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The difference of impedance characteristics in the production of impedance circu
  • Date:2020-10-29

What is the significance of PCB impedance for PCB? Why do PCB impedance? This paper first introduces what is impedance and the type of impedance, and then introduces why PCB should be impedance, and finally expounds the significance of impedance for PCB, so as to understand it with Xiaobian.
In circuits with resistance, inductance and capacitance, the blocking effect on alternating current is called impedance. Impedance is usually represented by Z, which is a complex number. The real part is called resistance, and the imaginary part is called reactance. The blocking effect of capacitance on alternating current in circuit is called capacitive reactance. The blocking effect of inductance on alternating current in circuit is called inductive reactance. The blocking effect of capacitance and inductance on alternating current in circuit is called reactance. The unit of impedance is the ohm.
(1) Characteristic impedance
In computer, wireless communication and other electronic information products, the energy transmitted by circuit board in the circuit is a square wave signal (called pulse) composed of voltage and time, and the resistance it encounters is called characteristic impedance.
(2) Odd mode impedance
The impedance Zo of one of the two lines to the ground is the same.
(3) Even mode impedance
The impedance zcom of two same signal waveforms with the same polarity input at the drive end when the two wires are connected together.
(4) Differential impedance
Two identical signal waveforms with opposite polarity are input at the driving end and transmitted by two differential lines respectively, and the two differential signals are subtracted at the receiving end. The differential impedance is the impedance zdiff between two lines.
(5) Common mode impedance
The impedance Zoe of one of the two lines to the ground is the same, which is usually larger than the odd mode impedance.
The impedance of PCB circuit board refers to the parameters of resistance and reactance, which hinder the alternating current. In PCB production, impedance processing is essential. The reasons are as follows:
1. PCB circuit board (bottom) should consider plug-in installation of electronic components, and consider conductivity and signal transmission performance after plug-in, so the lower the impedance is, the better, and the resistivity should be less than 1 & times; 10-6 per square centimeter.
2. PCB circuit board in the production process to go through copper deposition, tin plating (or electroless plating, or thermal spray tin), connector soldering and other processes, and the materials used in these links must ensure the low resistivity, in order to ensure that the overall impedance of the circuit board is low, meet the product quality requirements, and can operate normally.
3. The tinning of PCB is the most common problem in the production of PCB, and it is the key to affect the impedance. The biggest defects of electroless tin coating are easy to change color (easy to oxidize or deliquescence) and poor solderability, which will lead to difficult soldering of circuit board, high impedance and poor conductivity or instability of the whole board performance.
4. There will be a variety of signal transmission in the conductor of PCB circuit board. In order to improve its transmission rate, it is necessary to increase its frequency. If the line itself is different due to etching, stack thickness, wire width and other factors, it will cause the impedance value to change, make its signal distortion, and lead to the decline of circuit board performance. Therefore, it is necessary to control the impedance value within a certain range.
Impedance line in impedance circuit board
Impedance PCB diagram
For the electronic industry, according to the industry survey, the most fatal weaknesses of electroless tin coating are easy to change color (easy to oxidize or deliquescence), poor solderability leading to difficult soldering, high impedance leading to poor conductivity or instability of the whole board performance, and easy tin growth leading to PCB circuit short circuit, even burning or fire.
It is reported that Kunming University of science and technology was the first to study electroless tin plating in China in the early 1990s, followed by Guangzhou Tongqian chemical (enterprise) in the late 1990s. Up to now, the two organizations have been recognized in the industry for 10 years, and they are the best. Among them, according to our contact screening investigation, experimental observation and long-term endurance test of many enterprises, it is confirmed that the tin coating of Tongqian chemical is a pure tin layer with low resistivity, and the quality of electrical conductivity and brazing can be guaranteed to a higher level. No wonder they dare to guarantee that their coating can keep one year without any sealing and anti tarnishing agent protection Peeling, permanent no tin whiskers.
Later, when the whole social production industry developed to a certain extent, many later participants often plagiarized from each other. In fact, quite a number of enterprises did not have the ability of R & D or initiative. Therefore, the performance of many products and their users' electronic products (circuit board bottom or electronic products as a whole) was poor, and the main reason for the poor performance was because Impedance problem, because when the unqualified electroless tin plating technology is in use, the tin plated on PCB is not really pure tin (or pure metallic substance), but tin compound (that is, it is not metallic substance at all, but metal compound, oxygenate or halide, more directly, it belongs to non-metallic substance) or tin compound and tin metal monomer It's a mixture of different substances, but it's hard to find it with the naked eye.
Because the main circuit of PCB circuit board is copper foil, the solder joint of copper foil is tinned layer, and the electronic components are welded on the tinned layer through solder paste (or solder wire). In fact, the metal tin (that is, the metal element with good conductivity) is welded between the electronic components and the tin coating in the melting state of solder paste, so it can be simply pointed out that the electronic components are connected The tin coating is then connected with the copper foil at the bottom of the circuit board, so the purity and impedance of the tin coating are the key; in addition, before the electronic components are plugged in, we directly use the instrument

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