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PCB impedance knowledge
2021-01-23
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PCB PCB impedance knowledge

What is the significance of impedance to PCB circuit board, PCB circuit board why to do impedance?This paper first introduces what is impedance and the type of impedance, then introduces why PCB circuit board should do impedance, and finally describes the significance of impedance to PCB circuit board, specific to follow the small make up together to understand.

What is impedance

In circuits with resistors, inductors, and capacitors, the resistance to alternating current is called impedance.Impedance is often expressed by Z, is a complex number, the real part is called resistance, the virtual part is called reactance, the capacitor in the circuit to the alternating current of the obstruction is called reactance, the inductance in the circuit to the alternating current of the obstruction is called inductance, capacitance and inductance in the circuit to the alternating current caused by the obstruction is always called reactance.The unit of impedance is ohms.

Impedance type

(1) Characteristic impedance

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

(2) Differential impedance

The driver input two of the same signal waveforms with opposite polarity, respectively transmitted by two differential lines, at the receiving end the two differential signals subtraction.Differential impedance is the impedance Zdiff between two lines.

(3) Odd-mode impedance

Impedance ZOO of the first line to the ground in the two lines is consistent.

(4) even mode impedance

The driver input two of the same signal waveform with the same polarity, the impedance ZCOM when the two lines are connected together.

(5) Common mode impedance

The impedance ZOE of one of the two lines to the ground. The impedance values of the two lines are the same, usually greater than the odd-mode impedance.

Why do PCB boards need impedance

PCB circuit board impedance refers to the resistance and to the reactance of the parameters, the alternating current has a blocking effect.Impedance processing is essential in PCB circuit board production.Here's why:

1, PCB line (plate bottom) to consider the installation of electronic components, plug into the consideration of electrical conductivity and signal transmission performance and other issues, so it will require the lower the impedance, the better, the resistivity should be lower than 1× per square centimeter;10-6 below.

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

3. Tin plating of PCB circuit board is the most likely problem in the whole circuit board production, and it is the key link that affects the impedance.The biggest defect of electroless tin coating is easy to discolor (both easy oxidation or deliquation), poor brazing, will lead to the difficult welding of the circuit board, high impedance leads to poor conductivity or the instability of the whole board performance.

4, PCB circuit board in the conductor will have all kinds of signals, when the transmission rate and to raise its must raise its frequency, if the line itself due to factors such as etching, laminated thickness, wire width is different, will cause resistance to change, make the signal distortion, led to the decrease of the circuit board using performance, so they need to control the impedance value within a certain range.

The world's largest PCB comes from a flexible multilayer circuit board used in unmanned solar-powered aircraft by Johnson Electronics, a British firm.It is about 28 meters long.The circuit board made from soft polyimide as a base has better heat dissipation and higher conductivity density.Such a long circuit board is made by a special manufacturing process of section etching.