Why do multilayer circuit boards do impedance

Why do PCB multilayer circuit boards do impedance

PCB impedance refers to the resistance and reactance of the parameters, the alternating current plays an obstruction role. Impedance processing is essential in PCB circuit board production. Here's why:

1, PCB circuit (bottom of the board) to consider plug installation of electronic components, plug into the electrical performance and signal transmission performance, so the impedance will be required as low as possible.

2, PCB circuit board in the production process to experience copper plating, tin plating (or electroless plating, or hot spray tin), solder joints and other processes, and the materials used in these links must ensure the resistivity bottom, in order to ensure that the overall impedance of the circuit board is low to meet the product quality requirements, can operate normally.

3, the tin plating of PCB circuit board is the most prone to problems in the whole circuit board production, is the key link affecting the impedance. The biggest defect of electroless tin layer is easy to change color (easy oxidation or delixing), poor brazing, will lead to difficult welding circuit board, impedance is too high leading to poor conductivity or instability of the whole board performance.

4, PCB circuit board in the conductor will have a variety of signal transmission, when to improve its transmission rate and must improve its frequency, the line itself if the etch, laminated thickness, wire width and other factors are different, will cause the impedance value change, so that the signal distortion, resulting in circuit board performance decline, so it is necessary to control the impedance value in a certain range.

Two, the significance of impedance for PCB circuit boards

For the electronic 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 resulting in difficult welding, impedance is too high leading to poor conductivity or instability of the whole board performance, easy to lead to long tin PCB circuit short circuit and burn or fire events.

It is reported that the first study of chemical tin plating in China is the Kunming University of Science and Technology in the early 1990s, and then Guangzhou Tongqian Chemical (enterprises) in the late 1990s. Until now, the two institutions have been recognized within the industry for 10 years is 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 few enterprises themselves did not have the ability to research and develop or create, which resulted in the poor performance of many products and their users' electronic products (circuit board bottom or electronic products as a whole). And the main reason for poor performance is because of impedance problems, because when unqualified tin plating technology in the process of use, the printed board is plated with tin is not really pure tin (or pure metal elemental), but tin compounds (that is, not metal elemental at all, but metal compounds, oxides or halides, More directly, it is a nonmetallic substance) or a mixture of tin compounds and tin metals, but is difficult to detect with the naked eye... .

Because the main line of the PCB circuit board is copper foil, in the solder joint of the copper foil is tinned layer, and electronic components is through the solder paste (or solder line) welding on the tin layer above, in fact, solder paste in the melting state of welding to the electronic components and between the tin coating is metal tin (that is, a good conductive metal), so it can be simply pointed out, Electronic components are connected with copper foil at the bottom of PCB board through the tin coating, so the purity of the tin coating and its impedance is the key; But before inserting electronic components, we directly use the instrument to detect 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 printed circuit board tin coating and copper foil at the bottom of the PCB board to connect 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 (which is also the key to the existence of distribution capacity or transfer 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. So it will affect the performance of the circuit board and its whole 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: The first can not be seen by the naked eye (including its changes), the second can not be always measured, because it has changes with the change of time and environmental humidity, so it is always easy to be ignored.