Factors Affecting for PCB Impedance Control

In the process of PCB production, the circuit performance provided by PCB board must be able to make the signal in the transmission process does not reflect phenomenon, the signal remains complete, reduce transmission loss, play the role of matching impedance, so as to achieve complete, reliable, accurate, no interference, noise transmission signal.The relationship between characteristic impedance and pcb base raw material is very important, so the substrate material selection is very important in PCB production process.

The main factors affecting the characteristic impedance are:

1.The dielectric constant of the material and its influence

The speed of signal transmission in the dielectric material will decrease with the increase of the dielectric constant. Therefore, in order to obtain high signal transmission speed, the dielectric constant of the material must be reduced.At the same time, to obtain high transmission speed, high characteristic resistance must be used, and the high characteristic resistance must choose low medium constant material.

2. Influence of wire width and thickness

The permissible wire width variation during PCB production will inevitably lead to a great change in impedance value.Therefore, it is required that the production personnel should ensure that the line width meets the design requirements in the PCB production process, and make it change within the tolerance range, so as to adapt to the requirements of impedance.It should be noted that the surface of the wire should be clean before electroplating, and there should be no residue and black oil for repairing the plate. Otherwise, it will lead to uneven copper plating and local wire thickness changes, thus affecting the characteristic impedance value.In addition, in the process of washing the plate, we must be careful to operate, do not therefore change the wire thickness, resulting in changes in the impedance value.

3. the influence of medium thickness

The characteristic impedance is proportional to the natural logarithm of the thickness of the medium. The thicker the thickness of the medium, the greater the impedance, so the thickness of the medium is another major factor affecting the characteristic resistance value.Since the wire width and the dielectric constant of the material are determined before production, controlling the laminate thickness (medium thickness) is the main step in controlling the characteristic impedance during PCB production, and the variation of laminate thickness between each layer will cause a large change in the impedance value.Therefore, the characteristic impedance value will increase with the increase of medium thickness during PCB production.