What are the factors that affect PCB impedance

In PCB production, the circuit performance provided by the PCB board must be able to prevent the signal from being reflected during the transmission process, keep the signal intact, reduce the transmission loss, and play the role of matching impedance, so that it can be complete, reliable, accurate, and interference-free , Noisy transmission signal. The characteristic impedance has a very close relationship with the substrate material, so the selected substrate material is very important in pcb prototype service.

 
Generally speaking, the factors that affect the characteristic impedance of the PCB: dielectric thickness, copper thickness, trace width, trace spacing, dielectric constant of the material selected for the stack, and the thickness of the solder mask.
 
The greater the dielectric thickness and line spacing, the greater the impedance value;
The larger 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 main factors affecting characteristic impedance are:
1. The dielectric constant of the material and its influence
The signal transmission speed in the dielectric material will decrease with the increase of the dielectric constant. Therefore, to obtain a high signal transmission speed, the dielectric constant of the material must be reduced. At the same time, in order to obtain a high transmission speed, a high characteristic resistance must be used, and a low dielectric constant material must be used for a high characteristic resistance.
 
2. The influence of wire width and thickness
The allowable wire width change in PCB will inevitably lead to a great change in the impedance value. This requires the manufacturer to ensure that the line width meets the design requirements and changes it within the tolerance range to meet the impedance requirements. It should be noted that the surface of the wire must be clean before electroplating, and there should be no residue and repairing oil, otherwise the copper will not be plated during electroplating, which will change the thickness of the circuit and affect the characteristic impedance value. In addition, in the process of brushing, you must be careful not to change the thickness of the circuit  and cause the impedance value to change.

3. The influence of media thickness
The characteristic impedance is proportional to the natural logarithm of the dielectric thickness. The thicker the dielectric, the greater the impedance. Therefore, the dielectric thickness is another major factor affecting the characteristic resistance. Because the width of the wire and the dielectric constant of the material have been determined before production, controlling the thickness of the laminate (medium thickness) is the main means to control the characteristic impedance in PCB proofing. The change in the thickness of each layer will cause a large impedance value. change. Therefore, in prototype pcb manufacturing, the characteristic impedance value will increase as the thickness of the medium increases.
 
Therefore, for high-frequency circuits with strictly controlled characteristic impedance values, strict requirements should be imposed on the error of the dielectric thickness of the substrate material. Generally speaking, the change of the dielectric thickness should not exceed 10%. For multi-layer boards, the thickness of the dielectric is closely related to the multi-layer lamination process, so the characteristic impedance should be strictly controlled.
 
Impedance control is a special process, and some PCB manufacturers find it troublesome and unwilling to do it, but POE technology can make unconventional boards and special processes. At present, POE can carry out impedance control if the board is FR-4, FPC, soft and hard combination, etc.; the number of layers that can be achieved is 2-40 layers; the impedance tolerance is ±10% (limit ±8%).