FR-4 is the best circuit board material selection of high-speed PCB design?

When we are in charge of a project located outside our comfort zone, we have all experienced it once or once. For me, when my boss asked me to design a high -speed board, that day came. Although I think I am an experienced circuit designer, I know that there are many restrictions on high -speed PCB design. These restrictions are usually not encountered when designing ordinary circuits. Initially, I spent time making the principle diagram suitable for high-speed design; however, once I finished, I fully focused on whether I should use FR-4 or more professional materials for my high-speed PCB prototype. Before the knowledge, it is important to know that I refer to anything greater than 50 MHz in this article. These are the precautions that you should pay attention to when working in this frequency range.

Compared with other designs, high -speed design has stricter specifications in terms of signal integrity. Although the wiring of a high -speed signal is very careful to meet these requirements, it must be understood that the circuit board material itself is part of the entire signal integrity equation. Therefore, the circuit board material for high -speed design requires the characteristics of the dielectric constant with strict tolerance to help control the impedance. If the impedance changes in the entire design, the high -speed signal will start reflected the energy back when they pass through the line, and the signal will be distorted. In addition, it is also desirable to help maintain low -consumption factor of signal integrity. Finally, thermal stability is another important feature to ensure that the dielectric performance will not collapse.

FR-4: Its advantages and disadvantages

Since I designed a printing circuit board, FR-4 has been a standard material for making PCB. Back to the era of junior designers, we even developed a bad habit of referring to all circuit boards as "FR-4", whether or not it consists of it. FR-4 is a flame retardant type 4 glass fiber enhanced epoxy layer. It is a very cost -effective material, which is not only an excellent electrical insulation, and is very solid under dry and humid conditions. It also has good manufacturing performance, making it an ideal material for building PCB.

The disadvantage of FR-4 is that when it involves excessive power, voltage or heat, it has operating restrictions. If it exceeds its operational restrictions, the FR-4's dielectric performance will collapse. This means that the insulation of materials will be reduced, and it will start conducting electricity. Another problem of FR-4 is to maintain stable impedance for high-speed design. This is because the dielectric constant of the FR-4 will change with the length and width of the circuit board. In addition, with the improvement of the design speed, the acceptable signal loss in non-high-speed design will increase to the level unwanted in the FR-4 board.

What is the specific high -speed board material comparable to FR4?

Special high-speed board materials such as thermal solid hydrocarbons and PTFE layer pressure plates will have a better and more reliable feature than FR-4 in high-frequency design. There are some weighing, we will discuss it quickly, but first let us take a look at some of the advantages provided by high -speed board design materials:

1. Reduce signal loss. As the frequency of transmission line increases, signal loss has become a bigger problem. The dissipation factor of high-speed design board materials is much lower than that of FR-4. Some materials (such as almost pure PTFE layer pressure plates) are a magnitude magnitude. These lower dissipation factor are an important factor in reducing signal loss.

2. Strictly control the impedance. Traditional PCB materials such as FR-4 cannot accurately control the dielectric constant (DK) as high-speed board materials. FR-4 DK can change +/-10% or more, while the DK tolerances of ptfe and other materials remain at +/-2% or better.

3. Better thermal management. Some high-speed design board materials, such as thermosetable hydrocarbons, compact plates, are much better than FR-4. If your design is to deal with heat management problems, these circuit board materials are the materials to be studied.

4. Increase moisture absorption. Water has a dielectric characteristics. Even a small amount of water absorbed by PCB with high -frequency circuit will change the electrical properties of these circuits. Although the FR-4's moisture absorption rate is close to 50%, the hygroscopic rate of some PTFE materials is as low as 2%, and this problem should be considered.

5. Powerful size stability. For intensive high -speed design with strict tolerances, the demand for size stability has increased. Although FR-4 is known for its size stability, it lacks other advantages provided by high-speed materials. In this case, thermal solid hydrogen hydrocarbon compound plate may be a better choice