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1.Characteristic impedance resistance
When an AC current flows through a conductor, the resistance is called the impedance, corresponding to Z, and is measured in Ω.
The resistance at this point is different from the DC current. In addition to the resistance of the resistor, there is also the issue of inductive (XL) and capacitive (XC) resistance.
In order to distinguish between DC resistance, the resistance encountered in AC is called impedance (z).
Z=√ R2 + (XL - XC)2
2.Impedance Characteristics of Characteristic Impedance (z)
In recent years, with the improvement of IC integration and application, the frequency and speed of signal transmission are getting higher and higher. Therefore, when the signal transmission reaches a certain value, it is affected by the PCB wire itself, resulting in serious distortion or complete loss of the transmitted signal. It can be seen that the ‘stuff’ flowing through the PCB wire is not current, but the transmission of square wave signals or pulses in energy.
3.Characteristic Impedance Control (Z0)
The resistance of the transmission of the above ‘signal’ is also called ‘characteristic impedance’ and is indicated by the symbol Z0.
Therefore,it is necessary to control the characteristic impedance of the lead wires not only to solve the problems of ‘on’, ‘off’ and ‘short circuit’ on the PCB lines. In other words, the quality of the transmission line for high-speed transmission and high-frequency signal transmission is much more stringent than that of the transmission line. It is no longer a case of passing an ‘open/short’ test, or having notches and burrs not exceeding 20% of the wire width.It must be required to measure the characteristic impedance value, and the impedance must be controlled within the tolerance, otherwise it can only be scrapped,not reworked.
Why do we need to control the characteristic impedance?
1.When an electronic device (computer, communicator) is in operation,the signal from the driver travels through the PCB transmission line to the receiver.When the signal is transmitted on the signal line of the PCB, 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 fully transmitted.
2.If the quality of the printed circuit board is poor and Z0 exceeds the permitted value,the transmitted signal will suffer from reflection, dispersion, attenuation or delay.In serious cases,it will transmit wrong signals and cause the computer to crash.
3.Strict selection of boards,strict control of the production process, multi-layer boards on the Z0 can meet the customer's specifications. The higher the electronic impedance, the faster the transmission speed. Therefore, the Z0 of PCB must be improved to meet the requirements of matching components. Only if Z0 is qualified, it is considered a qualified product for high-speed or high-frequency signal requirements.
Relationship between PCB characteristic impedance Zo and PCB material and PCB process
PCB microstrip structure characteristic impedance Z0 formula:Z0 = 87 / R + 1.41ln5.98h / (0.8W + T)
Where:ε R - dielectric constant H - dielectric thickness W - conductor width T - conductor thickness
The lower the ε R of the circuit board, the easier it is to increase the Z0 value of the PCB circuit to match the output impedance value of the high-speed components.
1.Characteristic impedance Z0 is inversely proportional to the εR of the board.
Z0 increases with increasing dielectric thickness. Therefore, for high-frequency circuits with stringent Z0, there are strict requirements on the dielectric thickness error of the laminate substrate. In general, the change of dielectric thickness should not exceed 10%.
2.Effect of dielectric thickness on characteristic impedance Z0
With the increase in line density, the increase in dielectric thickness will cause an increase in electromagnetic interference.Therefore,as the density of conductor wiring increases, the thickness of the dielectric should be reduced to eliminate or reduce the spurious signals or crosstalk caused by electromagnetic interference, or to reduce ε R,so as to select a low ε r substrate.
According to the microstrip line structure characteristic impedance Z0, the formula is: Z0 = 87 / R + 1.41 ln5.98h / (0.8W + T)
The thickness of the copper foil (T) is an important factor affecting Z0. The greater the wire thickness, the smaller the Z0. However, the range of variation is relatively small.
3.Effect of copper foil thickness on characteristic impedance Z0
The thinner the thickness of the copper foil, the higher the Z0 value can be obtained, but the variation of the thickness contributes little to the Z0.
The contribution of thin copper foils to Z0 is more accurate than that of thin copper foils when making fine lines to increase or control Z0.
According to the formula:
Z0 = 87/r +1.41 ln5.98H / (0.8W+T)
The smaller the wire width W is, the larger Z0 is; reducing the wire width can increase the characteristic impedance.
The effect of line width change on Z0 is much more obvious than the effect of line width.
4.The effect of conductor width on the characteristic impedance Z0
Z0 increases rapidly with the narrowing of the wire width W. Therefore, to control Z0, it is important to have a small wire width. Therefore, in order to control Z0, the line width must be strictly controlled. Currently, most high-frequency lines and high-speed digital lines have a signal transmission line width W of 0.10 or 0.13 mm; traditionally, the line width control deviation is ±20%. For conventional electronic products without transmission lines, the PCB alignment (line length ‘1/7 of the signal wavelength’) can meet the requirements, but for Z0-controlled signal transmission lines, the PCB alignment width deviation of ±20% cannot meet the requirements. This is 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℃, 55±5%), dustproof, line width compensation.
2.PCB panel design
Panel edges can not be too narrow, plating should be uniform, the use of electroplating and false cathode to disperse the current;
Standard samples ( Coupons ) are designed to test Z0.
3.PCB Etching
Strict process parameters to reduce side corrosion, first inspection;
Reduce copper residue at wire edge.Copper slag.Copper chips;
Check the line width, control within the required range (± 10% or ± 0.02mm).
4.pcbaoi examination
For 2GHz high-speed signals, even 0.05mm gap, must be scrapped; the key is to control the inner layer of the line width and defects.
5.PCB lamination
Vacuum laminator, reduce pressure, reduce glue flow, try to retain as much resin as possible, because resin affects εR, the more resin is retained, the lower εr will be.Control the lamination thickness tolerance.Because the board thickness is not uniform, which means that the dielectric thickness changes will affect Z0.
6.Selection of PCB substrate
Strictly follow the customer's requirements for board layout.Wrong model, wrong ε R, wrong board thickness, correct PCB manufacturing process, same scrap.Because Z0 is greatly affected by εR.
7.PCB Solder Resist Layer
Theoretically, the thickness of soldermask should not be too thick, but in practice,the effect is not very large.The surface of the copper conductor is exposed to air (ε r = 1), so the measured value of Z0 is higher.However, the value of Z0 decreases by 1-3 Ω after soldering the resistor, because the εr of resistor soldering is 4.0,which is much higher than that of air.
8.Water absorption of circuit boards
Finished multilayer boards should avoid absorbing water as much as possible, because the εr=75 of water will bring a big difference and instability to Z0.
The PCB characteristics that affect the impedance are the thickness of the dielectric,followed by the dielectric constant,line width and line thickness.Line width and line thickness.When choosing the substrate, the variation of ε R and H is small and t is easy to control, but the line width W is difficult to control within ±10%.In addition,wire width problems include pinholes,nicks and dents in the wire.Notches and depressions in the wire.In a sense, an effective and important way to control impedance is to control and adjust the PCB line width.