The Difference between High Speed PCB Board and High Frequency PCB Board

With the development and construction of 5G communication, the electronic equipment industry needs more and more high frequency board and high-speed board. Because of the different use environment, high frequency board and high speed board have many common characteristics, but also have some differences. In this paper, the characteristics of high frequency and high speed plates are described, and the future development of high frequency and high speed plates is predicted.

I. Demand of 5G network for high frequency and high speed pcb

5G, the fifth generation of mobile communications. Cellular communications have undergone four upgrades from analog communications (1G) to LTE (4G). Research and testing of 5G networks has progressed rapidly since 2012. In the past, from 1G to 4G, the main scene was the network communication between people, but 5G network will meet the Internet of everything, and open a new round of information network revolution. The 5G communication industry chain mainly includes the following five important links:

(1) Network planning and design (preliminary technical research and network construction planning);

(2) Wireless main equipment (core network, base station antenna, RF device, optical device/optical module, small base station, etc., wireless supporting, network coverage and optimization have been deployed);

(3) Transmission equipment (after wireless equipment, wired transmission links are required, by optical fiber and cable, system integration, IT support, value-added services, etc.);

(4) Terminal equipment (chip and terminal matching);

(5) Operators.

In addition to the above five important aspects, the following two aspects are also important:

(6) PCB/CCL industry chain (used for base station RF, baseband processing unit, IDC and core network router, etc.);

(7) Dielectric waveguide filter (base station radio frequency).

In the process of 5G construction, products in different industries use different frequency bands, which leads to different requirements for high frequency and high speed plates for different products in different industries. It can be seen that 5G network is the comprehensive application of multi-frequency microwave. Therefore, products in different industries choose high-speed board and high-frequency board will be different.

2, the characteristics of high frequency board and high-speed board

2.1 Dielectric constant Dk and dielectric loss Df of materials

When it comes to high frequency high-speed board, it is inevitable to talk about two concepts "dielectric constant -Dk" and "dielectric loss -Df". The PCB dielectric layer used for high-speed digital signal transmission not only acts as an insulating layer between conductors, but more importantly as a "characteristic impedance", which also affects signal transmission speed, signal attenuation and heating.

The size of the dielectric loss (Df) indicates the attenuation degree of signal transmission. This attenuation of signal transmission is often consumed by heat generation. With the transmission of high frequency and high speed digital signal, signal attenuation and heat loss will increase rapidly with the transmission of high frequency and high speed digital signal. For high frequency and high speed digital signal transmission, the smaller the medium loss (Df), the better.

In the development process of high speed and high frequency products, the dielectric constant (Dk) and dielectric loss (Df) of plates are required to develop in a smaller direction. But there are still some differences between high frequency products and high speed products.

2.2 High-speed material characteristics

High-speed products pay more attention to the dielectric loss (Df) of the plate. The grades of high-speed materials commonly used in the market are also based on the dielectric loss (Df). Different substrate materials are divided into conventional loss, medium loss, low loss, very low loss and ultra-low loss according to the dielectric loss of the substrate. The five transmission signal losses correspond to levels.

2.3 High frequency material characteristics

Compared with high-speed materials, high frequency materials pay more attention to the size and variation of material dielectric constant (Dk). High-frequency products are very sensitive to changes in the dielectric constant (Dk) of materials. Therefore, the focus of high frequency materials is the stability of dielectric constant (Dk), as well as the thickness of the material medium, the temperature drift coefficient of the material and the stasmatic performance of the material. There is no clear classification standard for hf materials, but many PCB manufacturers roughly classify HF boards based on the material's dielectric constant (Dk). Materials with the same permittivity (Dk) are considered similar and interchangeable.

In the field of high frequency materials, there is also a conventional division, that is, materials are divided into PTFE materials and non-PTFE materials. This is closely related to the application field of high-frequency products. The current RF landscape can be divided into two parts. One is the frequency below 6GHZ commonly used frequency 3.5GHZ, 2.7GHZ, 1.8GHZ. The main products are power amplifier, antenna calibrator, matrix and so on. The other part is 20GHZ and above millimeter wave field commonly used frequency 24GHZ, 66GHZ, 77GHZ, the main products are radar products. This is mainly because with the increase of frequency, the stasmatic effect and dielectric loss of non-PTFE products have a sharp increase in the impact of signal transmission, and PTFE materials have better performance characteristics.

3, high frequency high-speed board development prospects

The traditional copper clad plate material has high transmission loss and cannot meet the requirements of high frequency signal transmission quality. Therefore, the most important performance of PCB substrate materials used in 5G communication is to meet the requirements of high frequency and high speed, as well as the requirements of integration, miniaturization, lightweight, multi-function and high reliability. In particular, resin materials require low dielectric constant (Dk), low dielectric loss (Df), low thermal expansion coefficient (CTE) and high thermal conductivity. At present, the hard copper-clad boards represented by thermoplastic materials such as PTFE and hydrocarbon resin (PCH) occupy most of the market of HIGH frequency/high speed PCB substrates due to their unmatched low dielectric properties. In recent years, polyphenylene ether (PPO or PPE), bismaleimide (BMI), cyanate ester (CE), triazine resin (BT), benzoxazine (BOZ) and benzcyclobutane (BCB) and other new resin materials and related modification of high frequency/high speed PCB substrate.

Polyphenylene ether (PPO or PPE), whose dielectric properties are second only to PTFE, has attracted much attention in the industry in recent years

In addition, the processability of PPO material is much better than that of PTFE material, so at present, the Very Low Loss and Ultra Low Loss of high-speed board are mostly modified PPO resin, such as PANASONIC M6, M7N, LianMao IT968, IT988GSE. The resin system of high frequency plate is mainly polytetrafluoroethylene (PTFE) thermoplastic material and hydrocarbon resin (PCH). Although very low dielectric loss (Df) and stable dielectric constant (Dk) can be obtained, the poor machinability of the material is not suitable for high multilayer board, let alone HDI board processing products. With the development of 5G communication, THE PCB complexity of high-frequency products is getting higher and higher (traditional HIGH-FREQUENCY PCB is mainly single-sided board and double-sided board, and the development of multi-layer board even requires HDI design). In recent years, material developers also use PPO resin to make HIGH-FREQUENCY boards. In order to ensure the board has very low dielectric loss (Df) and stable dielectric constant (Dk) at the same time, to obtain good PCB processability. For example, the IT-88GMW, IT-8300GA, IT-8350G, IT-8338G, IT-180A and other high-frequency plates launched by Lianmao are a mixture of modified PPO resin and hydrocarbon resin. The machinability of the material is greatly enhanced while meeting the requirements of high frequency signal transmission.

On the one hand, the development of 5G communication to higher speed and higher frequency inevitably requires the material dielectric loss (Df) and dielectric constant (Dk) to develop in a smaller direction; On the other hand, 5G products require miniaturization and more unification, and PCB will inevitably develop towards higher multilayer and even HDI, which requires good machinability of materials. At present, the use of polyphenylene ether (PPO or PPE) resin is a good development direction no matter from high frequency materials or high speed materials.