The surface characteristic impedance of the printed circuit board (PCB) and characteristic impedance control
1, resistance
Alternating current flows through a conductor, the resistance is called Impedance (Impedance), meet for Z, unit or Ω.
At this time of the resistance is different with dc current encountered resistance, in addition to the resistance of the resistance, and inductance (XL) and the capacitive reactance (XC) the resistance of the problem.
For the difference between a direct current resistance, called an alternating current resistance encountered by impedance (Z).
R2 + Z =) (XL - XC) 2
2, the impedance (Z)
In recent years, the improvement of IC integration and application of the signal frequency and the rate is higher and higher, consequently in PCB conductor, signal transmission (emission) after high to a certain value, will be affected by PCB wire itself, leading to the serious distortion or complete loss of lose signal. This shows that the PCB traces "flow" by the "thing" is not current, but the square wave signals, or pulses on the energy transmission.
3, the characteristic impedance control (Z0)
The above this happens when the "signal transmission" the resistance, the other is called "property resistance", on behalf of the symbol for Z0.
So, PCB traces on the single solution to "tong", "broken" and "short circuit" problem is not enough, but also control the characteristic impedance of the wire problem. That is to say, high-speed transmission, the transmission line of high frequency signal transmission, far more than the transmission conductor is strict on quality. Is no longer "open/short circuit test pass, or gap, burr is not more than 20% of line width, can receive. Must demand the determination of characteristic impedance value, also want to control in the tolerance to the impedance, otherwise, only scrap, rework is not allowed.
Second, the signal propagation and transmission line
1, the signal transmission line definition
(1) according to the principle of electromagnetic wave, the shorter the wavelength (lambda), the higher the frequency (f). Both the product of the speed of light. The C = lambda. F = 3 x 1010 cm/s
(2) any components, although has the very high frequency signal transmission, but after the PCB wire transfer, the original high transmission frequency will drop down, or a time delay.
Therefore, the wire length as short as possible.
(3) improve the density of PCB wiring or shorten the wire size is good. However, with the speeding up of the frequency components, or pulse cycle shortened, conductor length close to signal wavelength (speed) of a certain range, the components on the PCB guide line transmission, will also appear obvious "distortion".
(4) the IPC - 2141 3.4.4 put forward: when the signal transmission in the wire, if the wire length is close to 1/7 of the signal wavelength, wire is seen as a signal transmission line at this time.
(5), for example:
A component signal transmission frequency (f) for 10 MHZ, PCB, wire length 50 cm, characteristic impedance control should be considered?
Solution: C = lambda. F = 3 x 1010 cm/s
Lambda = C/f = (3 x 1010 cm/s)/(1 x 107 / s) = 3000 cm
Wire length/signal wavelength = 50/3000 = 50/3000
Because: 1/60 < 1/7, so the wire for the common wires, don't have to consider the characteristic impedance.
In electromagnetic theory, maxwell's equation tells us: sine wave signal propagation velocity VS in the medium is proportional to the speed of light C, and is inversely proportional to the transmission medium of dielectric constant.
V epsilon r = C /)
When the epsilon r = 1, signal transmission to the speed of light, namely 3 x 1010 cm/s.
2, transmission rate and dielectric constant
Different plate under 30 MHZ signal transmission speed
Dielectric constant dielectric materials Tg (° C) signal transmission speed (m/(including s)
Vacuum / 1.0 300.00
Ptfe / 2.2 202.26
Thermosetting poly propylene ether 2.5 189.74 210
Cyanate ester resin 225 3.0 3.0
Ptfe resin + / 2.6 2.6 E glass cloth
Cyanate ester resin + glass cloth 225 3.7 155.96
Polyimide 230 4.5 4.5 + glass cloth
Quartz / 3.9 151.98
Epoxy glass cloth 130 + 5 4.7 4.7
Aluminum / 9.0 9.0
From the table above shows, with the increase of dielectric constant (epsilon r), signal transmission speed decrease in medium quality material. To obtain high signal transmission speed, the surface characteristic impedance of the need to use high value; The characteristics of high impedance, must choose low dielectric constant (epsilon r) materials; Dielectric constant of ptfe (Teflon) minimum epsilon (r), degree of the fastest transmission speed.
