The impedance of pcb circuit board 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.The PCB circuit (bottom of the board) should consider plugging and installing electronic components. After plugging, the conductivity and signal transmission performance should be considered. Therefore, the lower the impedance, the better, and the resistivity should be less than 1&mes;10 per square centimeter -6 or less.
2. During the production process, PCB circuit boards have to go through process steps such as copper sinking, tin electroplating (or chemical plating, or thermal spray tin), connector soldering, etc. The materials used in these process must ensure that the resistivity is low, in order 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 PCB circuit boards is the most prone to problems in the entire circuit board production, and it is a key process that affects 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 are various signal transmissions in the conductors in the PCB circuit board. When it is necessary to increase its frequency in order to increase its transmission rate. If the circuit is different due to factors such as etching, stack thickness, wire width, etc., the impedance value will change. So that its signal is distorted and the performance of the circuit board is degraded, so 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, possible long tin whiskers cause short circuit of PCB circuit and even burn out or catch fire.
When the entire social production industry develops to a certain extent, many subsequent manufacture often copy each other. In fact, a considerable number of companies themselves do not have the R&D or initiative capabilities themselves. Therefore, many products and their users’ electronic products (circuit board boards) The main reason for the poor performance is the impedance problem, because when the unqualified electroless tin plating technology is in use, the tin plated on the PCB circuit board is actually not good. Not really pure tin (or pure metal element), but tin compound (that is, it is not a metal element at all, but a metal compound, oxide or halide, or more directly a non-metal substance) or tin compound It is a mixture with tin metal, but it is difficult to find with the naked eye.
Because the main circuit of the PCB circuit board is copper foil, there is a tin-plated layer on the solder joints of the copper foil, and the electronic components are soldered on the tin-plated layer through solder paste (or solder wire). In fact, the solder paste that is soldered between the electronic components and the tin plating layer in the molten state is metallic tin (ie, a metal element with good conductivity). Therefore, it can be pointed out simply that the electronic components are connected to the copper foil at the bottom of the PCB through tin plating, so the purity and impedance of the tin plating is 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 foil on the bottom of the PCB board. The tin plating on the surface is then connected to the copper foil at the bottom of the PCB to communicate the current. Therefore, tin plating is the key that affects the impedance and affects the performance of the entire PCB, and it 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). The natual 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 PCB board are the most important and most direct reasons affecting the characteristic impedance of the entire PCB. However, because of its variability with the aging of the coating and the damp electrolysis, the worrying impact of its impedance becomes more invisible and variable. The main reason for its concealment is: first, it cannot be seen by the naked eye (including its changes). Second, it cannot be measured constantly, because it has the variability that changes with time and environmental humidity, so it is always easy to be ignored.
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