1. Resistance
When AC current flows through a conductor, the resistance it encounters is called impedance, which is Z and expressed in ohms.
The resistance at this point is different from the resistance encountered by direct current. In addition to the resistance of the resistance, there are also resistance issues with inductive reactance (XL) and capacitive reactance (XC).
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 have become increasingly high. Therefore, when the signal transmission (emission) in printed circuit board wires reaches a certain value, it will be affected by the printed circuit board wires themselves, leading to serious distortion or complete loss of the transmission signal. This indicates that the "thing" that flows through PCB wires is not current, but rather the transmission of square wave signals or pulses in energy.
3. Characteristic impedance control (Z0)
The resistance of the above "signal" transmission is also called "characteristic impedance", and the symbol is Z0.
Therefore, it is not enough to solve the problems of "on", "off" and "short circuit" on PCB wires, but also to control the characteristic impedance of wires. That is to say, the quality of high-speed transmission and high-frequency signal transmission lines is much stricter than that of transmission wires. It is no longer necessary to pass the "open circuit/short circuit" test, or if the gap or burr does not exceed 20% of the line width, it can be accepted. The characteristic impedance value must be measured, and this impedance must also be controlled within the tolerance, otherwise, only scrap is allowed and rework is not allowed.
2、 Signal propagation and transmission lines
1. Definition of Signal Transmission Line
(1) According to the principle of electromagnetic waves, wavelength（ λ） The shorter, the higher the frequency (f). The product of the two is the speed of light. I.e. C= λ. f ＝3 × 1010 cm/s
(2) Any component, although having a high signal transmission frequency, will experience a decrease in the originally high transmission frequency or time delay after being transmitted through PCB wires.
Therefore, the shorter the wire length, the better.
(3) Improving PCB wiring density or shortening wire size is beneficial. However, as the frequency of the component increases or the pulse period shortens, the wire length approaches a certain range of signal wavelength (speed), and the component will experience significant "distortion" when transmitted through the PCB wire.
(4) IPC-2141's 3.4.4 states that when a signal is transmitted in a wire, if the wire length is close to 1/7 of the signal wavelength, the wire is considered a signal transmission line.
(5) Example:
The signal transmission frequency (f) of a component is 10MHZ, and the length of the conductor on the PCB is 50cm. Should characteristic impedance control be considered?
Solution: C= λ. f ＝3 × 1010 cm/s
λ＝ C/f＝（3 × 1010 cm/s）/（1 × 107 /s ）＝3000cm
Wire length/signal wavelength=50/3000=1/60
Because: 1/60<1/7, this conductor is an ordinary conductor, and characteristic impedance need not be considered.
In electromagnetic wave theory, the Maxwell formula tells us that the propagation speed VS of a sine wave signal in a medium is proportional to the speed C of light, and inversely proportional to the dielectric constant of the transmission medium.
VS ＝C/√ ε r
When ε When r=1, the signal transmission reaches the propagation speed of light, which is 3 × 1010 cm/s 。
2. Transmission rate and dielectric constant
Signal transmission speed of different plates at 30MHZ
Dielectric constant signal transmission speed (m/µ s) of dielectric material Tg (° C)
Vacuum/1.0 300.00
PTFE/2.2 202.26
Thermosetting Polypropylene Ether 210 2.5 189.74
Cyanate ester resin 225 3.0 173.21
PTFE resin+E glass cloth/2.6 186.25
Cyanate ester resin+glass cloth 225 3.7 155.96
Polyimide+glass cloth 230 4.5 141.42
Quartz/3.9 151.98
Epoxy resin glass cloth 130 ± 5 4.7 138.38
Aluminum/9.0 100.00
As can be seen from the above table, as the dielectric constant increases（ ε r) With the increase of, the transmission speed of the signal in the dielectric material decreases. To obtain high signal transmission speed, high characteristic impedance value is required; High characteristic impedance requires low dielectric constant（ ε r) Materials; Dielectric constant of polytetrafluoroethylene (Teflon)（ ε r) Smallest, fastest transmission speed.
