High frequency circuit board
1. 5G network’s demand for high-frequency and high-speed circuit boards
5G, the fifth generation of mobile communications. Cellular mobile communications have developed from analog communications (1G) to the now popular LTE (4G), and have experienced four upgrades. Since 2012, research and testing of 5G networks have progressed rapidly. In the past, from 1G to 4G, the main scenario was network communication between people. The 5G network will satisfy the interconnection of everything and start a new round of information network revolution. The 5G communications industry chain mainly includes the following five important links:
(1) Network planning and design (preliminary scientific and technological research and network construction planning).
(2) Main wireless equipment (core network, base station antennas, radio frequency devices, optical devices/optical modules, small base stations, etc., wireless supporting, network coverage and optimization links have begun to be deployed).
(3) Transmission equipment (wireless equipment then requires wired transmission links, followed by optical fiber cables, system integration, IT support, value-added services, etc.).
(4) Terminal equipment (chip and terminal matching).
(5) Operator.
In addition to the above five important links, the following two links are also very important:
(6) PCB/CCL industry chain (for base station radio frequency, baseband processing unit, IDC and core network router, etc.).
(7) Dielectric waveguide filter (base station radio frequency).
During the 5G construction process, products in different industries work in different frequency bands, which leads to different products in different industries having different requirements for high-frequency and high-speed circuit boards. It can be seen that 5G network is a comprehensive application of multi-band microwave. Therefore, the selection of high-speed circuit boards and high-frequency circuit boards for products in different industries will be different.
2. Characteristics of high-frequency and high-speed circuit boards
2.1 Dielectric constant and dielectric loss (Dk Df) of high-frequency and high-speed materials
When it comes to high-frequency and high-speed circuit boards, 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 functions as an insulating layer between conductors, but more importantly, it plays the role of "characteristic impedance", which also affects signal transmission rate, signal attenuation and heat generation.
The size of dielectric loss (Df) indicates the degree of attenuation of signal transmission. This attenuation of signal transmission is often generated and consumed by heat. With the transmission of high-frequency and high-speed digital signals, signal attenuation and heat consumption will inevitably increase rapidly with the transmission of high-frequency and high-speed digital signals. For high-frequency and high-speed digital signal transmission, the smaller the dielectric loss (Df), the better.
In the development process of high-speed products and high-frequency products, the dielectric constant (Dk) and dielectric loss (Df) of the board are required to develop in a smaller direction. However, there are still certain differences in the demand for plates between high-frequency products and high-speed products.
2.2 High-speed material properties
High-speed products pay more attention to the dielectric loss (Df) of the board. The grades of high-speed materials commonly used in the market are also divided according to the size of dielectric loss (Df). Different substrate materials are divided into regular loss, medium loss, low loss, extremely low loss and ultra-low loss according to the dielectric loss of the substrate. ) Five corresponding levels of transmission signal loss.
High frequency circuit board
2.3 High-frequency material properties
Compared with high-speed materials, high-frequency materials pay more attention to the size and change of the material's dielectric constant (Dk). High frequency products are very sensitive to changes in the material's dielectric constant (Dk). Therefore, the focus of high-frequency materials is the stability of the dielectric constant (Dk), as well as the material's dielectric thickness, the temperature drift coefficient of the material, and the stroboscopic efficiency of the material. There is no clear classification standard for high-frequency materials in the industry, but many PCB manufacturers roughly classify high-frequency circuit boards based on the dielectric constant (Dk) of the material. Materials with the same dielectric constant (Dk) are considered similar and can be substituted for each other.
There is also a commonly used classification pipeline in the field of high-frequency materials, which is to divide materials into PTFE materials and non-PTFE materials. This is closely related to the application fields of high-frequency products. The current radio frequency field can be divided into two parts. First, the commonly used frequencies below 6GHZ are 3.5GHZ, 2.7GHZ, and 1.8GHZ. The main products are power amplifiers, antenna calibrators, arrays and other products. The other part is that the commonly used frequencies in the millimeter wave field above 20GHZ are 24GHZ, 66GHZ, and 77GHZ, and the main products are radar products. This is mainly because as the frequency increases, the stroboscopic effect and dielectric loss of non-PTFE products have a sharp increase in their impact on signal transmission, while PTFE materials have better performance characteristics.
3. Development prospects of high-frequency and high-speed circuit boards
Traditional copper-clad laminate materials have large transmission losses and cannot meet the requirements for high-frequency signal transmission quality. Therefore, the most important performance of PCB substrate materials used in 5G communications 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 coefficient of thermal expansion (CTE) and high thermal conductivity. At present, rigid copper-clad laminates, represented by polytetrafluoroethylene (PTFE) thermoplastic materials and hydrocarbon resin (PCH) thermosetting materials, account for the vast majority of high-frequency/high-speed PCB substrates due to their unparalleled low dielectric performance. market. In recent years, polyphenylene ether (PPO or PPE), bismaleimide (BMI), cyanate ester (CE), triazine resin (BT), benzoxazine (BOZ) and benzocyclobutene ( BCB) and related modifications and other new resin materials for high-frequency/high-speed PCB substrates.
In addition, the processability of PPO materials is much better than that of PTFE materials, so currently very low loss (Very Low Loss) and ultra low loss (Ultra Low Loss) in high-speed circuit boards mostly use modified PPO resin, such as Panasonic M6, M7N, Lianmao’s IT968, IT988GSE. The resin system of high-frequency circuit boards is mainly polytetrafluoroethylene (PTFE) thermoplastic material and hydrocarbon resin (PCH). Although extremely low dielectric loss (Df) and stable dielectric constant (Dk) can be obtained, the material's poor processability is not suitable for high-multilayer boards, let alone HDI board processing products. With the development of 5G communications, the PCB complexity of high-frequency products is also getting higher and higher (traditional high-frequency PCBs are mainly single- and double-sided, and the development of multi-layer boards even has HDI design requirements). In recent years, material developers have also adopted PPO resin is used to make high-frequency circuit boards. While ensuring that the board has extremely low dielectric loss (Df) and stable dielectric constant (Dk), it can achieve good PCB processability. For example, the IT-88GMW, IT-8300GA, IT-8350G, IT-8338G, IT-8615G and other high-frequency circuit boards launched by Lianmao use a mixed system of modified PPO resin and hydrocarbon resin. While meeting the requirements for high-frequency signal transmission, the processability of the material is greatly enhanced.
On the one hand, the development of 5G communications towards higher speed and higher frequency will inevitably require the material dielectric constant and dielectric loss (Dk Df) to develop in a smaller direction. On the other hand, 5G products require miniaturization and more uniformity, and corresponding PCBs will inevitably develop in the direction of high multi-layer or even HDI, which requires materials with good processability. At present, whether it is from the perspective of high-frequency circuit boards or high-speed circuit boards, the use of polyphenylene ether (PPO or PPE) resin is a good development direction.