Impedance for PCB board

What is the importance of PCB impedance and why the PCB should be impedance? Impedance - actually refers to the resistance and reactance parameters, because the PCB line (board) needs to consider the insertion and installation of the inserted electronic components, conductivity and signal transmission performance, so it is necessary to require lower impedance, better, resistivity Less than the negative sixth power of 1 * 10 per square centimeter.
On the other hand, the board must be copper deposited, tinned (or electroless or hot sputtered), the soldering of the connector and the materials used in the process must ensure the bottom resistivity to ensure that the overall impedance board is low enough Can meet the product quality requirements, otherwise the circuit board will not work properly.
In addition, in the electronics industry, PCB board factories are the most prone to tin plating problems, which is a key link affecting impedance. Since electroless tin plating technology has been widely used for tin plating of circuit boards to achieve tin plating purposes, as a recipient of the electronics industry, we have contacted and observed the electronic or circuit board manufacturing industry for more than 10 years, domestic energy. There are not many companies in electroless tin plating (for circuit boards or electronic tin plating), because electroless tin plating technology is a rising star in China, and the technical level of many companies is uneven...
For the electronics industry, according to the survey, the most deadly weakness of the electroless tin plating layer is that it is easy to change color (easy to oxidize or deliquesce), poor solderability leads to soldering difficulties, high impedance leads to poor conductivity or unstable performance of the whole board, and tin tends to grow and cause PCB Line short circuit or even burning or ignition event...
According to reports, the first chemical tinning study in China was Kunming University of Science and Technology in the early 1990s, followed by Guangzhou Tongqian Chemical Industry (Enterprise) in the late 1990s. Among them, according to our contact screening survey, experimental observation and long-term durability test of many enterprises, it is confirmed that the tin plating layer of Tongqian Chemical Co., Ltd. is a low-resistivity pure tin layer, and the conductivity and brazing quality can guarantee a higher level. . No wonder they dare to ensure that the tin coating remains the same, no blistering and no discoloration for a year without any seal and anti-tarnish protection. Peeling, never growing tin whiskers.
Later, when the entire social production industry developed to a certain extent, many participants often copied each other. In fact, quite a few companies have no ability to develop or innovate. As a result, many products and their users' electronic products (PCB boards or electronic products as a whole) perform poorly. The main cause of poor performance is impedance, because when using unqualified electroless tin plating technology, PCB boards Tin plating is not really pure tin (or pure metal), but tin compounds (that is, not metal elements at all, but metal compounds, oxides or halides, more directly belong to non-metallic substances) or tin and tin metal elements compound of. Compound, but hard to find with the naked eye...
Since the main circuit of the PCB circuit board is copper foil, the solder joint of the copper foil is a tin plating layer, and the electronic components are soldered on the tin plating layer by solder paste (or solder wire). In fact, the solder paste soldered between the electronic component and the tin coating in the molten state is metallic tin (ie, a metal element having good electrical conductivity), so that it can be simply and succinctly pointed out that the electronic component is pasted by solder (or solder wire) ). The tin coating is then attached to the copper foil on the bottom of the PCB, so the purity and impedance of the tin coating is critical; moreover, before we insert the electronic components, when we use the instrument directly to detect the impedance, the instrument's probe (or pen) The ends are connected by contacting the tin coating on the surface of the copper foil at the bottom of the PCB. Then use the copper foil on the bottom of the PCB board. Tin plating is therefore the key to affecting impedance, the key to board performance, and the key to being overlooked.
It is well known that in addition to metallic elements, their compounds are not good electrical conductors, or even electrically conductive (this is also the key to the presence of distribution capacity or transmission capacity in the circuit). Therefore, when such a compound or a mixture of tin which is similar in conductivity but not electrically conductive exists in the tin coating, their resistivity or resistivity which is easily obtained after the oxidation reaction of moisture and moisture in the future and their corresponding Resistivity. The impedance is very high (sufficient to affect the level or signal transmission in the digital circuit) and its characteristic impedance is inconsistent. This can affect the performance of the board and the entire machine.
Therefore, in terms of current social production phenomena, the coating material and properties at the bottom of the PCB are the most important and direct causes of the characteristic impedance of the PCB. However, as it changes with coating aging and moisture electrolysis, the effects of impedance become more subtle and variable. The main reasons for hiding are as follows: First, the naked eye cannot ignore it. Secondly, what we see (including its changes) cannot be measured continuously because it changes with time and ambient humidity and is therefore always easy to ignore.