1、 Why do PCB boards need impedance
PCB circuit board impedance refers to the parameters of resistance and reactance, which hinder alternating current. In PCB production, impedance treatment is essential. The reasons are as follows:
1. PCB circuit (board bottom) should consider the installation of electronic components, after the plug-in consideration of conductivity and signal transmission performance and other issues, so the lower the impedance, the better, the resistivity should be less than 1 & times; 10-6 per square centimeter.
2. PCB circuit board in the production process has to go through copper deposition, electro tinning (or electroless plating, or hot spray tin), connector soldering process, and the materials used in these links must ensure the low resistivity, to ensure that the overall impedance of the circuit board is low, meet the product quality requirements, and can operate normally.
3. The tin plating of PCB circuit board is the most easy problem in the production of PCB, and it is the key link to affect the impedance. The biggest defect of electroless tin coating is easy to change color (easy to oxidize or deliquescence), poor solderability, which will lead to the circuit board difficult to weld, high impedance, poor conductivity or instability of the whole board performance.
4. There will be various signal transmission in the conductor of PCB circuit board. When the frequency must be increased in order to improve the transmission rate, if the circuit itself is different due to different factors such as etching, stack thickness, wire width, etc., the impedance value will change, the signal will be distorted and the performance of the circuit board will be reduced. Therefore, it is necessary to control the impedance value within a certain range.
2、 The significance of impedance to PCB
For the electronic industry, according to the industry survey, the most fatal weakness of electroless tin coating is easy to change color (i.e. easy to oxidize or deliquescence), poor solderability leads to difficult soldering, high impedance leads to poor conductivity or instability of the whole board performance, and easy to grow tin leads to PCB circuit short circuit, burn out or fire events.
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 institutions have been recognized in the industry as 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 industry is pure tin layer with low resistivity, and the quality of electric conduction and brazing can be guaranteed to a higher level. No wonder they dare to guarantee that the coating will not change color, blister or blister for one year without any sealing and anti tarnishing agent protection Peeling, permanent no whiskers.
Later, when the whole social production industry developed to a certain extent, many later participants often copied from each other. In fact, quite a number of enterprises themselves did not have the ability of research and development 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 poor performance was resistance Anti problem, because when unqualified electroless tin plating technology is used, the tin plated on PCB is not really pure tin (or pure metal element), but tin compound (that is, it is not metal element at all, but metal compound, oxide or halide, more directly speaking, it belongs to non-metallic substance) or the mixture of tin compound and tin metal element Things, but it is difficult to find with the naked eye.
Because the main circuit of PCB circuit board is copper foil, the solder joint of copper foil is tinning layer, and electronic components are welded on the tin coating through solder paste (or solder wire). In fact, the solder paste is soldered between the electronic components and tin coating in the melting state, which is the metal element with good conductivity. Therefore, it can be simply pointed out that the electronic components are plated by tin The layer is then connected with the copper foil at the bottom of the PCB, so the purity and impedance of the tin coating are the key. However, before the electronic components are inserted, when we directly use the instrument to detect the impedance, in fact, both ends of the instrument probe (or probe) are connected with the current by contacting the tin coating on the copper foil surface at the bottom of the PCB and then connecting with the copper foil at the bottom of the PCB. Therefore, tin coating is the key, which affects the impedance and the performance of PCB, and is also the key to be ignored.
It is well known that, except for metal elements, their compounds are poor conductors of electricity or even non-conductive (and this is also the key to the existence of distribution capacity or transmission capacity in the circuit). Therefore, when there is such a conductive rather than non-conductive tin compound or mixture in the tin coating, the ready-made resistivity or the resistivity after the electrolytic reaction of future oxidation and damp and its corresponding The impedance is quite high (enough to affect the level or signal transmission in the digital circuit), and its characteristic impedance is not consistent. So it will affect the performance of the circuit board and its whole machine.
Therefore, as far as the current social production phenomenon is concerned, the coating material and performance on the bottom of PCB board are the most important and direct reason for influencing the characteristic impedance of the whole PCB. However, due to its variability with the aging of the coating and the electrolysis affected by moisture, the worrying influence of the impedance becomes more recessive and changeable. The main reason for its concealment is: first, it can not be seen by the naked eye Second, what we see (including its change), can not be measured constantly, because it has the variability with the change of time and environmental humidity, so it is always easy to be ignored.
