Why PCB circuit board should do impedance control

Why PCB circuit board should do impedance control

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

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

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

Please see the following professional academic analysis:

Impedance-actually refers to the parameters of resistance and reactance, because the PCB circuit (bottom) should consider plugging and installing electronic components, and considering the conductivity and signal transmission performance after plugging, so the lower the impedance, the better. The resistivity should be lower than the negative 6th power of 1×10 per square centimeter.

On the other hand, in the production process of PCB circuit boards, it is necessary to go through the steps of sinking copper, electroplating tin (or electroless plating, or thermal spray tin), connector soldering, etc. and the materials used in this link must ensure the resistivity bottom to ensure The overall impedance of the circuit board is low to meet the product quality requirements, otherwise the circuit board will not operate normally.

In addition, from the overall view of the electronics industry, PCB circuit boards are the most prone to problems in tin plating, and are the key link that affects impedance. Because the tin plating of circuit boards, it is now popular to use chemical tin plating technology to achieve tin plating. However, as a recipient of the electronics industry, we have been contacting and observing the electronics or electronic processing industry for more than 10 years, and there are not many enterprises that can do chemical tin plating (for PCB or electronic tin plating) in China. The chemical tin plating process is a rising star in China, and the technical level of various enterprises is uneven...

For the electronics industry, according to industry surveys, the most fatal weakness of electroless tin plating is easy to discolor (both easily oxidized or deliquesced), poor solderability leads to difficult soldering, high impedance leads to poor conductivity or unstable overall board performance 1. Yi Changxi must cause PCB circuit short circuit to burn or fire...

It is reported that the first domestic research on chemical tin plating was Kunming University of Science and Technology in the early 1990s, followed by Guangzhou Tongqian Chemical (enterprise) in the late 1990s. It has been recognized in the industry for the past 10 years that these two institutions are doing The best. Among them, according to our contact screening investigations, experimental observations and long-term endurance tests for many companies, it is confirmed that the tin plating layer of Tongqian Chemical is a low-resistivity pure tin layer, and the quality of conductivity and brazing can be guaranteed to a high level. No wonder they dare to guarantee to the outside that their coatings can remain unchanged for one year without any sealing and protection from discoloration agents, without blistering, without peeling, and without permanent tin whiskers.

Later, when the entire social production industry developed to a certain level, many later participants often belonged to plagiarism. In fact, a considerable number of companies themselves did not have 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 because of the impedance problem, because when the unqualified chemical tin plating technology is in use, it is actually the tin plated on the PCB circuit board. It is not really pure tin (or pure metal element), but a tin compound (that is, it is not a metal element at all, but a metal compound, oxide or halide, which is more directly a non-metallic substance) or tin It is a mixture of compound and tin metal element, but it is hard to find by naked eyes...

And because the main circuit of the PCB circuit board is copper foil, the tinned layer is on the solder joint of the copper foil, and the electronic component is soldered on the tinned layer through solder paste (or solder wire). In fact, the solder paste is in It is metal tin (that is, a metal element with good conductivity) that is welded between the electronic component and the tin plating layer in the molten state, so it can be briefly pointed out that the electronic component is connected to the copper foil on the bottom of the PCB board through the tin plating layer, so The purity of the tin coating and its impedance are the key; but, before we plug in the electronic components, when we directly use the instrument to detect the impedance, in fact, the two ends of the instrument probe (or called the test pen) also touch the bottom of the PCB board first The tin coating on the surface of the copper foil communicates with the copper foil on the bottom of the PCB. Therefore, tin plating is the key, the key that affects the impedance and the performance of the entire PCB, and it is also the key that is easily overlooked.

Also, it is well known that, except for the elemental metal, its compounds are electrical poor conductors or even non-conductive (again, this is also the key to the distribution capacity or spreading capacity in the circuit), so there is this kind of seemingly conductive in the tin plating layer. For non-conductive tin compounds or mixtures, their ready-made resistivity or future oxidation, resistance after electrolysis due to moisture, and their corresponding impedance are quite high (which has affected the level or signal transmission in digital circuits, ) And its characteristic impedance is also inconsistent. So it will affect the performance of the circuit board and the whole machine.

Therefore, as far as the current social production phenomenon is concerned, the coating material and performance on the bottom of the PCB board are the most important and direct reasons affecting the characteristic impedance of the entire PCB board, but due to its aging with the plating layer and the electrolysis of moisture Variability, so the anxiety effect of its impedance becomes more recessive and volatile, the main reason for its concealment is: the first cannot be seen by the naked eye (including its changes), and the second cannot be measured constantly because it has The variability changes with time and environmental humidity, so it is always easy to be ignored. Or pass the cause wrong.

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