Why should pcb circuit boards do impedance

A common impedance term for PCBS is "characteristic impedance".

Take, for example, the characteristic impedance of a transmission line on a PCB board: Generally, if the characteristic impedance is consistent everywhere on the same PCB interconnection line, such a transmission line on a PCB board is of high quality. What kind of circuit board is called a controlled impedance circuit board? A circuit board with controlled impedance means that the characteristic impedance of all transmission lines on the PCB board conforms to a uniform target specification, usually meaning that the characteristic impedance of all transmission lines is between 25Ω and 70Ω.

What is the main cause of high impedance PCB? Or PCB board itself quality can not see the problem but over time the problem (including impedance) is greater, or the main reason for the unstable performance?

Please refer to the following professional academic analysis:

Impedance -- in fact, refers to the resistance and the parameters of the reactance, because PCB line (bottom of the board) to consider plug installation components, plug into the electrical conductivity and signal transmission performance and other issues, so it is necessary to impedance as low as possible, resistivity is lower than 1×10 minus 6 power per square centimeter below.

On the other hand, PCB circuit board in the production process to experience copper plating, tin plating (or electroless plating, or hot spray tin), solder joints and other links and the link used in the material must ensure that the resistivity bottom, to ensure that the overall impedance of the circuit board is low and meet the product quality requirements, otherwise the circuit board will not operate normally.

In addition, the industry as a whole, PCB circuit board in the tin plating link is the most prone to problems, is the key link affecting impedance, because the circuit board tin plating link, has been popular to use chemical tin plating technology to achieve the purpose of tin plating, but we as the industry employers, throughout the domestic can do a good job of chemical tin plating (for PCB or tin plating field) of the enterprise is not many, Because the chemical tin plating process is a rising star in China, and the technical level of the enterprises is uneven.

For the industry, according to the investigation in the industry, the most fatal weakness of the chemical tin layer is easy to discolor (easy oxidation or moisture), poor brazing leading to difficult welding, impedance is too high leading to poor conductivity or instability of the whole board performance, easy long tin to lead to PCB circuit short circuit and burn or fire.

It is reported that the first study of chemical tin plating in China is the Kunming Science and technology in the early 1990s, and then the Guangzhou Tongqian Chemical industry (enterprises) in the late 1990s. Until now, the two institutions have been recognized within the industry for 10 years as the best. Among them, according to our contact screening investigation of many enterprises, experimental observation and long-term endurance test, confirmed that the same Qian chemical tinning layer is a pure tin layer of low resistivity, conductive and brazing quality can be guaranteed to a higher level, no wonder they dare to guarantee its coating without any closure and anti-color agent protection, Can maintain a year without discoloration, no foaming, no peeling, permanent not long tin beard.

Later, when the whole social manufacturing industry developed to a certain extent, many participants were often copied from each other. In fact, quite a number of enterprises themselves did not have the ability to research and develop or create, so many products and their users' products (circuit board bottom or overall product) performance is poor, and the main reason for poor performance is because of the impedance problem. Because when unqualified chemical tin plating technology is in use, the tin plated on the PCB board is not really pure tin (or pure metal elemental), but a tin compound (that is, not a metal elemental at all, but a metal compound, oxide or halide, more directly belongs to non-metallic substances) or a mixture of tin compounds and tin gold. But it's hard to see with the eye alone.

The main circuit of PCB circuit board is copper foil, in the solder joint of copper foil is tinned layer, and the element is through the solder paste (or solder line) welding on the tin layer above, in fact, solder paste in the melting state of the element and between the tin coating is metal tin (that is, a good conductive metal), so it can be simply pointed out, Components are connected with copper foil at the bottom of the PCB board through the tin coating, so the purity of the tin coating and its impedance is the key; But before the plug element, we directly use the instrument to detect the impedance, in fact, both ends of the instrument probe (or pen) is also through the first contact with the copper foil surface at the bottom of the PCB board tin coating and copper foil at the bottom of the PCB board to communicate the current. Therefore, tin coating is the key, is the key to affect the impedance and the key to affect the performance of the entire PCB board, is also easy to be ignored.

It is well known that, except for the metal element, its compounds are bad conductors of electricity or even non-conductive (again, this is the key to the existence of distribution capacity or distribution capacity in the line), so the presence of such seemingly conductive rather than conductive tin compounds or mixtures in the coating, Its ready-made resistivity or the resistivity after the electrolytic reaction caused by oxidation or moisture in the future and its corresponding impedance are quite high (enough to affect the level or signal transmission in digital circuits) 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 material and performance on the bottom of PCB board is the most important and direct reason affecting the characteristic impedance of PCB board, but because of its change with the coating aging and moisture electrolysis, so the impact of its impedance becomes more recessive and variable, the main reason for its hidden is: First, it can not be seen by the eye (including its changes), and second, it can not be always measured, because it has the change with the change of time and environmental humidity, so it is always easy to be ignored. Or pass on the cause error.

Therefore, after knowing the cause of high impedance, solving the coating problem is the key to the impedance problem.