What should I do if the impedance in PCB design is not continuous?

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What should I do if the impedance in PCB design is not continuous? [Copy link]

What should I do if the impedance in PCB design is not continuous?

(The following text is reproduced from the Internet. You are welcome to add to it and make corrections.)

Characteristic impedance: Also known as "characteristic impedance", it is not DC resistance, but a concept in long-distance transmission. In the high-frequency range, when the signal reaches the place where the signal line reaches, an instantaneous current will be generated between the signal line and the reference plane (power supply or ground plane) due to the establishment of an electric field.

If the transmission line is isotropic, then as long as the signal is being transmitted, there will always be a current I. If the output voltage of the signal is V, during the signal transmission process, the transmission line will be equivalent to a resistor with a magnitude of V/I. This equivalent resistance is called the characteristic impedance Z of the transmission line.

During the transmission process of a signal, if the characteristic impedance on the transmission path changes, the signal will be reflected at the node where the impedance is discontinuous.

Factors that affect characteristic impedance include: dielectric constant, dielectric thickness, line width, and copper foil thickness.

【1】 Gradient line

Some RF devices have small packages. The width of the SMD pad may be as small as 12 mils, while the width of the RF signal line may be more than 50 mils. Gradient lines should be used to prohibit sudden changes in line width. The gradient line is shown in the figure, and the line of the transition part should not be too long.

[2] Corner

If the RF signal line in the PCB runs at a right angle, the effective line width at the corner will increase, the impedance will be discontinuous, and signal reflection will occur. In order to reduce the discontinuity, the corners must be processed. There are two methods: corner cutting and corner filleting. The radius of the arc angle should be large enough. Generally speaking, it should be ensured that: R>3W. As shown in the right figure.

【3】Large solder pad

When there is a large pad on a 50 ohm microstrip line, the large pad is equivalent to distributed capacitance, which destroys the continuity of the characteristic impedance of the microstrip line. Two methods can be used to improve it at the same time: first, thicken the microstrip line medium, and second, hollow out the ground plane under the pad, both of which can reduce the distributed capacitance of the pad. See the figure below.

【4】Via

PCB vias are metal cylinders plated outside of through holes between the top and bottom layers of a board. Signal vias connect transmission lines on different layers. Via stubs are unused portions of a via. Via pads are donut-shaped pads that connect vias to top or inner transmission lines. Spacer pads are annular gaps within each power or ground plane to prevent shorts to the power and ground planes.

Parasitic parameters of vias

After rigorous physical theory derivation and approximate analysis, the equivalent circuit model of the via can be an inductor with a grounding capacitor connected in series at both ends, as shown in Figure 1.

Equivalent circuit model of vias

From the equivalent circuit model, we know that the via itself has parasitic capacitance to the ground. Assuming that the diameter of the via anti-pad is D2, the diameter of the via pad is D1, the thickness of the PCB is T, and the dielectric constant of the substrate is ε, the parasitic capacitance of the via is approximately:

The parasitic capacitance of vias can cause the signal rise time to be prolonged and the transmission speed to be slowed down, thus deteriorating the signal quality. Similarly, vias also have parasitic inductance. In high-speed digital PCBs, the harm caused by parasitic inductance is often greater than parasitic capacitance.

Its parasitic series inductance will weaken the contribution of the bypass capacitor, thereby weakening the filtering effect of the entire power system. Assume that L is the inductance of the via, h is the length of the via, and d is the diameter of the center drill hole. The approximate parasitic inductance of the via is approximately:

Vias are one of the important factors that cause impedance discontinuity in RF channels. If the signal frequency is greater than 1GHz, the impact of vias must be considered.

Common methods to reduce the impedance discontinuity of vias include: using padless technology, selecting the routing method, optimizing the anti-pad diameter, etc. Optimizing the anti-pad diameter is one of the most commonly used methods to reduce impedance discontinuity. Since the characteristics of vias are related to the structural dimensions such as aperture, pad, anti-pad, stacking structure, routing method, etc., it is recommended to use HFSS and Optimetrics for optimization simulation according to the specific situation every time you design.

When a parametric model is used, the modeling process is simple. During the review, the PCB designer is required to provide the corresponding simulation documents.

The via diameter, pad diameter, depth, and anti-pad will all cause changes in impedance discontinuity, reflection, and insertion loss severity.

【5】Through-hole coaxial connector

Similar to via structures, through-hole coaxial connectors also have impedance discontinuity, so the solution is the same as that of vias. The common methods to reduce the impedance discontinuity of through-hole coaxial connectors are also: using a padless process, a suitable wire outlet method, and optimizing the anti-pad diameter.

lkh747566933

Very good post, I'll learn from it, thank you very much for sharing!

led2015

Thanks for sharing. Modern PCB design software usually provides impedance control tools to help designers perform impedance matching and analysis.

fengerbbz

When you encounter a situation where impedance cannot be continuous in PCB design, you can consider the following methods to solve the problem: Use transition structure: During the transmission of the signal line, if you transition from one impedance value to another, you can use transition structure to achieve a smooth transition. For example, you can use transition line segments, transition devices, or transition layers to gradually change the impedance. Add impedance matching elements: Add appropriate impedance matching elements such as series resistors or parallel capacitors/inductors to the signal line to achieve the desired impedance matching. Adjust line width/line spacing: Change the impedance value by adjusting the width and line spacing of the signal line. Wider line width and larger line spacing will reduce impedance, while narrower line width and smaller line spacing will increase impedance. Use special materials: Choose special materials with the desired impedance characteristics to make PCBs. For example, using materials with specific dielectric constants can achieve the desired impedance. Use impedance control layers/planes: Adding impedance control layers or planes in PCB design can help achieve the desired impedance matching. By adding ground planes or power planes above or below the signal layer, the impedance of the signal line can be effectively controlled. Use simulation and verification tools: Using professional PCB simulation and verification tools, such as SI/PI simulation software, can help predict and optimize impedance matching problems in PCB design. Please note that depending on the specific design requirements and circuit characteristics, it may be necessary to combine multiple methods to solve the problem of impedance discontinuity. In addition, for high-frequency signals and high-speed transmission lines, impedance control is more important, so it should be carefully considered and optimized accordingly during the design process. If professional guidance and support are needed, it is recommended to consult a circuit design expert or PCB design engineer.

一叶知秋霜

I'm a beginner, come here to get some experience points, thanks for sharing, let's make progress together, thank you la la la la

lkh747566933

This is a good piece of knowledge. I will also learn from it and improve myself.