When selecting substrate materials for PCBs used in high-frequency circuits, special attention must be paid to the material’s dielectric constant (DK) and its variation characteristics at different frequencies. For applications emphasizing high-speed signal transmission or controlled characteristic impedance, the focus should be on the dielectric loss (DF) and its performance under various conditions, such as frequency, temperature, and humidity.
Standard substrate materials typically exhibit significant variations in DK and DF values as frequency changes. This variation is particularly pronounced within the 1 MHz to 1 GHz frequency range. For example, a standard epoxy-glass fiber laminate substrate (standard FR-4) has a DK value of 4.7 at 1 MHz, which changes to 4.19 at 1 GHz. Above 1 GHz, the trend in DK values tends to flatten. The trend is that as frequency increases, the change in DK decreases (though the magnitude of the change is small). For example, at 10 GHz, the DK value of standard FR-4 is 4.15. In contrast, substrate materials with high-speed and high-frequency characteristics exhibit minimal changes in DK values under varying frequencies; within the 1 MHz to 1 GHz range, DK typically remains within a 0.02 variation range. Its DK value shows a slight downward trend across different frequency ranges from low to high.
For general-purpose substrate materials, the dielectric loss factor (DF) is more significantly affected by frequency changes (especially within the high-frequency range) than the DK value. The trend is toward an increase; therefore, when evaluating the high-frequency characteristics of a substrate material, the primary focus should be on the variation in its DF value. Substrate materials with high-speed, high-frequency characteristics generally fall into two distinct categories regarding their high-frequency behavior: one category exhibits very little change in its DF value with frequency, while the other category, although similar in the magnitude of change to general-purpose substrate materials, has a lower inherent DF value.
