Introduction to High Speed PCB Design: Is FR-4 the Best Board Material Choice for High Speed PCB Design?

Introduction to High Speed PCB Design: Is FR-4 the Best Board Material Choice for High Speed PCB Design?

Is FR-4 the best board material choice for high-speed PCB design?

We've been there at one time or another when we were tasked with a project that was beyond our comfort zone. For me, that was the day my boss asked me to design a high speed board. While I consider myself an experienced circuit designer, I know that high-speed PCB design has many constraints that are not typically encountered when designing an average circuit. Initially, I took the time to make schematics suitable for high-speed designs; however, once that was done, I was completely focused on understanding whether my high-speed PCB prototype should use FR-4 or more specialized materials. Before learning what you've learned, it's important to understand that "high-speed" in this article refers to anything greater than 50 MHz. These are the material considerations to be aware of when working in this frequency range.

High-speed designs have stricter specifications for signal integrity than other designs. While extra care must be taken in routing high-speed signals to meet these requirements, it must be understood that the board material itself is part of the overall signal integrity equation. Therefore, circuit board materials for high-speed designs require properties such as dielectric constant with tight tolerances to help control impedance. If the impedance is allowed to vary throughout the design, high-speed signals will start reflecting energy back as they pass through the line, and the signal will be distorted. Also, a low dissipation factor is desired to help maintain signal integrity. Finally, thermal stability is another important property to ensure that the dielectric properties are not destroyed.

FR-4: Pros and Cons

FR-4 has been the standard material for making PCBs since I've been designing printed circuit boards. When we were junior designers, we were even used to calling all boards "FR-4" regardless of whether they were made of it or not. FR-4 is a flame retardant Type 4 glass fiber reinforced epoxy laminate. It is an extremely cost-effective material that is both an excellent electrical insulator and very strong in both dry and wet conditions. It also has good manufacturing properties, making it an ideal material for constructing PCBs.

The downside of FR-4 is that when there is too much power, voltage or heat, there are operational limitations. If its working limit is exceeded, the dielectric properties of FR-4 will degrade. This means that the insulating properties of the material will decrease and it will begin to conduct electricity. Another problem with FR-4 is maintaining stable impedance in high-speed designs. This is because the dielectric constant of FR-4 varies over the length and width of the board. Also, as design speeds increase, signal losses that are acceptable in non-high-speed designs will increase to undesirable levels on FR-4 boards.

Comparison of specific high-speed board materials with FR4

Specialized high-speed board materials, such as thermoset hydrocarbon and PTFE laminates, will perform better and more reliably than FR-4 in higher frequency designs. We'll weigh in on the trade-offs later, but first let's look at some of the advantages that high-speed board design materials offer:

1. Reduce signal loss. As transmission line frequencies increase, signal loss becomes a bigger problem. High-speed design board materials have much lower dissipation coefficients than FR-4, and some of them, such as near-pure PTFE laminates, are an order of magnitude better. These lower dissipation factors are an important factor in reducing signal loss.

2. Impedance is more tightly controlled. Traditional PCB materials such as FR-4 do not provide the precise control over dielectric constant (Dk) that high-speed board materials can. FR-4 Dk varies by +/- 10% or more, while materials such as PTFE keep their Dk tolerance at +/- 2% or more.

3. Better thermal management. Certain high-speed design board materials (eg, thermoset hydrocarbon laminates) conduct much better heat than FR-4. If your design deals with thermal management, these board materials are the ones to look into.

4. Increase moisture absorption. Water has dielectric properties and even small amounts of moisture absorbed into PCBs with high frequency circuits can alter the electrical properties of those circuits. Although FR-4 has a moisture absorption rate of nearly 50%, some PTFE materials have moisture absorption rates as low as 2% and should be considered to address this issue.

5. Strong dimensional stability. For dense high-speed designs with tight tolerances, the need for dimensional stability increases. Although FR-4 is known for its dimensional stability, it lacks other advantages offered by high-speed materials. In this case, a thermoset hydrocarbon laminate may be a better choice.