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## PCB News

Talk about the impedance knowledge of multilayer circuit boards
2021-08-27
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Author：Aure

Talk about the impedance knowledge of multilayer circuit boards

What is the significance of impedance to multilayer circuit boards, and why do multilayer circuit boards need impedance? This article first introduces what impedance and its types are, then it introduces why multilayer circuit boards need impedance, and finally explains the meaning of impedance to PCB multilayer circuit boards. Follow the editor to learn more about it.

In a circuit with resistance, inductance, and capacitance, the obstacle to alternating current is called impedance. Impedance is often represented by Z, which is a complex number. The real part is called resistance, and the imaginary part is called reactance. The hindering effect of capacitance on alternating current in the circuit is called capacitive reactance, and the hindering effect of inductance on alternating current in the circuit is called For inductive reactance, the obstructive effect of capacitance and inductance on alternating current in the circuit is collectively called reactance. The unit of impedance is ohms.

(1) Characteristic impedance

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

(2) Odd Mode Impedance

The impedance value of one line to the ground of the two lines is the same as the impedance value of the two lines.

(3) Even mode impedance

The two same signal waveforms with the same polarity are input to the drive terminal, and the impedance Zcom when the two wires are connected together.

(4) Differential impedance

Two identical signal waveforms with opposite polarities are input at the driving end, 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.

(5) Common mode impedance

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

Multilayer circuit board impedance refers to the parameters of resistance and reactance, which hinder the alternating current. In the production of pcb circuit boards, impedance processing is essential. The reasons are as follows:

1. For multi-layer circuit boards (bottom of the board), consider plugging and installing electronic components. After plugging, consider issues such as electrical conductivity and signal transmission performance. Therefore, the lower the impedance, the better, and the resistivity should be less than 1 per square centimeter.&TImes ; Below 10-6.

2. The production process of multi-layer circuit boards has to go through the process of copper sinking, tin electroplating (or chemical plating, or thermal spray tin), connector soldering, etc., and the materials used in these links must ensure that the resistivity is low to ensure The overall impedance of the circuit board is low to meet the product quality requirements and can operate normally.

3. The tinning of multilayer circuit boards is the most prone to problems in the production of the entire circuit board, and it is the key link that affects the impedance. The biggest defect of the electroless tin coating is easy discoloration (easy to be oxidized or deliquescent) and poor solderability, which will lead to difficult soldering of the circuit board, high impedance, poor electrical conductivity, or instability of the overall board performance.

4. There will be various signal transmissions in the conductors of the multilayer circuit board. When the frequency must be increased to increase its transmission rate, the circuit itself will cause impedance to be worthwhile due to factors such as etching, laminate thickness, and wire width. The change causes the signal to be distorted and the performance of the circuit board is degraded. Therefore, it is necessary to control the impedance value within a certain range.

For the electronics industry, according to industry surveys, the most fatal weakness of the electroless tin coating is its easy discoloration (easy to be oxidized or deliquescent), poor brazing properties leading to difficult soldering, and high impedance leading to poor electrical conductivity or instability of the overall board performance. 、Easy long tin whiskers cause short circuit of PCB circuit and even burn out or catch fire.

It is reported that the first domestic research on chemical tin plating was Kunming University of Science and Technology in the early 1990s, followed by Guangzhou Tongqian Chemical (enterprise) in the late 1990s. Up to now, the industry has recognized that these two institutions are The best. Among them, according to our contact screening investigations, experimental observations and long-term endurance tests of many companies, it is confirmed that the tin layer of Tongqian Chemical is a pure tin layer with low resistivity, and the quality of conductivity and brazing can be guaranteed to a high level. No wonder they dare to guarantee to the outside that the coating can keep its color for one year, no blistering, no peeling, and permanent tin whisker without any sealing and anti-tarnish protection.

Later, when the entire social production industry developed to a certain extent, many subsequent participants often copied each other. In fact, a considerable number of companies themselves did not have the R&D or initiative capabilities themselves. Therefore, many products and their users’ electronic products (circuit boards) The bottom of the board or the overall electronic product) performance is poor, and the main reason for the poor performance is the impedance problem, because when the unqualified electroless tin plating technology is in use, it is plated on the PCB multilayer circuit board. Tin is not really pure tin (or pure metal element), but a compound of tin (that is, it is not a metal element at all, but a metal compound, oxide or halide, or more directly, it is a non-metal substance) Or a mixture of tin compound and tin metal element, but it is difficult to find with the naked eye...

Because the main circuit of the multi-layer circuit board is copper foil, the solder joints of the copper foil are tin-plated, and the electronic components are soldered on the tin-plated layer through solder paste (or solder wire). In fact, the solder paste is melting. The molten state soldered between the electronic component and the tin plating layer is metal tin (that is, the metal element with good conductivity), so it can be simply pointed out that the electronic component is connected to the copper foil at the bottom of the circuit board through the tin plating layer, so tin The purity of the coating and its impedance are the key; but before the electronic components are plugged in, when we directly use the instrument to detect the impedance, in fact, the two ends of the instrument probe (or called the test lead) first touch the copper on the bottom of the PCB board. The tin-plated layer on the surface of the foil is then connected to the copper foil at the bottom of the circuit board to communicate the current. Therefore, tin plating is the key, the key that affects the impedance and the key that affects the performance of the entire circuit board, and it is also the key that is easy to be ignored.

As we all know, except for the simple substance of metal, its compounds are poor electrical conductors or even non-conductive (also, this is the key to the distribution capacity or spreading capacity in the circuit), so there is this kind of quasi-conductive rather than conductive in the tin plating layer In the case of tin compounds or mixtures, the existing resistivity or the resistivity after the electrolysis reaction due to future oxidation or damp and the corresponding impedance are quite high (enough to affect the level or signal transmission in the digital circuit) and The characteristic impedance is also not consistent. So it will affect the performance of the circuit board and the whole machine.

Therefore, as far as the current social production phenomenon is concerned, the coating material and performance on the bottom of the circuit board are the most important and the most direct reason that affects the characteristic impedance of the entire circuit board. Because of its variability, the worrying impact of its impedance has become more invisible and variable. The main reasons for its concealment are: firstly, it cannot be seen by the naked eye (including its changes), and secondly, it cannot be measured constantly because of its concealment. There is variability that changes with time and environmental humidity, so it is always easy to be ignored.