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Circuit board impedance

By:PCBBUY 04/28/2021 11:00

In a circuit with resistance, inductance, and capacitance, the impediment to alternating current is called impedance. The impedance is usually expressed by Z. Impedance is composed of resistance, inductive reactance and capacitive reactance, but they are not simply added together. The unit of impedance is ohm. In direct current, the effect of an object on current obstruction is called resistance. All substances in the world have resistance, but the difference in resistance value.

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Substances with very low resistance conduct good conductors, such as metals, etc.; substances with extremely high resistance conduct conductors, such as wood and plastics. There is also a conductor between the two called a semiconductor, and a superconductor is a substance whose resistance value is almost zero. However, in the field of alternating current, in addition to resistance inducting current, capacitance and inductance will also interfere with the flow of current. This effect is enough to make reactance, which means resistance to current.


Their unit of measurement is ohms as the resistance, and its value is related to the frequency of the alternating current. The higher the frequency, the capacitive reactance, the inductance, the capacitance and the inductance, and the phase angle. There is a relationship between the vectors. Will say: resistance is the sum of resistance and reactance on the vector.


For a specific circuit, the impedance is not constant, but changes with frequency. In a series circuit of resistance, inductance and capacitance, the impedance of the circuit is generally larger than the resistance. That is, the impedance is reduced to the minimum. In the parallel circuit of inductance and capacitance, the impedance increases to the maximum value at resonance, which is the opposite of the series circuit.


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 for 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 Chang tin must cause PCB circuit short circuit to burn or catch fire.


Later, when the entire social production industry developed to a certain extent, 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. 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 compounds and tin metal, but it is difficult to find by naked eyes alone.


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 simply 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 test lead) also contact 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.


As we all know, 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 distribution capacity in the circuit), so there is such a conductive rather than conductive in tin plating Tin compounds or mixtures, their ready-made resistivity or future oxidation, the resistivity after electrolysis caused by moisture, and their corresponding impedance are quite high (which has affected the level or signal transmission in digital circuits), and The 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 that affect the characteristic impedance of the entire PCB board. However, due to its aging with the coating layer and electrolysis due to 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.

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