What is the impedance test in PCB?
What is the significance of impedance to PCB circuit boards, and why should PCB circuit boards be characteristic impedance PCB? This article first introduces what impedance and impedance types are, secondly, why PCB circuit boards should be characteristic impedance PCB, and finally explain the characteristics of characteristic impedance PCB to PCB circuits. The meaning of the board.
01
What is characteristic impedance PCB
In a circuit with Electronic Resistor and Capacitor, the obstacle to alternating current is called impedance. Impedance is often represented by Z, which is a complex number. The real part is called resistance, and the imaginary part is called reactance. The hindering effect of capacitance on alternating current in the circuit is called capacitive reactance, and the hindering effect of inductance on alternating current in the circuit is called For inductive reactance, the obstructive effect of capacitance and inductance on alternating current in the circuit is collectively called reactance. The unit of impedance is ohms.
02
Impedance type
(1) Characteristic impedance PCB In electronic information products such as computers and wireless communications, the energy transmitted in the PCB circuit is a square wave signal (called pulse) composed of voltage and time. The resistance it encounters is called characteristic impedance.
(2) Two identical signal waveforms with opposite polarities are input to the differential impedance driving end, which are respectively transmitted by two differential lines, and the two differential signals are subtracted at the receiving end. The differential impedance is the impedance Zdiff between the two wires.
(3) The impedance value of one line to the ground of the two lines of odd mode impedance is the same as the impedance value of the two lines.
(4) Impedance Zcom when two identical signal waveforms with the same polarity are input to the even mode impedance drive terminal, and the two wires are connected together.
(5) Common mode impedance The impedance Zoe of one line to the ground of the two lines of common mode impedance, the impedance value of the two lines is the same, usually larger than the odd mode impedance.
03
Why do PCB circuit boards need impedance
PCB circuit board impedance refers to the parameters of resistance and reactance, which hinder the alternating current. In custom pcb fabrication, impedance processing is essential. The reasons are as follows:
1. The PCB circuit (bottom of the board) should consider plugging and installing Electronic Components. After plugging, the conductivity and signal transmission performance should be considered. Therefore, the lower the impedance, the better, and the resistivity should be less than 1 per square centimeter. -6 or less.
2. In the production process of PCB circuit boards, they have to go through processes such as copper sinking, electrolytic tin (or chemical plating, or thermal spray tin), connector soldering, etc., and the materials used in these links must ensure that the resistivity is low in order to ensure The overall impedance of the circuit board is low to meet the product quality requirements and can operate normally.
3. The tin plating of PCB circuit boards is the most prone to problems in the entire Quick-turn PCB fabrication, and it is a key link that affects impedance. The biggest defect of electroless tin coating is easy discoloration (easy to be oxidized or deliquescent) and poor solderability, which can lead to difficult soldering of circuit boards, high impedance, poor electrical conductivity, or instability of overall board performance.
4. There are various signal transmissions in the conductors in the PCB circuit board. When the frequency must be increased in order to increase its transmission rate, if the circuit itself is different due to factors such as etching, stack thickness, wire width, etc., the impedance value will be changed. The change causes the signal to be distorted and the performance of the circuit board is degraded. Therefore, it is necessary to control the impedance value within a certain range.
04
The meaning of impedance to PCB circuit board
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-tarnishing 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 (circuit boards) 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 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 tin compound (that is, it is not a metal element at all, but a metal compound, oxide or halide, or more directly a non-metal substance) or tin A mixture of compound and tin metal element, but it is difficult to find with the naked eye.
Because the main circuit of the PCB circuit board is copper foil, the solder joints of the copper foil are the tin-plated layer, and the Electronic Components are soldered on the tin-plated layer through solder paste (or solder wire). In fact, the solder paste is melting. The state soldered between the Electronic Components and the tin plating layer is metal tin (that is, a good conductive metal element), so it can be pointed out simply that the Electronic Components are connected to the copper foil at the bottom of the PCB through the tin plating layer, so the tin plating layer The purity and impedance of the instrument is 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) first touch the copper foil on the bottom of the PCB board. The tin plating on the surface is then connected to the copper foil at the bottom of the PCB 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 PCB, 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.
05
How to test impedance
A batch, automated, accurate and reliable PCB impedance testing program-automatic impedance tester. It is dedicated to the characteristic impedance test of PCB single-ended or differential lines, which solves the low efficiency and low efficiency of circuit board characteristic impedance testing in the prior art. The problem of not being able to test the internal impedance strips of circuit boards serves the high frequency board vendors in the 5G era.
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