and use. Thermoset hydrocarbon laminate materials are a good choice for mechanical stability.

• Decreased Moisture: Another consideration is the moisture absorption rating of the materials used in the circuit board. Even a small amount of moisture can change the electrical performance of high-frequency circuits. Traditional FR-4 can have a moisture absorption rating as high as 50% while materials such as PTFE can be as low as 2%.

• Thermal Management: High-speed designs will often deal with more heat in their operation, and materials such as polyimides are very robust with excellent thermal properties. This also makes them a good choice for boards that are going to be used in harsh environments.

The above list highlights a handful of the benefits that high-speed board materials can give you. There are however other considerations as well.

Choose the right PCB materials for your high-frequency designs

Other Considerations of Using High-Speed Board Materials

Unfortunately, there is a price to pay for all of the benefits of these materials, and the first to consider is the actual cost. High-speed PCB materials are more expensive than regular materials. You need to look at the trade-offs between cost and performance and decide which is the better way to go. Another consideration is to figure out how much performance improvement your circuits really need. It may be that good ol’ standard FR-4, or one of its enhanced versions will do an adequate job for you.

Another consideration is that the high-speed board materials are often more difficult for the PCB fabricators to work with. Some of the PTFE laminates for instance, which have great electrical properties, require non-standard fabrication processes. This will also drive up your costs which must be factored into your overall decision as well.

The best thing that you can do is to consult your contract manufacturer before you start your PCB layout to determine which are the best material choices. Your CM can also help you to determine the appropriate board layer stackup strategy which they can send direct to your CAD tools through an IPC-2581 formatted file.

Another way that you can help your overall PCB design process is to use PCB design tools that are up to the challenges of working with high-frequency designs. OrCAD PCB Designer is an excellent tool for this work, and comes with the high-speed design constraints and IPC-2581 interfaces that you need to get the job done right.

If you’re looking to learn more about how Cadence has the solution for you, talk to us and our team of experts.