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Engineering Technology
14Nov

# PCB impedance board impedance line design

What is the meaning of impedance to PCB circuit boards, why PCB circuit boards do impedance? This paper first introduces what is impedance and impedance type, then introduces the PCB circuit board why to do impedance, describes the significance of impedance for PCB circuit board, the specific follow me together to understand.

What is the meaning of impedance to PCB circuit boards, why PCB circuit boards do impedance? This paper first introduces what is impedance and impedance type, then introduces the PCB circuit board why to do impedance, and finally expounds the significance of impedance for PCB circuit board, the specific follow Xiaobian together to understand.

What is impedance

In a circuit with resistance, inductance, and capacitance, the resistance to alternating current is called impedance. Impedance is often represented by Z, is a complex number, the real part is called resistance, the imaginary part is called reactance, where the capacitor in the circuit to AC current obstruction effect is called capacitive reactance, inductance in the circuit to AC current obstruction effect is called inductive reactance, capacitance and inductance in the circuit to AC current caused by obstruction effect is called reactance. The unit of impedance is ohms.

Impedance type

(1) Characteristic impedance

In computer, wireless communication and other electronic information products, PCB line transmission energy, is a square wave signal composed of voltage and time (known as pulse), it encountered resistance is called characteristic impedance.

(2) differential impedance

Two identical signal waveforms of opposite polarity are input at the driving end, which are respectively transmitted by two differential wires. The two differential signals are subtracted at the receiving end. Differential impedance is simply the impedance Zdiff between two lines.

(3) odd-mode impedance

The impedance of one of the two lines to the ground Zoo, the impedance values of the two lines are the same.

(4) even mode impedance

The driving end inputs two identical signal waveforms of the same polarity, and the impedance Zcom when the two lines are connected.

(5) common-mode impedance

The impedance of one line of the two lines to the ground is Zoe, and the impedance values of the two lines are the same, usually larger than the odd-mode impedance.

Why should PCB circuit boards do impedance

pcb circuit board impedance refers to the resistance and reactance of the parameter, the alternating current plays an obstruction role. Impedance processing is essential in pcb circuit board production. Here's why:

1, PCB line (bottom of the board) to consider plug installation of electronic components, plug into the electrical performance and signal transmission performance, so it will require the lower the impedance, the better, resistivity is lower than 1&TImes per square centimeter; Below 10-6.

2, PCB circuit board in the production process to experience copper sinking, electroplating tin (or electroless plating, or hot spray tin), solder joints and other processes, and the materials used in these links must ensure the resistivity bottom, in order to ensure that the overall impedance of the circuit board is low to meet the product quality requirements, can operate normally.

3, the tin plating of PCB circuit board is the most prone to problems in the production of the whole circuit board, is the key link affecting the impedance. The biggest defect of electroless tin layer is easy to change color (easy oxidation or delixing), poor brazing, which will lead to the welding of the circuit board, high impedance leading to poor conductivity or instability of the whole board performance.

4, PCB circuit board in the conductor will have a variety of signal transmission, when to improve its transmission rate and must improve its frequency, the line itself if the etching, laminated thickness, wire width and other factors are different, will cause impedance value change, so that the signal distortion, resulting in circuit board performance decline, so it is necessary to control the impedance value in a certain range.

Impedance for PCB circuit board significance

For the electronic industry, according to the investigation in the industry, the most fatal weakness of the chemical tin layer is easy to discolor (easy oxidation or moisture), poor brazing resulting in difficult welding, impedance is too high leading to poor conductivity or instability of the whole board performance, easy to lead to long tin PCB circuit short circuit and burn or fire events.

It is reported that the first study of chemical tin plating in China is the Kunming University of Science and Technology in the early 1990s, and then Guangzhou Tongqian Chemical (enterprises) in the late 1990s. Until now, the two institutions have been recognized within the industry for 10 years is the best. Among them, according to our contact screening investigation of many enterprises, experimental observation and long-term endurance test, confirmed with the same small chemical tinning layer is a pure tin layer of low resistivity, conductive and brazing quality can be guaranteed to a higher level, no wonder they dare to guarantee its coating without any closed and anti-color agent protection, can maintain a year without discoloration, no foaming, no peeling, forever A tin beard does not grow long.

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