PCB Technical - The meaning of impedance to PCB circuit board

The meaning of impedance to PCB circuit board

What is the significance of impedance to PCB circuit boards, and why do PCB circuit boards need to be impedance. Impedance-actually refers to the resistance and reactance parameters, because the PCB circuit (board bottom) should consider plugging and installing electronic components, and the conductivity and signal transmission performance should be considered after plugging. Therefore, the lower the impedance, the better. The resistivity should be lower than the minus 6th power of 1*10 per square centimeter.

On the other hand, in the production process, the circuit board has to go through the links of copper sinking, tin plating (or electroless plating, or thermal spray tin), connector soldering, and the materials used in this link must ensure the low resistivity to ensure the circuit The overall impedance of the board is so low that it meets the product quality requirements, otherwise the circuit board will not operate normally.

In addition, from the perspective of the electronics industry as a whole,PCB manufacturer are the most prone to problems in the tin plating process, which is the key link that affects impedance. Because the circuit board tin plating process, chemical tin plating technology is now popular to achieve tin plating. However, as an employer in the electronics industry, we have contacted and observed the electronics or circuit board manufacturing and processing industries for more than 10 years. Looking at the domestic companies that can do chemical tinning (used in the field of circuit boards or electronic tinning), there are no How many, because the electroless tin plating process is a rising star in China, and the technical level of the companies is uneven...

For the electronics industry, according to industry surveys, the most fatal weakness of the electroless tin coating is its easy discoloration (easy to be oxidized or deliquescent), poor brazing properties leading to difficult soldering, and high impedance leading to poor electrical conductivity or instability in overall board performance., Easy long tin whiskers cause short circuit of PCB circuit and even burn out or catch 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 10 years that these two institutions are The best. Among them, according to our contact screening investigations, experimental observations and long-term endurance tests of many companies, it is confirmed that the tin layer of Tongqian Chemical is a pure tin layer with low resistivity, and the quality of conductivity and brazing can be guaranteed to a high level. No wonder they dare to guarantee to the outside that the coating can keep its color for one year, no blistering, no peeling, and permanent tin whisker without any sealing and anti-tarnish protection.

Later, when the entire social production industry developed to a certain extent, many subsequent participants often copied each other. In fact, a considerable number of companies themselves did not have the R&D or initiative capabilities themselves. Therefore, many products and their users’ electronic products (PCB circuits) The bottom of the board or the overall electronic product) performance is poor, and the main reason for the poor performance is the impedance problem, because when the unqualified electroless tin technology is in use, it is the tin plated on the PCB circuit board. In fact, it is not really pure tin (or pure metal element), but a compound of tin (that is, it is not a metal element at all, but a metal compound, oxide or halide, or more directly, it is a non-metal substance) or A mixture of tin compound and tin metal element, but it is difficult to find with the naked eye...

And because the main circuit of the PCB circuit board is copper foil, the solder joints of the copper foil is the tin-plated layer, and the electronic components are soldered on the tin-plated layer by solder paste (or solder wire). In fact, the solder paste is The molten state welded between the electronic component and the tin plating layer is metal tin (ie, a good conductive metal element), so it can be simply pointed out that the electronic components are connected to the copper foil at the bottom of the PCB through the tin plating layer, so The purity of the tin plating and its impedance are the key; but before the electronic components are plugged in, when we directly use the instrument to detect the impedance, in fact, the two ends of the instrument probe (or called the test lead) first touch the bottom of the circuit board. The tin plating on the surface of the copper foil is connected to the copper foil at the bottom of the PCB board to communicate the current. Therefore, tin plating is the key, the key that affects the impedance and the key that affects the performance of the entire circuit board, and it is also the key that is easy to be ignored.

As we all know, in addition to the simple substance of metal, its compounds are poor electrical 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 quasi-conductive rather than conductive in the tin plating layer In the case of tin compounds or mixtures, the existing resistivity or the resistivity after the electrolysis reaction due to future oxidation and damp and the corresponding impedance are quite high (enough to affect the level or signal transmission in the digital circuit) and The characteristic impedance is also not consistent. 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 most direct reasons affecting the characteristic impedance of the entire PCB. Variability, so the worrying impact of its impedance has become more invisible and changeable. The main reasons for its concealment are: firstly, it cannot be seen by the naked eye (including its changes), and secondly it cannot be measured constantly, because it has The variability changes with time and environmental humidity, so it is always easy to be ignored.