What does PCB board impedance control mean? How to make impedance on PCB?
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What does PCB board impedance control mean? How to make impedance on PCB?

Posted Date: 2024-01-18

What does PCB board impedance control mean? How to make impedance on PCB?

PCB board impedance control refers to ensuring that the design meets specific impedance requirements by optimizing electrical characteristics and signal integrity during the PCB (Printed Circuit Board) design and manufacturing process. In high-speed digital and analog circuits, impedance control is very important because it directly affects the speed, quality and stability of signal transmission.

Impedance on a PCB board refers to the resistance and inductance that current encounters when it flows along a wire or current trace. Their presence suppresses current fluctuations and signal distortion. In PCB design, impedance control mainly focuses on three parameters: resistance (R), capacitance (C) and inductance (L). PCB designers need to determine the appropriate impedance level based on the requirements of the circuit, especially the needs of high-frequency and high-speed signals.

To achieve impedance control, here are some common methods:

1. Based on circuit board level design:

PCB designers can take full advantage of the PCB's hierarchical structure to achieve impedance control. By placing different signal layers in different layers, interlayer capacitance and inductance can be effectively controlled. Generally speaking, high-impedance materials are used for the inner layer and low-impedance materials for the outer layer to reduce the effects of reflections and crosstalk.

2. Use differential signal transmission lines:

Differential signal transmission lines can provide better immunity to interference and lower crosstalk risk. Differential signal transmission lines are a pair of parallel wires with opposite voltages but equal sizes, which provide better signal integrity and immunity to interference. The impedance of differential signal transmission lines is usually controlled by the choice of conductor spacing, width, and ground plane.

3. Control routing geometry:

Geometric parameters such as PCB line width, spacing and arrangement can also be used to control impedance. For common microstrip lines, thicker line widths and larger spacing can reduce impedance. With coaxial lines, a smaller inner conductor diameter and a larger outer conductor radius can increase impedance. The choice of routing geometry needs to be optimized based on specific impedance requirements and signal frequency.

4. PCB material selection:

The dielectric constant of the PCB material also affects impedance. Selecting materials with stable dielectric properties is part of impedance control. In high frequency and high speed applications, commonly used materials are FR-4 (fiberglass reinforced), PTFE (polytetrafluoroethylene) and RF (radio frequency) laminates.

5. Use simulation and design tools:
Using simulation and design tools can help designers quickly and accurately verify and optimize impedance before proceeding with PCB design. These tools can simulate circuit behavior, signal transmission losses and electromagnetic interactions to determine optimal PCB design parameters. Some common simulation tools include CST Studio Suite, HyperLynx, and ADS.

In short, PCB board impedance control is a vital task, which plays a key role in high-speed digital and analog circuits. Through proper hierarchical design, use of differential signal transmission lines, controlled routing geometry, selection of appropriate PCB materials, and use of simulation and design tools, precise impedance control can be achieved to improve circuit performance and signal integrity.


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