Microwave Tech - Characteristics of dielectric constant and dielectric loss of high frequency pcb board

As high-frequency PCB technology and products occupy an increasingly important position, the development of high-frequency PCB plates has also increased at a high speed. One of the more important aspects is the selection of materials with low dielectric constant and low dielectric loss factor. This is an important performance item for PCB high-frequency plates to achieve high-speed and high-frequency. The article discusses the relationship between the dielectric constant of the substrate material and the dielectric loss, and elaborates the relationship between them and the external environment, so that various substrate materials can be reasonably and correctly evaluated and used in the manufacture of PCB.

Preface

At present, there are three main types of commercial high-frequency plates:

(1)polytetrafluoroethylene (PTFE) plates;(2) thermosetting PPO (Polyphenyl Oxide);(3) cross-linked polybutadiene substrates and epoxy resin composite substrates (FR -4). PTFE substrate has the advantages of small dielectric loss, small dielectric constant and small change with temperature and frequency, and close to the thermal expansion coefficient of metal copper foil which has been widely used. Substrates prepared by collocation of PTFE, glass fiber and ceramics, such as RO3200, RO3210, RO4003 and other series have been able to meet the requirements of dielectric constant 2.2 to 10.8, and working frequency range of 30 MHz to 30 GHz . Although the manufacturing of PTFE microwave boards is developing rapidly, the corresponding process is improved from the traditional FR-4 printed circuit manufacturing process . At present, the rapid development of electronic information products, especially microwave devices, the greatly improved integration and the requirements of digitization, high frequency, multi-function and application in special environments have challenged the general PTFE high-frequency board and manufacturing process. . For the high-speed and high-frequency characteristics of microwave PCBs, it is mainly through two technical approaches: on the one hand, this development is made into high-density wiring, fine wires and spacing, small apertures, thinness, and high reliability of conduction and insulation. Sex. In this way, the signal transmission distance can be further shortened to reduce its loss in transmission. On the other hand, it is necessary to use substrate materials with high-speed and high-frequency characteristics. The realization of the latter requires the industry to develop a more in-depth understanding of such substrate materials, research work to find out and master accurate control process methods, so as to achieve the selected substrate materials and the manufacturing process, performance and cost requirements. To achieve the purpose of reasonable matching.

Characteristics of dielectric constant and dielectric loss of high frequency PCB board

It can be seen that the lower the dielectric constant of the substrate, the faster the signal propagates. Therefore, in order to obtain a high signal transmission rate, it is necessary to research and develop a substrate material with a low dielectric constant.

In addition to directly affecting the transmission speed of the signal, the dielectric constant also determines the characteristic impedance to a large extent. It can be expressed as
It can be seen that the main factors affecting the characteristic impedance are: (1) the dielectric constant ε r of the substrate; (2) the dielectric thickness h; (3) the wire width w; (4) the wire thickness t and so on. The smaller the dielectric constant of the substrate, the greater the characteristic impedance.

In high-speed circuits, high characteristic impedance values are required, and materials with low dielectric constants must be researched and developed. At the same time, in order to achieve continuous and stable impedance in the high-speed digital circuit PCB, the substrate material is required to have a stable dielectric constant.

1 The relationship between the signal transmission loss on the PCB and the properties of the high-frequency sheet

The dielectric loss in the transmission loss on the conductor circuit is mainly dominated by the dielectric constant (εr) and the dielectric loss factor (tan δ) of the insulating layer of the substrate material. The influence on the transmission loss is proportional to the magnitude of ε r and tan δ, and is related to the frequency of the medium when it is working. Under the same ε r or tan δ, the higher the frequency, the greater the transmission loss.