Selection of PCB materials in high frequency electronic products

(1) Considerations for key items affecting dielectric properties

In consideration of the dielectric properties of the insulating substrate material, it is first necessary to select a substrate material having a small dielectric loss factor. The resin molecular structure of the substrate material has a polar structure portion, and the vibration, heat, and clutter changes to the frequency signal under the high frequency condition, so that the signal voltage is attenuated.

In addition, it is necessary to select a substrate material having a small median constant. The dielectric constant is related to the dielectric loss, the transmission speed of the signal, and the shortening rate of the wavelength of the signal. The substrate material has a high dielectric constant value and a large wavelength shortening rate. For example, a general epoxy resin-glass fiber cloth-based copper clad plate (commonly known as FR-4) has a dielectric constant (nominal value) of "4.7", and is "4.0" for easy calculation. The wavelength of the 1 GHz analog signal is 30 mm in air (the dielectric constant of air is 1), and the number of amplitudes is 10 when the transmission path is 300 mm long. The wiring pattern using the FR-4 substrate material has a dielectric constant that is four times larger than that of air. In the case of the same length (30 mm) transmission path, the number of amplitudes is increased to 20. In terms of the amplitude of the signal, the amount of signal attenuation is greater than the amount of attenuation transmitted in the air due to the relationship between reflection and absorption.

RF circuits and antenna circuits are often designed with a wavelength (λ) of λ/4. Since there is a relationship in which the dielectric constant is proportional to the shortening rate of the wavelength, the design of the dielectric constant is required for the substrate material using these two circuit designs. In the era of KHZ and MHZ frequency circuit design, ceramic substrate materials with high dielectric constant and low dielectric loss factor have been used (its dielectric is usually 10, and dielectric loss is 0.0002). At a transmission frequency of 1 GHz, the signal wavelength is 30 mm in air, and the signal wavelength on the wiring pattern using the FR-4 substrate material is 15 mm. In the λ/4 design, the wiring length is shortened to 3.75mm. From the viewpoint of the precision of the manufacturing quality of the printed circuit board, it is difficult to obtain the required high frequency performance by using a substrate material having a high dielectric constant. Therefore, in the case of a circuit requirement of 1 GHz or more, it is necessary to manufacture a PCB using a substrate material having a low dielectric constant. The use of an organic resin-based substrate makes it easier to achieve a lower dielectric constant and a low dielectric loss factor of the substrate material than the ceramic substrate. Table 1 shows the low dielectric constant substrate material products of the representative organic resins currently on the market.

(2) Consideration of dielectric properties under frequency changes

The typical substrate materials currently used, FR-4 copper clad laminates and specific representative low dielectric constant base materials, have different dielectric constants and dielectric loss factors at different frequency conditions.

Low dielectric constant substrate materials have little change in dielectric properties above 1 GHz. In the frequency range of 1MHZ -1MHZ, its dielectric characteristic measurement value is "chaotic" - the variation is large.

The dielectric constant of the FR-4 substrate material was 4.7 at 1 MHZ and 4.3 at 1 GHz. There is a slight change with the frequency conditions. And the frequency from 1MHZ to 1GHZ increased, it showed a slight downward trend.

(3) Consideration of the characteristics of the medium under environmental changes

When selecting the substrate material for the high-frequency circuit, attention should also be paid to the change in the dielectric properties of the substrate material in the case where the high-frequency component generates a large amount of heat and in a high-temperature, high-humidity environment. Generally, a substrate material having a small change in dielectric characteristics under the above environmental changes is selected. In the case of a high temperature, high humidity environment, the ε and tan δ values of a general substrate material are increased. Therefore, it is important to grasp the amount of change in the dielectric properties of the base material depending on the temperature and humidity changes in the environment used.

(4) Evaluation of the stability of dielectric properties under changes in frequency, temperature and humidity

For the evaluation of the stability of the dielectric properties of the substrate material under the change of frequency and temperature and humidity, the test-specific printed circuit board is first made of the substrate material. Then, the S parameter is measured by means of circuit analysis (S21: amount of attenuation, S11: amount of reflection). And check its phase characteristics, VSWR, characteristic impedance and so on. Attention must be paid to the degree of influence in the test during the test. This kind of detection is abrupt, and the impact on the accuracy of the test results is crucial.