1.factors affecting the dielectric constant
The dielectric constant is related to dielectric loss, signal transmission speed, and the shortening rate of the wavelength of the signal. The dielectric constant of the substrate material is high, and the rate of shortening of the wavelength is high. RF circuit and antenna circuit adopt the design of wavelength (lambda) as lambda /4, which requires low dielectric constant of substrate material. In the past, with the design of KHZ and MHZ frequency circuits, ceramic substrate materials with high dielectric constant and low dielectric loss factor were used (10 dielectric constant and 0.0002 dielectric loss factor). At the 1GHZ transmission frequency, the signal wavelength is 30cm in the air, and the signal wavelength of the wiring pattern of the FR-4 substrate material is 15cm. In the design of lambda /4, the wiring length is shortened to 3.75cm. Considering the accuracy of printed circuit board manufacturing, it is very difficult to obtain high frequency performance if the substrate material with high dielectric constant is used. Therefore, it is necessary to make PCB with substrate materials with low dielectric constant when the circuit is more than 1GHZ. The use of organic resin matrix material is easier than ceramic substrate to achieve low dielectric constant of substrate material and low dielectric loss factor requirements.
2. frequency change on dielectric properties
The dielectric constant and dielectric loss factor change in different frequency conditions. The substrate material with low dielectric constant, above 1GHZ, has little change in dielectric properties. In the low frequency range, the dielectric properties measured by the system are "chaotic" - the change is greater. The dielectric constant of FR-4 substrate is 4.7 at 1MHZ, and slightly varies with frequency in 1GHZ. And the frequency from 1MHZ to 1GHZ increased, and it showed a slight downward trend. Therefore, high frequency RF board usually adopts special RF board.
3. the influence of environmental change on dielectric properties
In order to select the substrate materials for high frequency circuits, it is also necessary to investigate the changes in dielectric properties of the substrate material under high temperature and high humidity conditions when the heat generation of the high frequency components is large. Generally speaking, the substrate material with small change of dielectric properties should be selected under the above environmental changes. Under high temperature and high humidity conditions, the general epsilon and tan delta values of the substrate material are on the rise. Therefore, it is very important to master the change of dielectric properties of the base material according to the changes of temperature and humidity in the environment used.
4. evaluate the stability of dielectric properties under the change of frequency, temperature and humidity
In order to evaluate the stability of dielectric properties of substrate materials under frequency, temperature and humidity, a printed circuit board for testing purpose must be fabricated with this kind of substrate material. Then, the S parameters are measured by means of circuit analysis (S21: attenuation, S11: Reflection). Its phase characteristics, VSWR and characteristic impedance are also examined. In testing, attention must be paid to the impact of the test. (this detection delay is bad) and is critical to the accuracy of the results.