Generally speaking, the factors that affect the characteristic impedance of the PCB are: dielectric thickness, copper thickness, trace width, trace spacing, dielectric constant Er of the material selected for the stack, and thickness of the solder mask.
In general, the greater the dielectric thickness and line spacing, the greater the impedance value; the greater the dielectric constant, copper thickness, line width, and solder mask thickness, the smaller the impedance value. The relationship between these factors and characteristic impedance is shown in the figure below.
The first one: medium thickness, increasing the medium thickness can increase the impedance, and decreasing the medium thickness can reduce the impedance; different prepregs have different glue contents and thicknesses. The thickness after pressing is related to the flatness of the press and the procedure of the pressing plate; for any type of plate used, it is necessary to obtain the thickness of the media layer that can be produced, which is conducive to design calculation, and engineering design, pressing plate control, incoming Tolerance is the key to media thickness control.
The second: line width, increasing the line width can reduce the impedance, reducing the line width can increase the impedance. The control of the line width needs to be within a tolerance of +/- 10% to achieve the impedance control. The gap of the signal line affects the entire test waveform. Its single-point impedance is high, making the entire waveform uneven, and the impedance line is not allowed to make up. Line, the gap can not exceed 10%. The line width is mainly controlled by etching control. In order to ensure the line width, according to the etching side etching amount, the light drawing error, and the pattern transfer error, the process film is compensated for the process to meet the line width requirement.
The third: copper thickness, reducing the line thickness can increase the impedance, increasing the line thickness can reduce the impedance; the line thickness can be controlled by pattern plating or selecting the corresponding thickness of the base material copper foil. The control of copper thickness is required to be uniform. A shunt block is added to the board of thin wires and isolated wires to balance the current to prevent the uneven copper thickness on the wire and affect the extremely uneven distribution of copper on the cs and ss surfaces. It is necessary to cross the board to achieve the purpose of uniform copper thickness on both sides.
The fourth: dielectric constant, increasing the dielectric constant can reduce the impedance, reducing the dielectric constant can increase the impedance, the dielectric constant is mainly controlled by the material. The dielectric constant of different plates is different, which is related to the resin material used: the dielectric constant of FR4 plate is 3.9-4.5, which will decrease with the increase of the frequency of use, and the dielectric constant of PTFE plate is 2.2- To get a high signal transmission between 3.9 requires a high impedance value, which requires a low dielectric constant.
Fifth: the thickness of the solder mask. Printing the solder mask will reduce the resistance of the outer layer. Under normal circumstances, printing a single solder mask can reduce the single-ended drop by 2 ohms, and can make the differential drop by 8 ohms. Printing twice the drop value is twice that of one pass. When printing more than three times, the impedance value will not change.