When selecting the substrate used for high-frequency PCBs, special consideration should be given to the change characteristics of the material DK at different frequencies. As for the requirements that focus on high-speed signal transmission or characteristic impedance control requirements, the focus is on the DF and its performance under conditions such as frequency, temperature and humidity.
The general substrate material shows the rule that the DK and DF values change greatly under the condition of frequency change. Especially in the frequency range from 1 MHz to 1 GHz, their DK and DF values change more obviously. For example, the general epoxy resin-glass fiber cloth-based substrate material (general FR-4) has a DK value of 4.7 at a frequency of 1 MHz, and a DK value change of 4.19 at a frequency of 1 GHz. Above 1GHz, the change trend of its DK value is flat. The change trend is to decrease with the increase of frequency (but the change range is not large), for example, at 10GHz, the DK value of FR-4 is generally 4.15, and the substrate material with high speed and high frequency characteristics changes in frequency In the case of DK, the change in DK value is small. At a change frequency from 1MHz to 1GHz, DK keeps the change in the range of 0.02. The DK value tends to decrease slightly under different frequency conditions from low to high.
The dielectric loss factor (DF) of general substrate materials is affected by frequency changes (especially changes in the high-frequency range) and the change in DF value is greater than DK. The change law tends to increase. Therefore, when evaluating the high-frequency characteristics of a substrate material, the key point to be investigated is the change of its DF value. For substrate materials with high-speed and high-frequency characteristics, there are two distinct types of general-type substrate materials in terms of changing characteristics at high frequencies: One is that the (DF) value changes little with the change of frequency . Another category is that although the variation range is similar to that of general substrate materials, its own (DF) value is low.
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