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Why do PCB boards need to be impedance

o-leading. o-leading.com 2019-03-04 18:53:24

What is the significance of impedance for PCB boards? Why do PCB boards need to be impedance? Let me introduce you to the following.

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What is impedance?

In circuits with resistance, inductance, and capacitance, the obstruction to alternating current is called impedance. The impedance is usually represented by Z, which is a complex number. The real part is called a resistor, and the imaginary part is called a reactance. The blocking effect of the capacitor on the alternating current in the circuit is called the capacitive reactance. The inductance acts on the alternating current in the circuit. For inductive reactance, the blocking effect of capacitors and inductors on the AC in the circuit is collectively called reactance. The unit of impedance is Europe.

Impedance type

(1) Characteristic impedance

In electronic information products such as computers and wireless communications, the energy transmitted in the circuit of the PCB is a square wave signal (called pulse pulse) composed of voltage and time, and the resistance encountered is It is called characteristic impedance.

(2) Differential impedance

The driver inputs two identical signal waveforms of opposite polarity, which are respectively transmitted by two differential lines, and the two differential signals are subtracted at the receiving end. The differential impedance is the impedance Zdiff between the two wires.

(3) odd mode impedance

The impedance of the first line to the ground in the two lines is Zoo, and the impedance values ​​of the two lines are the same.

(4) Even mode impedance

The drive inputs two identical signal waveforms of the same polarity, and the impedance Zcom when the two wires are connected together.

(5) Common mode impedance

The impedance of the first line to ground in the two lines, Zoe, the impedance values ​​of the two lines are the same, usually larger than the odd mode impedance.

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Why do PCB boards need to be impedance?

The pcb board impedance refers to the resistance and the parameters of the reactance, which hinder the AC. Impedance processing is essential in the production of pcb boards. The reasons are as follows:

1. The PCB line (the bottom of the board) should be considered for the installation of electronic components. After plugging, consider the conductive performance and signal transmission performance. Therefore, the lower the impedance, the better the resistivity should be lower than 1/TImes per square centimeter; -6 or less.

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

3. The tin plating of the PCB circuit board is the most prone to problems in the whole circuit board production, and is the key link affecting the impedance. The biggest defect of the electroless tin plating layer is that it is easy to change color (both easy to oxidize or deliquesce), and the brazing property is poor, which may cause the circuit board to be difficult to solder, and the impedance is too high, resulting in poor electrical conductivity or instability of the whole board performance.

4. There are various signal transmissions in the conductors of the PCB circuit board. When the transmission rate is increased, the frequency must be increased. If the line itself is different due to etching, laminate thickness, wire width, etc., the impedance value will change. The signal is distorted, resulting in a decline in the performance of the board, so it is necessary to control the impedance value within a certain range.

Impedance for PCB board

For the electronics industry, according to an industry survey, the most deadly weakness of the electroless tin plating layer is that it is easy to change color (both easy to oxidize or deliquesce), poor solderability leads to difficult soldering, high impedance, poor conductivity, or unstable overall board performance. Yi Changxi must cause short circuit of the PCB line to burn or fire.

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When the entire social production industry develops to a certain extent, many later participants often belong to each other's plagiarism. In fact, quite a few enterprises themselves do not have the ability to develop or innovate, so many products and their users' electronic products (circuit board boards) Bottom or electronic products as a whole have poor performance, and the most important cause of poor performance is due to impedance problems, because when the unqualified electroless tin plating technology is in use, the tin plated on the PCB board is actually It is not a true pure tin (or pure metal element), but a tin compound (that is, it is not a metal element at all, but a metal compound, an oxide or a halide, more directly, a non-metallic substance) or a tin compound. A mixture with a single element of tin metal, but it is difficult to find with the naked eye alone...

Because the main circuit of the PCB circuit board is copper foil, the solder joint on the copper foil is tinned, and the electronic components are soldered on the tin plating layer by solder paste (or solder wire). In fact, the solder paste is melted. The state is soldered between the electronic component and the tin plating layer, which is metallic tin (that is, a good conductive metal element), so it can be easily pointed out that the electronic component is connected to the copper foil of the bottom of the PCB through the tin plating layer, so the tin plating layer The purity and its impedance are the key; in addition, before we plug in the electronic components, we use the instrument to detect the impedance directly. In fact, the instrument probe (or the test pen) is also connected to the copper foil at the bottom of the PCB. The tin plating on the surface is connected to the copper foil at the bottom of the PCB to connect the current. Therefore, tin plating is the key, the key to affecting the impedance and the key to the performance of the PCB. It is also the key to easy to ignore.


It is well known that, except for metal simple substances, their compounds are poor conductors of electricity or even non-conductive (again, this is also the key to the distribution capacity or distribution capacity in the circuit), so there is such a conductive and non-conductive in the tin plating. The compound or mixture of tin, its ready-made resistivity or future oxidation, the resistivity after the electrolytic reaction caused by moisture and its corresponding impedance is quite high (since has affected the level or signal transmission in the digital circuit) Its characteristic impedance is also not consistent. Therefore, it will affect the performance of the circuit board and its complete machine.

Therefore, in terms of the current social production phenomenon, the coating material and performance on the bottom of the PCB is the most important and most direct cause of the characteristic impedance of the PCB, but it has the same aging and moisture electrolysis. Variability, so the impact of its impedance becomes more recessive and versatile. The main reasons for its concealment are: the first can not be seen by the naked eye (including its changes), the second can not be measured constantly, because it has It varies with time and environmental humidity, so it is always easy to be ignored.

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