Other factors that affect the characteristic impedance of printed circuit boards (on)

Abstract: Following the literature [1], this paper further analyzes other factors that affect the characteristic impedance, and focuses on the influence of the dielectric constant on the characteristic impedance of the PCB.

Keywords: printed circuit board characteristic impedance dielectric constant

Overview

With the rapid increase of signal transmission speed and the wide application of high-frequency circuits, higher requirements have been put forward for printed circuit boards. The circuit performance provided by the printed circuit board must enable the signal to not be reflected during the transmission process, the signal should remain intact, and the transmission loss should be reduced to play a role in matching the impedance so that a complete, reliable, accurate, interference-free, noise-free transmission can be obtained. signal. Based on the literature [1], this paper further discusses other factors that affect the PCB's characteristic impedance, especially the influence of the dielectric constant.

1 surface microstrip and characteristic impedance

The characteristic impedance value of the surface microstrip line is relatively high and widely used in practice. The outer layer of the microstrip line is the signal line surface for controlling the impedance, and is separated from the adjacent reference plane by an insulating material, as shown in FIG.

The formula for calculating the characteristic impedance is:

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: thickness of printed wire

It can be seen from Figure 1 and Formula (1) that the main factors affecting the characteristic impedance are: (1) dielectric constant εr; (2) dielectric thickness h; (3) wire width w; (4) wire thickness t and so on. Therefore, it can be seen that the relationship between the characteristic impedance and the substrate material (copper-clad plate) is very close. Therefore, it is very important to select the substrate material in the PCB design. In (1), we have explored the effects of several important parameters on Z0. Below we continue to explore the influence of other factors on Z0.

2 The effect of copper foil thickness on Z0

From the formula (1), it can be seen that the thickness of the copper foil is also an important factor affecting Z0. The larger the copper foil thickness, the smaller the characteristic impedance, but the range of variation is relatively small. as shown in picture 2.

As can be seen from FIG. 2, although a thinner ZF value can be obtained with a thinner copper foil thickness, its thickness variation does not greatly contribute to the Z0 value, and at the same time, its thickness variation range is not large, so a thin copper foil pair is used. The contribution of Z0 is more to be attributed to the fact that thin copper foil contributes much more precisely to the manufacture of fine wires to increase or control the characteristic impedance value. In fact, the thickness of the wire of a PCB product is not only the thickness of the copper-clad laminate, but it also includes the thickness variation during the manufacturing process of the board. 3PCB production and processing on Z0

When the board material type and PCB design for high-frequency line or high-speed digital line are selected, the expected characteristic impedance value has been determined, but the expected characteristic impedance or the actual control control is within the range of the expected characteristic impedance value. Internally, only the management and control of the PCB production process can be achieved. There are many factors that affect the influence of PCB production and processing on Z0, and it is also very complicated. Z0 control of the PCB in the future will inevitably become one of the most prominent and biggest problems in PCB production.