FR - 4 board, is made of epoxy resin and E glass cloth joint, dielectric constant (epsilon r) is 4.7. Signal transmission speed of 138 m/mu s. Change the resin system, can easily change the dielectric constant (epsilon r).
Third, characteristic impedance value control
1, the reason why a
Electronic devices (computer, communication machine), signals emitted from drive component (Driver), will arrive via PCB line receiving components (Receiver). Signal transmission in the line of PCB, the characteristic impedance value Z0 must with end components "electronic impedance can match," signal in the "energy" will have to complete transmission.
2, why
Once appear, PCB quality bad, Z0 beyond tolerance, can appear the preaching of signal Reflection (Reflection), lost (Dissipation), Attenuation (Attenuation) or Delay (Delay), serious when the wrong letter Number, crashing.
3, why three
Strict selection sheet and control production processes, laminated on Z0 can meet the requirements of the customer specifications. Elements of electrical impedance is higher, its transmission speed will be faster, thus PCB Z0 also want to rise, can meet the requirements of matching components. Z0 qualified sandwich plate, is what is required by the high speed or high frequency signal on the nonconforming product.
Four, the characteristic impedance ZO relationship with plate and process
The characteristic impedance of microstrip line structure Z0 formula: Z0 = 87 + 1.41 ln5.98 H/r/(0.8 W + T)
Among them: epsilon r - H - medium dielectric constant thickness W - wire width T - wire thickness
Plank the epsilon r is lower, the more likely it is to improve the PCB line Z0 value, with high speed component output impedance values match.
1, the characteristic impedance Z0 is inversely proportional to the plate epsilon r
Z0 increased with the increase of medium thickness increased. Therefore, for the high frequency circuit Z0 strictly, error of medium thickness of sheet base material, strict requirements are put forward. Usually, the medium thickness changes should not exceed 10%.
2, medium thickness affect Z0 characteristic impedance
With the increase of linear density, medium thickness increase will cause the increase of electromagnetic interference. As a result, the high frequency circuit and high speed digital circuit of signal transmission lines, with the increase of the density of the conductor wiring, should reduce the medium thickness, in order to eliminate or reduce the electrical magnetic interference noise brought by the letter or crosstalk problems, or reduce the epsilon r, choose low epsilon r base material.
According to the characteristic impedance of the microstrip line structure Z0 formula: Z0 = 87 + 1.41 ln5.98 H/r/(0.8 W + T)
Copper foil thickness (T) is an important factor to influence Z0, the thickness of the wire is more big, the smaller the Z0. But its range is relatively small.
3, the influence of the copper foil thickness of Z0 characteristic impedance
The thinner the thickness of copper foil, can get higher Z0 values, but its thickness change on Z0 contribution is not big.
Using thin copper foil's contribution to the Z0, is due to the thin copper foil in fabrication of fine line, to improve or control Z0 and contribute to a more exact.
According to the formula:
Z0 = 87 + 1.41 ln5.98 H/r/W (0.8 + T)
Line width W is smaller, the greater the Z0; Reduce wire width can improve the characteristic impedance.
Line width thicker than the line changes affect Z0 much more obvious.
4, the influence of the wire width of characteristic impedance Z0
Z0 with the rapid increase of narrow line width W and, therefore, to control the Z0, must strict control line width. At present, most of the high frequency circuit and high speed digital circuit of signal transmission lines W 0.10 or 0.13 mm wide. Traditionally, the line width control deviation of plus or minus 20%. For the transmission line products, the conventional electric son of PCB wire (wire length < 1/7 of the signal wavelength) can meet the require, but to have Z0 control signal transmission line, PCB wire width deviation plus or minus 20%, can not meet the requirements. Because, at this time of Z0 error has more than plus or minus 10%.
For example, are as follows:
A PCB microstrip line width is 100 microns, the line is 20 microns thick, medium thickness is 100 microns, assuming that the finished PCB copper thickness is constant, ask line width change plus or minus 20%, