What is The Meaning of Impedance For PCB Circuit Board

What is the meaning of impedance for PCB circuit boards, and why should PCB circuit boards do impedance? This article first introduces what is the impedance and the type of impedance, then introduces the PCB circuit board why to do impedance, and finally explains the significance of impedance for PCB circuit board, and the specific with Xiaobian to understand.

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

1. Characteristic impedance

In electronic information products such as computers and wireless communications, the energy transmitted in the PCB line is a squarewavesignal (squarewavesignal, known as pulse pulse) composed of voltage and time, and the resistance it encounters is called the characteristic impedance.

2. Odd-mode impedance

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

3. Differential impedance

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

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 to the ground in the two lines is Zoe, and the impedance values of the two lines are the same, usually greater than the odd-mode impedance.

PCB impedance refers to the parameters of resistance and reactance, which plays an obstruction role in alternating current. In PCB circuit board production, impedance treatment is essential. Here's why:

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

2. PCB circuit board in the production process to experience copper sinking, tin plating (or electroless plating, or hot spray tin), solder solder and other process production links, and the materials used in these links must ensure that 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 tinning of PCB board is the most prone to problems in the production of the entire circuit board, and it is the key link affecting the impedance. The biggest defect of the chemical tin plating layer is easy discoloration (both easy oxidation or delixing), poor brazing, which will lead to difficult welding of the circuit board, high impedance leading to poor electrical conductivity or instability of the whole board performance.

4. the conductor in the circuit board factory will have a variety of signal transmission, when to improve its transmission rate and must increase its frequency, the line itself if the etching, laminated thickness, wire width and other factors are different, will cause the impedance worth changing, so that the signal distortion, resulting in the use of the circuit board performance decline, so you need to control the impedance value in a certain range.

For the electronics industry, according to the industry survey, the most fatal weakness of the chemical tin plating layer is easy discoloration (both easy oxidation or moisture), poor brazing resulting in difficult welding, high impedance resulting in poor electrical conductivity or instability of the whole board performance, easy long tin must lead to PCB circuit short circuit and burn or fire events.

It is reported that the first study of chemical tin plating in China was the Kunming University of Science and Technology in the early 1990s, followed by Guangzhou Tongqian Chemical (enterprise) in the late 1990s, and has been recognized in the industry for 10 years that the two institutions are the best. Among them, according to our contact screening investigations, experimental observations and long-term endurance tests of many enterprises, it has been confirmed that Tongqian Chemical's tin plating layer is a pure tin layer with low resistivity, and the quality of conductivity and brazing can be guaranteed to a higher level. No wonder they dare to ensure that their coating is protected without any sealing and anti-color agent. Can maintain a year without discoloration, no foaming, no peeling, never long tin whiskers.

Later, when the entire social production industry has developed to a certain extent, many later participants often belong to each other, in fact, a considerable number of enterprises themselves do not have the ability to research and develop or create, so many products and their users of electronic products (circuit board bottom or electronic products as a whole) performance is poor, The main reason for the poor performance is because of the impedance problem, because when the unqualified chemical tin plating technology is used in the process, the tin plated on the PCB board is not really pure tin (or pure metal element), but tin compounds (that is, it is not a metal element at all, but a metal compound, oxide or halide). More directly, it is a non-metallic substance) or a mixture of tin compounds and tin metals, but it is difficult to detect with the naked eye alone.

Because the main line of the PCB board is copper foil, the solder joint of the copper foil is a tin layer, and the electronic components are welded to the tin layer above the solder paste (or solder wire), in fact, the solder paste in the melting state is welded to the electronic components and tin coating between the metal tin (that is, a good conductive metal element), so it can be simply pointed out, The electronic components are connected to the copper foil at the bottom of the PCB through the tin coating, so the purity of the tin coating and its impedance are the key; Also, before plugging in the electronic components, when we directly use the instrument to detect the impedance, in fact, both ends of the instrument probe (or pen) are connected by first contacting the tin coating on the copper foil surface of the PCB bottom and then the copper foil at the bottom of the multi-layer PCB board to communicate the current. Therefore, the tin coating is the key, the key that affects the impedance and the key that affects the performance of the entire PCB board, and the key that is easy to be ignored.

As we all know, in addition to the metal element, its compounds are poor conductors of electricity or even non-conductive (in addition, this is also the key to the existence of distribution capacity or distribution capacity in the line), so the presence of such conductive rather than conductive tin compounds or mixtures in the tin coating, Its ready-made resistivity or the resistivity after the electrolytic reaction of future oxidation and moisture and its corresponding impedance are quite high (enough to affect the level or signal transmission in the digital circuit), and its characteristic impedance is 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 substance and performance on the bottom of the multi-layer PCB board is the main reason and the most direct reason for affecting the characteristic impedance of the multi-layer PCB board, but because of its variability with the aging of the coating and moisture electrolysis, the impact of its impedance becomes more recessive and variable, and the main reason for its concealment is: The first can not be seen by the naked eye (including its changes), the second can not be always measured, because it has changes with the change of time and environmental humidity, so it is always easy to be ignored.