PCB Tech - Why should the circuit board be designed for impedance?

What is the significance of impedance to PCB circuit boards, and why do PCB circuit boards need impedance? This article first introduces what impedance and its types are, then it introduces why the PCB circuit board should be impedance, and finally explains the meaning of impedance to the PCB circuit board.

01

What is impedance

In a circuit with resistance, inductance, and capacitance, 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

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 It is called characteristic impedance.

(2) Differential impedance

Two identical signal waveforms with opposite polarities are input at the 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) Odd mode impedance

The impedance value of one line to ground of the two lines is the same as the impedance value of the two lines.

(4) Even mode impedance

Two identical signal waveforms with the same polarity are input to the drive end, and the impedance Zcom when the two wires are connected together.

(5) Common mode impedance

The impedance Zoe of one of the two lines to the ground is the same as the impedance value of the two lines, which is usually greater than the odd mode impedance.

03

Why do PCB circuit boards need impedance

The impedance of PCB circuit board refers to the parameters of resistance and reactance, which hinder the alternating current. In the production of PCB circuit boards, impedance processing is essential. The reasons are as follows:

1. The PCB circuit bottom 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. During the PCB production process, PCB circuit boards have to go through process steps such as copper sinking, tin electroplating (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 tinning of PCB circuit boards is the most prone to problems in the entire circuit board production, and it is a key link that affects impedance. The biggest defect of the electroless tin coating is easy discoloration (easy to be oxidized or deliquescent) and poor solderability, which will lead to difficult soldering of the circuit board, high impedance, poor electrical conductivity, or instability of the 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 change., So that the signal is distorted and the performance of the circuit board is degraded, so 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 make it difficult to solder, and high impedance leads 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. Up to now, the industry has recognized that these two institutions are The best. Among them, according to our contact screening surveys, experimental observations and long-term endurance tests of many companies, it is confirmed that the tin-plated 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 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, there is a tin-plated layer on the solder joints of the copper foil, and the electronic components are soldered on the tin-plated layer by solder paste (or solder wire). In fact, the solder paste is melting. The state soldered between the electronic component and the tin plating layer is metal tin (that is, the metal element with good conductivity), 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 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 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 (also, 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 In the case of tin compounds or mixtures, the existing resistivity or the resistivity after the electrolysis reaction due to future oxidation or 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 variable. 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 of its concealment. The variability changes with time and environmental humidity, so it is always easy to be ignored.