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Why do PCB circuit boards have impedance

2020-03-23

First, why do PCB circuit boards have impedance?
pcb circuit board impedance refers to the parameters of resistance and reactance, which hinders AC power. In pcb circuit board production, impedance processing is essential. The reasons are as follows:

1. The PCB circuit (bottom) should consider plug-in installation of electronic components, and consider the issues of electrical conductivity and signal transmission after plug-in. Therefore, it is required that the lower the impedance, the better, and the resistivity should be lower than 1 & TImes; 10 per square centimeter. Below -6.

2. During the production process of the PCB circuit board, it needs to go through the process of sinking copper, electroplating tin (or chemical plating, or thermal spraying), connector soldering and other process manufacturing processes, and the materials used in these links must ensure the resistivity bottom to ensure The overall impedance of the circuit board is low enough to meet product quality requirements and can operate normally.

3. The tinning of PCB circuit boards is the most prone to problems in the production of the entire circuit board, and it is the key link that affects impedance. The biggest drawbacks of the electroless tin plating layer are easy discoloration (both easy to oxidize or deliquesce), poor solderability, which will make the circuit board difficult to solder, too high impedance, resulting in poor conductivity or instability of the entire board performance.

4. There will be various signal transmissions in the conductors of the PCB circuit board. When the frequency must be increased in order to increase its transmission rate, if the line itself is different due to factors such as etching, stack thickness, and wire width, it will cause the impedance to change. To make its signal distorted, leading to the degradation of the performance of the circuit board, it is necessary to control the impedance value within a certain range.

PCB circuit boards

The meaning of impedance for PCB circuit boards

For the electronics industry, according to industry surveys, the most lethal weaknesses of electroless tin plating are easy discoloration (both easy to oxidize or deliquesce), poor solderability leading to difficult soldering, high impedance leading to poor conductivity or instability of the entire board 2. Easy-to-chang tin must cause short circuit of PCB circuit and even burn or fire.

It is reported that the first study of chemical tin plating in China was Kunming University of Science and Technology in the early 1990s, and then Guangzhou Tongqian Chemical (Enterprise) in the late 1990s. Until now, the two institutions have recognized the two institutions as Get the best. Among them, according to our contact screening surveys, experimental observations, and long-term endurance tests on many enterprises, it was confirmed that the tin plating layer of Tongqian Chemical is a low-resistivity pure tin layer. The quality of conductivity and soldering can be guaranteed to a high level. No wonder they dare to guarantee to the outside that their coatings will not change color, no blistering, no peeling, and no long tin whisker for one year without any sealing and anti-discoloration agent protection.

Later, when the entire social production industry developed to a certain extent, many later participants often belonged to plagiarism. In fact, quite a few companies themselves did not have the R & D or pioneering capabilities. 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 due to the impedance problem, because when the unqualified electroless tin plating technology is used, it is actually the tin plated on the PCB circuit board. Not really pure tin (or pure metal elementary substance), but tin compounds (that is, not metal elementary substances at all, but metal compounds, oxides or halides, and more directly non-metallic substances) or tin A mixture of a compound and a tin metal element, but it is difficult to find with the naked eye ...

Because the main circuit of the PCB circuit board is copper foil, the soldering point of the copper foil is a tin plating layer, and the electronic components are soldered on the tin plating layer by a solder paste (or solder wire). In fact, the solder paste is melting. The state soldered between the electronic component and the tin plating layer is metal tin (ie, a conductive metal element), so it can be simply pointed out that the electronic component is connected to the copper foil on the bottom of the PCB through the tin plating layer, so the tin plating layer The purity and impedance are the key; but before we plug in the electronic components, we use the instrument to test the impedance directly. In fact, the two ends of the instrument probe (or test lead) also pass through the copper foil on the bottom of the PCB first. The tin plating on the surface communicates with the copper foil on the bottom of the PCB. So tin plating is the key, the key to the impedance, the key to the performance of the PCB, and the key to be easily overlooked.

As we all know, except metal simple compounds, their compounds are all poor conductors of electricity or even non-conductive (also, this is also the key to the distribution capacity or transmission capacity in the circuit), so this tin-like coating exists in this kind of conductive rather than conductive For tin compounds or mixtures, their ready-made resistivity or future oxidation and resistivity after the electrolytic reaction due to moisture and its corresponding impedance are quite high (which has affected the level or signal transmission in digital circuits), and The characteristic impedances are also inconsistent. So it will affect the performance of the circuit board and its entire machine.

Therefore, in terms of the current social production phenomenon, the coating material and performance on the bottom of the PCB are the most and most direct reasons affecting the characteristic impedance of the entire PCB, but because it has the ability to electrolyze with the aging of the coating and moisture. Because of its variability, the anxiety effect of its impedance becomes more recessive and changeable. The main reason for its concealment is that the first cannot be seen by the naked eye (including its changes), and the second cannot be measured constantly because it has Variability over time and ambient humidity, so it is always easy to ignore.