Microwave Tech - Why PCB board need to do impedance?

What is impedance

In circuits with resistance, inductance, and capacitance, the resistance to alternating current is called impedance. Impedance is commonly expressed as Z, is a complex number, the real part is called resistance, the virtual part is called reactance, capacitance in the circuit of alternating current of the blocking effect is called capacitance, inductance in the circuit of alternating current of the blocking effect is called inductive reactance, capacitance and inductance in the circuit of alternating current caused by the blocking effect are called reactance. Impedance is measured in ohm.

Impedance type

(1) Characteristic impedance

In computer, wireless communication, and other electronic information products, the energy transmitted in the circuit of PCB is a square wave signal (called pulse) composed of voltage and time, and the resistance it encounters is called characteristic impedance.

(2) differential impedance

The driving end input polarity of the two same signal waveform, respectively transmitted by two differential lines, at the receiving end of the two differential signal subtraction. Differential impedance is the impedance Zdiff between two wires.

(3) Odd mode impedance

The impedance zoo of one line to the ground of the two lines is consistent.

(4) Even mode impedance

The driver input two same signal waveform with the same polarity, the impedance Zcom when the two lines are connected together.

(5) Common mode impedance

The impedance Zoe of one of the two wires to the ground, the impedance value of the two wires is the same, usually larger than the odd mode impedance.

PCB board why to do impedance

PCB circuit board impedance refers to the resistance and reactance parameters, acting as an obstacle to ac. Impedance processing is essential in PCB production. Here's reason:

1, PCB line (bottom of the board) to consider the plug installation of electronic components, after plugging into the consideration of electrical conductivity and signal transmission performance and other problems, so it will require the impedance as low as possible, resistivity should be less than 1&TImes per square centimeter; 10-6 below.

2, PCB circuit board in the production process to experience copper precipitation, tin plating (or chemical plating, or hot spray tin), plug solder process production links, and the materials used in these links must ensure the resistivity of the bottom, in order to ensure the overall impedance of the circuit board to meet the quality requirements of the product, can run normally.

3, PCB tin plating is the most prone to problems in the production of the whole circuit board, is the key link to affect the impedance. The biggest defects of the electroless tin plating layer are easy to change color (easy oxidation or deliquescence), poor brazing, will lead to the difficult welding circuit board, impedance is too high to lead to poor conductivity or unstable performance of the whole board.

4,PCB circuit board in the conductor will have all kinds of signals, when the transmission rate and to raise its must raise its frequency, if the line itself due to factors such as etching, laminated thickness, wire width is different, will cause resistance to change, make the signal distortion, led to the decrease of the circuit board using performance, so they need to control the impedance value within a certain range.

Impedance for PCB board meaning

For the electronic industry, according to the investigation, the most fatal weakness of the chemical tin plating layer is easy to change color (easy oxidation or deliquescent), poor brazing lead to difficult welding, high impedance lead to poor conductivity or the instability of the whole board performance, easy to long tin must lead to PCB circuit short circuit and burn or fire events.

It is reported that the first research on electroless tin plating in China was Kunming University of Science and Technology in the early 1990s, and then Guangzhou Tongqian Chemical Industry (enterprise) in the late 1990s, has been recognized in the industry for 10 years, the two institutions are doing the best. Among them, according to our contact screening for many enterprises investigation, experimental observation, and long endurance test, confirm with modest chemical tin plating layer is the pure tin of low resistivity layer, conductive and brazing quality can ensure the high level, it is no wonder that they dare to external guarantee it's coating without any closed and its agent protection cases, Can keep for one year does not change color, does not blister, does not desquamate, does not grow tin beard permanently.

Later, when the development of the whole social production industry to a certain extent, many later participants are often copied from each other, in fact, a considerable part of the enterprise itself does not have the ability to research and develop or initiate, so, resulting in a lot of products and users of electronic products (circuit board bottom or electronic products as a whole) performance is not good, Caused by the poor performance of the main reasons is because the impedance problem, because when the unqualified chemical tin plating technology in use process, it's for PCB circuit board plating on the pure tin is not really true (or pure metal elemental), but the tin compounds (i.e. not metal simple substance, but metal compounds, oxide or halide, More directly nonmetallic) or a mixture of tin compounds and tin metals, but it's hard to spot with the naked eye...

Because the body of the PCB circuit board lines is copper foil and solder joints on copper foil are tin plating layer, and the electronic component is through the solder paste (or solder wire) welding on tin coating, welding in solder paste in melt state between electronic components and tin coating of metal tin (elemental) good conductive metal, so you can simply briefly points out, Electronic components are connected with copper foil at the bottom of PCB board through the tin coating, so the purity of tin coating and its impedance is the key; And, before the insertion of electronic components, when we directly use the instrument to test the impedance, in fact, the instrument probe (or called the pen) is also connected with the current by contacting the tin coating on the copper foil surface at the bottom of the PCB board first. So the tin coating is the key, is the key to affect the impedance, and the key to affecting the performance of the whole PCB board is also easy to be ignored.

As we all know, in addition to the metal elemental, its compounds are poor conductors of electricity or even non-conductive (and this is the key to the existence of distribution capacity or transmission capacity in the line), so there is a compound or mixture of tin coating that looks like conductivity but not conductivity, Its ready-made resistivity or the resistivity and corresponding impedance of future oxidation and moisture-induced electrolytic reaction is quite high (enough to affect the level or signal transmission in the digital circuit), and its characteristic impedance is not consistent. Therefore, it will affect the performance of the circuit board and the whole machine.

So, for the present phenomenon of social production, coating material on the bottom of the PCB and performance is the main factor affecting the characteristic impedance PCB board whole board and the most direct reason, but since it has with the variability of the electrolytic coating aging and be affected with damp, so the impedance of suffering influence become more hidden and changeability, the main causes of the hidden: The first can not be seen by the naked eye (including its change), the second can not be constant measurement, because it has a change with time and environmental humidity changes, so it is always easy to be ignored.