FR-4 board, composed of epoxy resin and E-grade glass cloth, with a dielectric constant（ ε r) It is 4.7. The signal transmission speed is 138m/ μ s。 Changing the resin system can easily change the dielectric constant（ ε r ）。
3、 Control reason of characteristic impedance value
1. Reason 1
When electronic devices (computers, communication machines) are operated, the signals emitted by the driver will reach the receiver through the PCB transmission line. When the signal is transmitted in the signal line of the printed board, its characteristic impedance value Z0 must match the "electronic impedance" of the head and tail components, so that the "energy" in the signal can be completely transmitted.
2. Reason 2
Once the quality of the printed board is poor and Z0 exceeds the tolerance, the transmitted signal will experience problems such as reflection, dispersion, attenuation, or delay. In severe cases, incorrect signals may be transmitted and the machine may crash.
3. Reason Three
Strict selection of boards and control of production processes are necessary to ensure that the Z0 on multi-layer boards meets the specifications required by customers. The higher the electronic impedance of the component, the faster its transmission speed will be. Therefore, the Z0 of the PCB must also be increased in order to meet the requirements of the matching component. Only qualified multilayer boards with Z0 can be considered as qualified products for high-speed or high-frequency signals.
4、 Relationship between characteristic impedance ZO and plate and process
Calculation formula of characteristic impedance Z0 of microstrip line structure: Z0=87/r+1.41 ln5.98H/(0.8W+T)
Among them: ε R - dielectric constant H - dielectric thickness W - wire width T - wire thickness
Sheet metal ε The lower r, the easier it is to increase the Z0 value of the PCB circuit and match the output impedance value of the high-speed component.
1. Characteristic impedance Z0 vs ε R is inversely proportional
Z0 increases with the increase of medium thickness. Therefore, for Z0 strict high-frequency circuits, strict requirements are put forward for the error in the dielectric thickness of the copper clad plate substrate. Usually, the variation in medium thickness should not exceed 10%.
2. Influence of dielectric thickness on characteristic impedance Z0
As the wire density increases, the increase in medium thickness will cause an increase in electromagnetic interference. Therefore, as the density of conductor wiring increases, the dielectric thickness of signal transmission lines for high-frequency and high-speed digital lines should be reduced to eliminate or reduce the problem of noise or crosstalk caused by electromagnetic interference, or to significantly reduce it ε r. Select Low ε R substrate.
According to the calculation formula of characteristic impedance Z0 of microstrip line structure: Z0=87/r+1.41 ln5.98H/(0.8W+T)
The thickness of copper foil (T) is an important factor affecting Z0, and the larger the wire thickness, the smaller its Z0. But its range of variation is relatively small.
3. Influence of copper foil thickness on characteristic impedance Z0
The thinner the copper foil thickness, the higher the Z0 value can be obtained, but its thickness variation has little contribution to Z0.
The contribution of using thin copper foil to Z0 is more precisely due to its contribution to improving or controlling Z0 in the manufacturing of fine wires.
According to the formula:
Z0 = 87/r +1.41 ln5.98H / （0.8W+T）
The smaller the line width W, the greater Z0; Reducing the wire width can improve the characteristic impedance.
The change in line width has a much more significant impact on Z0 than the change in line thickness.
4. Influence of conductor width on characteristic impedance Z0
Z0 rapidly increases as the line width W narrows, therefore, to control Z0, it is necessary to strictly control the line width. At present, the signal transmission line width W of most high-frequency lines and high-speed digital lines is 0.10 or 0.13mm. Traditionally, the deviation of line width control is ± 20%. For conventional electronic products without transmission lines, the PCB wire (wire length<1/7 of the signal wavelength) can meet the requirements, but for signal transmission lines with Z0 control, the PCB wire width deviation of ± 20% can no longer meet the requirements. Because at this point, the Z0 error has exceeded ± 10%.
