PCB Tech - What is characteristic impedance？

1. Resistance

When AC current flows through a conductor, the resistance is called impedance, which corresponds to Z, and the unit is Ω.

The resistance at this time is different from that of DC current. In addition to resistance of resistance, there are resistance problems of inductive reactance (XL) and capacitive reactance (XC).

In order to distinguish the resistance of direct current, the resistance encountered by alternating current is called impedance (z).

Z=√ R2 +(XL -XC)2

2. Impedance (z)

In recent years, with the improvement and application of IC integration, the signal transmission frequency and speed are getting higher and higher. Therefore, when the signal transmission (transmission) reaches a certain value, it will be affected by the PCB wire itself, resulting in serious distortion or complete loss of the transmission signal. This shows that the "thing" flowing through PCB wire is not the current, but the transmission of square wave signal or pulse in energy.

3. Characteristic impedance control (Z0)

The resistance of the above "signal" transmission, also known as "characteristic impedance", represents the symbol Z0.

Therefore, it is not enough to solve the problems of "on", "off" and "short circuit" on PCB wire, but also to control the characteristic impedance of wire. In other words, the quality of transmission lines for high-speed transmission and high-frequency signal transmission is much stricter than that of transmission wires. It is no longer "open / short circuit" test pass, or notch, burr does not exceed 20% of the line width. It must be required to measure the characteristic impedance value, and the impedance should be controlled within the tolerance, otherwise, it is only scrapped and cannot be reworked.

Why should PCB characteristic impedance be controlled

1. When the electronic equipment (computer, communication machine) is operating, the signal sent by the driver will reach the receiver through PCB transmission line. When the signal is transmitted in the signal line of the printed circuit board, the characteristic impedance value Z0 must match the "electronic impedance" of the head and tail components, so that the "energy" in the signal can be transmitted completely.

2. Once the quality of printed circuit board is poor and Z0 exceeds the tolerance, the transmitted signal will have problems such as reflection, dispersion, attenuation or delay. In serious cases, the wrong signal will be transmitted and the computer will crash.

3. Strict selection of plates and control of the production process, the Z0 on the multilayer board can meet the specifications required by customers. The higher the electronic impedance is, the faster the transmission speed will be. Therefore, the Z0 of PCB must be improved to meet the requirements of matching components. Only when Z0 is qualified, can it be regarded as the qualified product required by high-speed or high-frequency signals.

Relationship between characteristic impedance Zo of PCB and PCB material and PCB process

The formula of characteristic impedance Z0 of PCB microstrip line structure: Z0 = 87 / R + 1.41 ln5.98h / (0.8W + T)

Where: ε R - dielectric constant H - dielectric thickness W - conductor width T - conductor thickness

The lower the board's ε R, the easier it is to increase the Z0 value of PCB circuit and match with the output impedance value of high-speed components.

1. The characteristic impedance Z0 is inversely proportional to the plate's ε R

Z0 increases with the increase of medium thickness. Therefore, for Z0 strict high-frequency circuit, the error of medium thickness of copper clad laminate substrate is required strictly. In general, the change of medium thickness should not exceed 10%.

2. Influence of dielectric thickness on characteristic impedance Z0

With the increase of line density, the increase of medium thickness will cause the increase of electromagnetic interference. Therefore, with the increase of conductor wiring density, the thickness of medium should be reduced to eliminate or reduce the stray signal or crosstalk caused by electromagnetic interference, or to reduce the ε R, so as to select low ε r substrate.

According to the characteristic impedance Z0 of microstrip line structure, the formula is: Z0 = 87 / R + 1.41 ln5.98h / (0.8W + T)

The thickness of copper foil (T) is an important factor affecting Z0. The larger the wire thickness, the smaller Z0. But the range of variation is relatively small.

3. Influence of copper foil thickness on characteristic impedance Z0

The thinner the copper foil thickness, the higher Z0 value can be obtained, but the change of thickness has little contribution to Z0.

The contribution of thin copper foil to Z0 is more accurate than that of thin copper foil in manufacturing fine wires to improve or control Z0.

According to the formula:

Z0 = 87/r +1.41 ln5.98H / (0.8W+T)

The smaller the line width W, the larger Z0; reducing the wire width can improve the characteristic impedance.

The influence of line width change on Z0 is much more obvious than that of line thickness.

4. Influence of conductor width on characteristic impedance Z0

Z0 increases rapidly with the narrowing of line width W. therefore, in order to control Z0, the line width must be strictly controlled. At present, the signal transmission line width W of most high-frequency lines and high-speed digital lines is 0.10 or 0.13 mm. Traditionally, the line width control deviation is ± 20%. For conventional electronic products without transmission line, PCB wire (wire length "1 / 7 of signal wavelength") can meet the requirements, but for signal transmission line with Z0 control, the deviation of PCB wire width is ± 20%, which can not meet the requirements. Because the error of Z0 is more than ± 10%.

PCB characteristic impedance control and PCB process control

1. PCB film production management and inspection

Constant temperature and humidity room (21 ± 2 ° C, 55 ± 5%), dustproof, line width compensation.

2. PCB panel design

The edge of the panel should not be too narrow, the coating should be uniform, and the plating plus false cathode should be used to disperse the current;

The standard sample (coupon) was designed to test Z0.

3. PCB etching

Strict process parameters, reduce side corrosion and carry out the first inspection;

Reduce residual copper, copper slag and copper scrap at wire edge;

Check the line width and control it within the required range (± 10% or ± 0.02mm).

4. PCBAoi examination

For 2GHz high-speed signal, even if the gap is 0.05mm, it must be scrapped; the key is to control the line width and defects of the inner layer.

5. PCB lamination

Vacuum laminator, reduce the pressure, reduce the flow of glue, try to maintain more resin, because the resin affects the ε R, more resin preservation, ε r will be lower. Control lamination thickness tolerance. Because the plate thickness is not uniform, it means that the medium thickness change will affect Z0.

6. Select PCB substrate

Strictly according to the customer requirements of the plate type blanking. Wrong model, wrong ε R, wrong plate thickness, correct PCB manufacturing process, same scrap. Because Z0 is greatly affected by ε R.

7. PCB solder mask

In theory, the thickness of resistance welding should not be too thick, but in fact, the influence is not very great. The surface of copper conductor is exposed to air (ε r = 1), so the measured value of Z0 is higher. However, the value of Z0 will decrease by 1-3 Ω after welding resistance, because the ε r of resistance welding is 4.0, which is much higher than that of air.

8. Water absorption of PCB

The finished multilayer board should avoid water absorption as far as possible, because the ε r = 75 of water will bring great drop and unstable effect to Z0.

It is the thickness of dielectric that affects the characteristic impedance of PCB, followed by dielectric constant, wire width and wire thickness. When the substrate is selected, the change of ε R and H is small, and t is easy to control, but it is difficult to control the line width W within ± 10%. Moreover, the problems of line width include pinhole, notch and depression on the wire. In a sense, the effective and important way to control Z0 is to control and adjust the PCB line width.