Difference between 24G/77G microwave sheet and ordinary FR4 sheet
In the field of high frequency sheet, Rogers is the undoubted giant, typical products include 4000 series hydrocarbon resin and 3000 series PTFE, the latter is used for 77G automobile radar more due to lower DF, but due to poor processability, it is generally used for mixed pressure, and it is difficult to press and combine multilayer boards.
The main requirements of high frequency microwave boards are:
A. Low DF: Low dielectric loss.
B. The Appropriate DK: Low DK will enhance the signal group speed, and is conducive to antenna design, but too low will also lead to too wide a line width.
C. The Plate is Uniform and Stable: Generally the plate is composed of glass fiber and resin, so the DK,DF of the whole plate is not uniform, due to the 77GHz transmission line is very thin, so there will be impedance discontinuity. Corresponding solutions include the use of broken glass fibers, as far as possible, evenly spread.
D. The Roughness of The Copper Foil: To the millimeter wave band, the conductor loss in the overall loss of the proportion will rise greatly, low roughness of the copper foil in the technical difficulties, the difficulty is too smooth copper foil and dielectric layer bonding will become smaller, resulting in the peeling off of the Rogers on the special series of low roughness of the LoPro copper foil board core, said to be using a certain kind of potion.
E. The Coefficient of Thermal Expansion: Mainly considered in some of the pcb processing process, if the dielectric and copper coefficient of thermal expansion difference is large, it will produce uneven stress leading to conductor fracture, such as the famous 5880 series, DK is super low, DF is also super low, but the Z direction of the coefficient of thermal expansion is large, resulting in a greater risk of fracture of the conductor at the over-hole, so it is rarely used for mass production (too expensive is also one aspect), generally Used for antennas around the world student water thesis.
F. Water Absorption: Very simple, water absorption rate of the medium in the absorption of water vapor in the air after the DF, DK will deteriorate. One of the advantages of LCP, a particularly hot material this year, is that the water absorption rate is only 0.02%.
G. DK, DF thermal stability.
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