What is the significance of impedance for PCB circuit boards. Why do PCB circuit boards need impedance. Impedance—actually refers to the parameters of resistance and reactance. Because the PCB circuit (board bottom) should consider plugging and installing electronic components, and consider issues such as electrical conductivity and signal transmission performance after plugging, the lower the impedance, the better. The rate should be lower than 1×10 to the negative 6th power per square centimeter.
On the other hand, in the production process of the circuit board, it has to go through links such as copper sinking, electrolytic tin plating (or chemical plating, or thermal spray tin), connector soldering, etc., and the materials used in this link must ensure the low resistivity in order to ensure the circuit. The overall impedance of the board is low enough to meet 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 circuit board factories are the most prone to problems in the tinning process, which is the key link that affects impedance, because the tinning process of circuit boards is now popularly using chemical tinning technology to achieve the purpose of tinning ;But as a beneficiary of the electronics industry, we have been in touch with and observed the electronics or circuit board manufacturing and processing industry for more than 10 years. Throughout the country, there are not many companies that can do electroless tin plating (used in the field of circuit board or electronic tin plating) , because the electroless tin plating process is a rising star in China; and the technical level of various companies is uneven...
For the electronics industry, according to industry surveys, the most fatal weakness of electroless tin plating is easy discoloration (easy to oxidize or deliquescence), poor brazing performance leading to difficult soldering, high impedance leading to poor electrical conductivity or unstable performance of the entire board , Easy to grow tin whiskers, lead to short circuit of PCB circuit and even burn or catch fire...
It is reported that Kunming University of Science and Technology was the first to study electroless tin plating in the early 1990s, followed by Guangzhou Tongqian Chemical (Enterprise) in the late 1990s. Up to now, the two institutions have been recognized in the industry for 10 years. get the best. Among them, according to our contact screening investigation, experimental observation and long-term endurance test of many enterprises, it is confirmed that the tin plating layer of Tongqian Chemical Industry is a pure tin layer with low resistivity. They dare to guarantee to the outside world that its coating can keep for one year without any sealing and anti-tarnish protection, no discoloration, no blistering, no peeling, and no tin whiskers forever.
Later, when the entire social production industry developed to a certain level, many later participants were often plagiarized from each other. In fact, quite a few companies themselves did not have R&D or originality capabilities. Therefore, many products and their users' electronic products (PCB circuits) The main reason for the poor performance of the bottom of the board or the overall performance of the electronic product is the impedance problem, because when the unqualified chemical tin plating technology is in use, the tin plated on the PCB circuit board is actually 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, and more directly belongs to a non-metallic substance) or tin A mixture of compounds and tin metal elements, but it is difficult to detect with the naked eye alone...
And because the main line of the PCB circuit board is copper foil, the tin plated layer is on the solder joint of the copper foil, and the electronic components are welded on the tin plated layer through solder paste (or solder wire). In fact, the solder is melting The molten state is welded between the electronic components and the tin plating layer is metal tin (that is, a metal element with good conductivity), 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 tin The purity of the plating layer and its impedance are the key; but before the electronic components are connected, when we directly use the instrument to detect the impedance, in fact, the two ends of the instrument probe (or called the test lead) are also connected to the bottom of the circuit board first. The tin plating on the surface of the copper foil communicates with the copper foil on the bottom of the PCB to communicate with the current. Therefore, the tin plating layer is the key, the key that affects the impedance and the performance of the entire circuit board, and it is also the key that is easily overlooked.
As we all know, except for simple metals; its compounds are poor conductors of electricity or even non-conductive (also, this is also the key to the distribution capacity or transmission capacity in the line), so there is this kind of electrical conductivity in the tin plating layer. When the compound or mixture of tin is used, its current resistivity or the resistivity after the electrolytic reaction of oxidation and moisture and its corresponding impedance are quite high (enough to affect the level or signal transmission in digital circuits,) and its The characteristic impedance is also 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 material and performance of the coating on the bottom of the PCB are the most important and direct reasons that affect the characteristic impedance of the entire PCB; Therefore, the influence of its resistance becomes more hidden 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 constantly measured, because it has random It is always easy to be ignored because of its variability with the change of time and environmental humidity.
