Microwave Tech - The difference between 24G/77G microwave board and ordinary FR4 board

In the field of high-frequency sheet material, Rogers is an undoubted giant. Typical products include 4000 series of hydrocarbon resins and 3000 series of PTFE. The latter is used for 77G automotive radars because of its lower DF. However, due to poor processability, it is generally Both are used for mixed pressing, and it is difficult to press .
The main requirements of high-frequency PCB panels are:

1. Low DF: low dielectric loss
2. Appropriate DK: Low DK will increase the signal group speed and benefit the antenna design, but too low will also cause the line width to be too wide
3. The board is uniform and stable: Generally, the board is composed of glass fiber and resin, so the DK and DF of the entire board are not uniform. Because the 77GHz transmission line is very thin, there will be discontinuity in impedance. The corresponding solution is to use broken glass fiber to spread as evenly as possible.
4. The roughness of the copper foil: in the microwave board, the proportion of the conductor loss in the overall loss will greatly increase. The copper foil with low roughness is not technically difficult. The difficulty is the combination of the too smooth copper foil and the dielectric layer. The force will become smaller, causing peeling. Rogers specializes in the LoPro series of low-roughness copper foil cores, which are said to use some kind of potion.

5. Coefficient of thermal expansion: It is mainly considered in some processing processes of PCB. If the thermal expansion coefficient of the medium and copper is quite different, uneven stress will cause the conductor to break. For example, the famous Rogers5880 series has ultra-low DK and ultra-low DF., But the coefficient of thermal expansion in the Z direction is large, which leads to a greater risk of breaking the conductor at the via hole, so it is rarely used in mass production (too expensive is also on the one hand), and is generally used for student water thesis of antennas all over the world.

6. Water absorption rate: It is very simple. The DF and DK of the medium with high water absorption rate will deteriorate after absorbing water vapor in the air. One of the advantages of LCP, a material that is particularly hot this year, is that the water absorption rate is only 0.02%.
7. Thermal stability of DK and DF