Factors affecting the dielectric properties of the PCB substrate material have?

Number of visits:2111 Date:2018-01-29

Introduction to the:

Factors 1. The impact of permittivity
The dielectric constant and dielectric loss, transmission speed signal, the wavelength of the signal related shortening. The high dielectric constant of the substrate material, the wavelength reduction rate is large. RF circuit, an antenna circuit using multi-wavelength (λ) of λ / 4 of the design, the requirements of low dielectric constant substrate material. In the past with the KHZ, MHZ frequency circuit design era, we have used high dielectric constant, low dielectric loss factor of the ceramic substrate material (which is a dielectric constant of 10, the dielectric loss factor of 0.0002). In 1GHZ transmission frequency, signal wavelength in the air of 30cm, and the signal wavelength using FR-4 wiring pattern on the substrate material is 15cm. By design approach λ / 4, the wiring length is reduced to 3.75cm. From the printed circuit board manufacturing quality precision consider using a high dielectric constant of the substrate material, if you want to obtain the desired high frequency performance is very difficult. Therefore, in the case of more than 1GHZ circuit requirements, low dielectric constant of the substrate material is necessary to manufacture PCB. The use of an organic resin-based substrates, ceramic substrates easier to achieve than the low dielectric constant of the substrate material, low dielectric loss factor requirements.

2. The impact of changes in frequency dielectric properties
At different frequencies of the dielectric constant and dielectric loss factor will change. Low dielectric constant of the substrate material, 1GHZ on the dielectric properties of the above is a basic little change. While in the low frequency range, its dielectric properties measured value "chaos" - beating big change. The dielectric constant of FR-4 board material is 4.7 when in 1MHZ at 1GHZ frequency when conditions are as different and have changed slightly. And the frequency increased from 1MHZ to 1GHZ, which shows a slight downward trend. Therefore, the high-frequency RF board generally use a dedicated radio frequency sheet.

3. Impact of environmental changes on dielectric properties
Choose a high-frequency circuit board material, it should be noted in the case study of high-frequency components of heat capacity, as well as in high temperature, high humidity environment, the dielectric properties of the substrate material change in size. Generally choose small environmental changes in the above dielectric substrate material characteristics change. At high temperatures, high humidity environmental conditions, ε and tanδ values are generally substrate materials is rising. Thus, according to changes in the environment used in the temperature and humidity, to grasp the dielectric properties of the base material of the energy variation, it is very important.

4. Evaluation of the stability of dielectric properties at a frequency, temperature and humidity changes
The dielectric properties of the material evaluate the stability of the substrate at a frequency, temperature and humidity changes, the first to use such a substrate material to produce a test-specific printed circuit board. Then use means circuit analysis of S was measured (S21: minus decay amount, S11: reflection amount) parameter. And detects its phase characteristics, VSWR, characteristic impedance. It must be noted in the detection consider the impact of the detection. (This test abort poor), it affects the accuracy of test results is essential.

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