Other factors affecting the printed circuit board's characteristic impedance

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.

4 wire width control significance

4.1 The key to wire width control

The key to the control of the wire width is how to achieve the Z0 value or the control value Z0 of the OEM design within the range of change through the management and control of the whole process of PCB production and processing. Due to the proper choice of substrate material and the completion of the PCB design, the three parameters of dielectric constant, dielectric width, and wire width are basically fixed. Although wire width and dielectric thickness may be affected by PCB production and processing, mechanical polishing and micro-etching in the humidification process may make the copper foil layer thinner to facilitate the improvement of Z0, and the manufacture of buried blind via interconnects and outer layers The hole patterning and electroplating of the pattern can make the thickening of the copper foil layer unfavorable to Z0, so care should be taken to control it. However, the width of the wire is entirely produced by PCB production. At the same time, the manufacture of fine wires for the transmission of high-frequency signals and high-speed digital signals is still a key technology for today's high-density interconnect PCBs. The essence of fine wire manufacturing is fundamentally the control and management of fine wires. Therefore, as a signal transmission line application PCB manufacturing, the manufacturing of the wire width should be treated as a key issue.

4.2 Meaning of wire width control

High-frequency signals and high-speed digital (logic) signals are transmitted from the driving element and sent to the receiving element via the PCB signal transmission line. This is a signal transmission process. In this signal transmission process, if the PCB's signal transmission process, the PCB's signal transmission line's characteristic impedance value Z and the two components of the "electronic impedance" exactly match (in fact, the impedance of the receiving component is greater than the impedance of the drive component When it is only reasonable, the energy of the transmitted signal is completely transmitted. This situation is ideal. If the transmission line Z0 of the PCB does not match and there is a variation deviation or the variation deviation is too large, problems such as reflection, loss, attenuation, or time delay will occur during signal transmission. In severe cases, it may even cause complete "distortion" and fail to receive the original true signal.

Therefore, high-frequency signals and high-speed digital signals must be completely transmitted in the PCB transmission line, and it must be done that the characteristic impedance value Z0 at any point on the PCB transmission line should be equal, which means that in the PCB transmission line, The cross-sectional area (including no defect and ideal) in any one place must be the same. However, it is impossible to completely complete the actual production and processing of PCB transmission lines. Therefore, the control of the transmission line in the PCB, after the substrate material is determined, is essentially a control problem of the consistency of the cross-sectional area of ​​the transmission line in the double-sided board; in the multilayer board, the consistency of the cross-sectional area of ​​the transmission line is essentially The control of the uniformity of the medium thickness is mainly due to the consistency and integrity of the cross-sectional area of ​​the transmission line. Due to too many processing procedures and process parameters (especially dynamic process parameters) involved in the processing of the PCB transmission line, even full automation of production and processing is difficult to achieve. Therefore, one can only control the entire cross-sectional area of ​​the PCB transmission line for production and processing within the specified range. Therefore, the Z0 of the PCB transmission line can only be controlled within the design specification value according to the application object.

Traditionally, PCB wire width deviations have been allowed to be ±20%, which is already satisfactory for PCB wires (wire lengths less than one-seventh of the signal wavelength) for conventional electronic products that are not transmission lines. But for the signal transmission line with Z0 control requirements, PCB wire width deviation ± 20% can not meet the requirements, therefore, the error at this time has generally exceeded ± 10%, and Z0 error will also be thin with the media thickness and bias Big.

From the above theoretical calculations, such understanding and conclusion can be drawn. The traditional line width error precision control regulations are not applicable to the requirements of the transmission line. The error accuracy of the transmission line width must be determined according to the characteristics of the transmission line transmission signal. If the transmission line of the high frequency signal is transmitted, its precision control must be much stricter to achieve a smaller Z0 deviation value. These requirements can be calculated based on the formula (1) and the known medium thickness, wire thickness and Z0 bias value to calculate the wire's accuracy (error) control size.

Tian Li Cao Anzhao
(Anhui University of Engineering Science and Technology Department of Electrical Engineering