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Why should high-speed PCB design control impedance matching

Everyone will understand the concept of impedance when they conduct high-speed PCB design. So why does impedance need to be controlled in high-speed PCB design? Which signals require controlled impedance and what impact does not control impedance have on the circuit? Keep reading!

Table of Contents

What is impedance matching?

Impedance Matching
Impedance Matching

Impedance matching is mainly used on transmission lines to achieve the purpose that all high-frequency microwave signals can be transmitted to the load point, and no signal will be reflected back to the source point, so that the input section and output end of the transmission line are in impedance matching state.

impedance matching

Why do we need impedance matching?

In low-speed PCB design, impedance matching does not need to be done, but in high-speed PCB design, in order to obtain complete, reliable, accurate, interference-free, and noise-free transmission signals, it is necessary to ensure that the circuit performance provided by the printed circuit board ensures that the signal is transmitted during the transmission process. No reflection phenomenon occurs, the signal is complete, the transmission loss is low, and it plays the role of matching impedance. If the key signal does not achieve impedance matching, it may cause reflection loss of the signal, etc. The originally good signal waveform will be deformed, which will directly affact the performance and even functionality of circuits.

What are the factors that affect impedance?

Factors that usually affect impedance

1. Er–dielectric constant:

The dielectric constants of different plates are also different.

The current common boards include paper substrates (commonly known as cardboard, plastic boards, V0 boards, flame retardant boards, etc.) epoxy fiberglass cloth substrates (commonly known as: epoxy boards, fiberglass boards, fiberboards, FR4) composite substrates ( Commonly known as: powder board, etc., cem-1 board is also called 22F in some parts of the country).

Currently, most boards use FR-4. The Er characteristics of this material change with the loading frequency. Its Er is considered to be around 4.2 when the usage frequency is below 1GHZ and will drop slightly when the usage frequency is 1.5-2.0GHZ, so we need to pay attention to the usage frequency of the product in our actual application.

2. H—medium thickness:

This factor has the greatest impact on impedance control. For example, the accuracy of impedance is very high, so the design of this part should be accurate. The H component of FR-4 is composed of various prepregs. Usually, the media is divided into the media thickness of the inner core board and the media thickness of the laminate in the multilayer PCB.

3. W — trace width:

In PCB design, different line widths will also have an impact on impedance. We usually analyze and calculate the impedance based on the actual situation to obtain the appropriate line width.

4. T — trace thickness:

Generally, reducing the line thickness can increase the impedance, and increasing the line thickness can decrease the impedance.

What types of impedance lines are there?

Single-ended Impedance vs Differential Impedance
Single-ended Impedance vs Differential Impedance

Generally, impedance lines can be divided into single-ended impedance and differential impedance. In the processing of RF lines, layer reference will be carried out to ensure that the RF antenna reaches the optimal line width and achieves the best performance. And in order to achieve impedance matching when designing a double-layer PCB, coplanar impedance needs to be created. The so-called coplanar impedance means that single-ended lines or differential lines that require impedance matching usually refer to the copper on both sides of the signal line to achieve an impedance-matching purpose.

Which lines usually require impedance control?

We have learned that not all lines have impedance-matching requirements. Only the high-speed lines need to control impedance. The impedance values of different signals are different. Differential impedance includes 90om, 100om, 120om, etc. Generally, USB2.0 requires 90om impedance control, HDMI, USB3.0, MIPI, 100M network port, Gigabit network port, etc. require 100om impedance control, and RS422 generally requires 120om impedance control. Generally, the impedance of a single-ended line can be controlled to 50om.

FAQ

Impedance matching is mainly used on transmission lines to achieve the purpose that all high-frequency microwave signals can be transmitted to the load point, and no signal will be reflected back to the source point, so that the input section and output end of the transmission line are in impedance matching state.

  • 1. Er–dielectric constant
  • 2. H—medium thickness
  • 3. W — trace width
  • 4. T — trace thickness

Generally, impedance lines can be divided into single-ended impedance and differential impedance.

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Kerstin

Author

Kerstin

Hi, I am Kerstin, graduating from one of a well- known university in China and I has a master's degree in physics. I have more than 5 year's experience as a professional engineer in PCB industry and expertise in PCB design, PCB assembly, PCB manufacturing, etc. I am committed to offering services and solutions about PCB/PCBA for various industries for their projects with professional knowledge. During 5 years of engineering career, I have done different circuit designing projects for different companies such as electronics, industry and medical devices, winning a lot of reputation among many customers. Selected as an outstanding employee of IBE every year. I'm always here to provide you with fast, reliable, quality services about PCB/PCBA.

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