For example:
A PCB microstrip line with a width of 100 μ m. Line thickness is 20 μ m. Medium thickness is 100 μ m. Assuming that the copper thickness of the finished PCB remains uniform, can Z0 meet the requirement of ± 10% when the line width changes by ± 20%?
Solution: According to the formula
Z0 = 87/r +1.41 ln5.98H / （0.8W+T）
Substitution: Line width W0=100 μ m， W1 ＝ 80 μ m， W2 ＝ 120 μ m. Line thickness T=20 μ m. Medium thickness H=100 μ m. So: Z01/Z02=1.20
So, Z0 is just ± 10% and cannot reach<± 10%.
In order to achieve the characteristic impedance Z0<± 10%, the deviation of wire width must be further reduced and must be far less than ± 20%.
Similarly, to control Z0 ≤ 5%, the wire width tolerance must be controlled ≤ ± 10%.
Therefore, it is not difficult for us to understand why the polytetrafluoroethylene PCB and some FR-4PCBs require ± 0.02mm line width. The reason is to control the value of characteristic impedance Z0.
5、 Characteristic impedance control PCB process control
1. Film production management and inspection
Constant temperature and humidity room (21 ± 2 ° C, 55 ± 5%), dustproof; Line width process compensation.
2. Panel design
The edges of the panel should not be too narrow, the coating should be uniform, and a false cathode should be added during electroplating to disperse the current;
Design a standard sample (coupon) for testing Z0 on the edge of the panel.
3. Etching
Strictly control process parameters, reduce lateral corrosion, and conduct initial inspection;
Reduce residual copper, copper slag, and copper scrap at the wire edge;
Check the line width and control it within the required range (± 10% or ± 0.02mm).
4. AOI inspection
The inner layer board must identify wire gaps and protrusions. For 2GHZ high-speed signals, even if there is a 0.05mm gap, it must be scrapped; Controlling the inner line width and defects is crucial.
5. Lamination
Vacuum laminating machine, reduce pressure to reduce glue flow, and try to maintain a large amount of resin due to the influence of resin ε r. Save more resin, ε R will be lower. Control the thickness tolerance of the lamination. Because the plate thickness is uneven, it indicates that the change in medium thickness will affect Z0.
6. Choose a good substrate
Strictly follow the plate model required by the customer for cutting. Wrong model number, ε R is incorrect, the thickness of the board is incorrect, and the manufacturing process of the PCB is all correct. It is also scrapped. Because Z0 is affected by ε The impact of r is significant.
7. Resistance welding
The resistance welding on the board surface will reduce the Z0 value of the signal line by 1-3 Ω. In theory, the thickness of the resistance welding should not be too thick, but in fact, the impact is not significant. The surface of copper wire is in contact with air（ ε R=1), so the measured value of Z0 is relatively high. But after resistance welding, the measured Z0 value will decrease by 1-3 Ω, because of the resistance welding ε R is 4.0, much higher than air.
8. Water absorption rate
Finished multi-layer boards should try to avoid water absorption as much as possible, as water ε R=75, which will have a significant decrease and unstable effect on Z0.
6、 Summary
For the characteristic impedance Z0 of the multilayer board signal transmission line, the current required control range is usually 50 Ω± 10%, 75 Ω± 10%, or 28 Ω± 10%.
The controlled range of changes must consider four major factors:
(1) Signal line width W;
(2) Signal line thickness T;
(3) The thickness of the dielectric layer H;
(4) Dielectric constant ε r 。
The biggest impact is on the thickness of the medium, followed by the dielectric constant, wire width, and the smallest is wire thickness. After selecting the substrate, ε The change in r is very small, and the change in H is also small. T is easier to control, while controlling the line width W at ± 10% is difficult. Additionally, the problem of line width includes pinholes, notches, dents, and other issues on the wire. In a sense, the most effective and important way to control Z0 is to control and adjust the line width.
KINGFORD PCB ELECTRONICS CO.,LTD
Source from : http://www.kingfordpcb.com
E-mail: sales@kingfordpcb.com
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