PCB Blog - How to select high-frequency materials and high-speed materials

When selecting high frequency materials and high speed materials for PCB used in high frequency circuits, special consideration should be given to the DK value of PCB materials and their variation characteristics at different frequencies. For the requirements that focus on high-speed signal transmission or characteristic impedance control, DF and its performance under the conditions of frequency, temperature and humidity will be investigated.

Generally, high-frequency materials and high-speed materials show the law of large changes in DK and DF values under the condition of frequency changes. Especially in the frequency range from 1 MHz to 1 GHz, their DK and DF values change more obviously. For example, the DK value of general epoxy resin-glass fiber cloth based substrate material general FR-4 at lMHz frequency is 4.7, while the DK value changes to 4.19 at lGHz frequency. Above lGHz, its DK value changes slowly. The change trend is to decrease with the increase of frequency, but the change range is not large. For example, at l0GHz, the DK value of FR-4 is generally 4.15. The DK value of substrate materials with high speed and high frequency characteristics changes slightly with the change of frequency. At the change frequency from lMHz to lGHz, DK mostly remains within the range of 0.02. Its DK value has a slight downward trend under different frequency conditions from low to high.

The dielectric loss factor DF of high frequency materials and high speed materials changes more than DK due to the influence of frequency changes, especially in the high frequency range. Its change rule tends to increase. Therefore, when evaluating the high-frequency characteristics of a substrate material, the focus of its investigation is its DF value change. As for the substrate materials with high speed and high frequency characteristics, there are two distinct types of common substrate materials. One is that its DF value changes very little with the change of frequency. Another type is similar to the general substrate material in the range of change, but its DF value is low.

How to choose high-frequency materials and high-speed materials?

The selection of PCB materials must achieve a balance between meeting design requirements, mass production and cost. In short, design requirements include electrical and structural reliability. Usually, when designing a very high-speed PCB with a frequency greater than GHz, the PCB material problem will be more important. For example, the current FR-4 material has a large dielectric loss at several GHz frequencies, which may not be applicable.

What are the high-frequency materials and high-speed materials of PCB?

For example, 10Gb/S high-speed digital signal is a square wave, which can be seen as the superposition of sine wave signals with different frequencies. Therefore, 10Gb/S contains many different frequency signals. 5Ghz fundamental signal, 3rd order 15GHz, 5th order 25GHz, 7th order 35GHz signal, etc. The integrity of the digital signal and the steepness of the upper and lower edges are the same as that of the high-frequency harmonic part of the RF microwave digital signal, which reaches the low loss and low distortion transmission in the microwave frequency band. Therefore, in many aspects, the material selection of high-speed digital circuit PCB is similar to the requirements of RF and microwave circuits.

In actual engineering operations, the selection of high-frequency materials and high-speed materials seems simple, but there are many factors to be considered. Through the introduction of this article, as a PCB design engineer or high-speed project leader, I have a certain understanding of the characteristics and selection of plates. Understand the electrical properties, thermal properties, reliability, etc. of sheet metal. And use the stack reasonably to design a product with high reliability and good processability, and optimize the consideration of various factors.

The following will introduce the main considerations for selecting appropriate high-frequency materials and high-speed materials.

1. The manufacturability of PCB.

For example, how about multiple pressing performance, temperature performance, CAF/heat resistance, mechanical toughness, good reliability and fire rating.

2. PCB and product matching performance, such as electrical, performance stability, etc.

Low loss, stable Dk/Df parameter, low dispersion, small coefficient of variation with frequency and environment, small tolerance of material thickness and glue content, good impedance control, if the wiring is long, consider low roughness copper foil. In addition, high-speed circuit design needs simulation in the early stage, and the simulation results are the reference standard for design. "Xingsen Technology-Agilent High-speed/RF Joint Laboratory" has solved the performance problem of inconsistency between simulation results and testing, and has done a lot of closed-loop verification of simulation and actual testing, and can achieve consistency between simulation and actual measurement through unique methods.

3. Timely availability of PCB materials.

Many high-frequency materials and high-speed materials have a very long procurement cycle, even 2-3 months. In addition to the conventional high-frequency plate RO4350 in stock, many high-frequency plates need to be provided by customers. Therefore, high-frequency plates need to be communicated with the manufacturer in advance to prepare materials as soon as possible.

4. Cost factors of high-frequency materials and high-speed materials.

It depends on the price sensitivity of the product, whether it is a consumer product or a communication, medical, industrial and military application.

5. Applicability of laws and regulations, etc.

It should be integrated with environmental protection regulations of different countries to meet RoHS and halogen-free requirements.

Among the above factors, the operation speed of high-speed digital circuit is the main factor for PCB selection. The higher the circuit speed is, the smaller the Df value of the selected PCB should be. The circuit board with medium and low loss will be suitable for 10Gb/S digital circuit. The plate with lower loss is suitable for 25Gb/s digital circuit. The board with ultra-low loss will adapt to faster high-speed digital circuits, and its speed can be 50Gb/s or higher.

Considering the Df of high-frequency materials and high-speed materials

Df between 0.01-0.005 circuit board is suitable for 10Gb/S digital circuit.

The circuit board with Df between 0.005-0.003 is suitable for digital circuits with an upper limit of 25Gb/S.

The circuit board with Df no more than 0.0015 is suitable for 50Gb/S or even high-speed digital circuits.

Common high-frequency materials and high-speed materials include.

1. Rogers, RO4003, RO3003, RO4350, RO5880, etc

2. TUC: Tuc862, 872SLK, 883, 933, etc

3. Panasonic, Megtron 4, Megtron 6, etc

4. Isola, 370hr, FR408HR, IS620, IS680, etc

5. Nelco, N4000-13, N4000-13EPSI, etc

6. Dongguan Shengyi, Taizhou Wangling, Taixing Microwave, etc

Of course, there are many other high-frequency boards, such as Arlon, which was acquired by Rogers last year, and Taconic, which are all RF microwave board factories with guaranteed performance.

Here we share how to select high-frequency materials and high-speed materials, as well as PCB design considerations. You need to analyze specific cases in practical application.