What is the significance of impedance to PCB, and why does PCB need impedance?
What is impedance?
In a circuit with resistance, inductance and capacitance, the resistance to alternating current is called impedance. Impedance is often represented by B, which is a complex number. The real part is called resistance and the imaginary part is called reactance. The impeding effect of capacitor on alternating current in the circuit is called capacitive reactance, and the impeding effect of inductor on alternating current in circuit is called capacitive reactance. Inductive reactance, the resistance caused by capacitance and inductance to alternating current in a circuit is collectively called reactance. The unit of impedance is ohms.
Impedance type
1.Characteristic impedance In electronic information products such as computers and wireless communications, the energy transmitted in PCB lines is a square wave signal (called pulse) composed of voltage and time. The resistance encountered is called characteristic impedance.
2.The differential impedance driving end inputs two same signal waveforms with opposite polarities, which are transmitted by two differential lines respectively. The two differential signals are subtracted at the receiving end. Differential impedance is the impedance between two lines Zdiff
3.The impedance of one of the two odd-mode impedance lines to ground is Zoo, and the impedance values of the two lines are the same.
4.When two identical signal waveforms with the same polarity are input to the even-mode impedance driving terminal, the impedance Zcom is obtained when the two lines are connected together.
5.The impedance Zoe of one of the two lines of common mode impedance to ground, the impedance value of the two lines is the same, usually larger than the odd mode impedance.
Why does PCB need impedance?
PCB impedance refers to the parameters of resistance and reactance, which play a hindering role in alternating current. In PCB circuit board production, impedance processing is essential. The reasons are as follows:
1.The bottom of the PCB board should consider plugging and installing electronic components. After plugging, consider issues such as conductive performance and signal transmission performance. Therefore, the lower the impedance, the better, and the resistivity should be lower than 1×10-6 per square centimeter. the following.
2.During the production process, PCB circuit boards must go through manufacturing processes such as copper immersion, electroplating tin (or chemical plating, or thermal spray tin), connector soldering, etc., and the materials used in these processes must ensure low resistivity to ensure The overall impedance of the circuit board is low and meets product quality requirements and can operate normally.
3.The tin plating of PCB circuit boards is the most prone to problems in the entire circuit board production, and is a key link that affects impedance. The biggest drawbacks of the electroless tin plating layer are that it is easy to change color (easy to oxidize or deliquesce) and has poor solderability, which will make the circuit board difficult to solder. High impedance will lead to poor conductivity or unstable performance of the entire board.
4.Various signals are transmitted in the conductors in the PCB circuit board. In order to increase the transmission rate, the frequency must be increased. If the line itself is different due to factors such as etching, lamination thickness, wire width, etc., the impedance value will change. , which distorts the signal and leads to a decrease in the performance of the circuit board, so the impedance value needs to be controlled within a certain range.
What does impedance mean for PCB?
For the electronics industry, according to industry surveys, the most fatal weaknesses of chemical tin plating are that it is easy to change color (easy to oxidize or deliquesce), poor solderability makes it difficult to weld, and high impedance leads to poor conductivity or unstable overall board performance. , It is easy for tin whiskers to grow, causing PCB circuits to short-circuit and even burn or catch fire.
When the entire social production industry develops to a certain level, many subsequent participants often copy each other. In fact, a considerable number of companies themselves do not have R&D or originality capabilities. Therefore, many products and their users’ electronic products (circuit boards) (or the overall electronic product) has poor performance, and the main reason for poor performance is the impedance problem, because when unqualified chemical tin plating technology is used, the tin plated on the PCB circuit board is not 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, more directly a non-metallic substance) or a tin compound In a mixture with tin metal, it is difficult to detect problems just by looking at them.
Because the main circuit of the PCB circuit board is copper foil, the solder joints of the copper foil are tin-plated layers, and the electronic components are welded on the tin-plated layer through solder paste (or solder wire). In fact, the solder paste is melting What is soldered between the electronic components and the tin plating layer is metallic 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 board through the tin plating layer, so the tin plating layer The purity and impedance are the key; also, before connecting the electronic components, when we directly use the instrument to detect the impedance, in fact, both ends of the instrument probe (or test lead) are also first contacted by the copper foil at the bottom of the PCB board. The tin plating on the surface connects the current with the copper foil on the bottom of the PCB board. Therefore, the tin plating layer is the key. It is the key that affects the impedance and the performance of the entire PCB board. It is also the key that is easily ignored.
