2021-10-16

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Author：Aure

Influencing factors of **PCB** impedance value

1. Surface microstrip line and characteristic impedance

The characteristic impedance of the surface microstrip line is relatively high and is widely used in practice. Its outer layer is the signal line surface for impedance control, and it is separated from the adjacent reference surface with an insulating material. It can be seen from the formula that the characteristic impedance has a very close relationship with the substrate material (copper-clad plate), so the choice of substrate material is very important in **PCB design**.

Attachment: The calculation formula of characteristic impedance is:

Z0=87/SQRT(εr+1.41)*ln[(5.98h)/(0.8w+t)] (1)

Z0: Characteristic impedance of printed wire:

εr: Dielectric constant of insulating material:

h: The thickness of the medium between the printed wire and the reference plane:

w: Width of printed wire:

t: the thickness of the printed wire.

Second, the dielectric constant of the material

The dielectric constant of the material is measured and determined by the material manufacturer at a frequency of 1 MHz. The same materials produced by different manufacturers are different due to their different resin content.

The dielectric constant decreases as the frequency increases, so in actual applications, the dielectric constant of the material should be determined according to the working frequency, and the average value is generally used to meet the requirements. Increasing the dielectric constant can reduce the impedance, and reducing the dielectric constant can increase the impedance. The dielectric constant is mainly controlled by the material. The signal transmission speed in the dielectric material will decrease as the dielectric constant increases. Therefore, to obtain a high signal transmission speed, the dielectric constant of the material must be reduced. At the same time, a high characteristic resistance value must be used to obtain a high transmission speed, and a low dielectric constant material must be used for a high characteristic impedance.

3. The influence of wire width and thickness

Wire width is one of the main parameters that affect the change of characteristic impedance. Increasing the thickness of the medium can increase the impedance, and reducing the thickness of the price can reduce the impedance; When the wire width changes by 0.025mm. It will cause a corresponding change of 5 to 6Ω in the impedance value. In actual production, if 18um copper foil is used to control the impedance of the signal line surface, the allowable wire width variation tolerance is ±0.015mm. If the variation tolerance of the control impedance is 35um copper foil, the allowable wire width variation tolerance is ±0.003 mm.

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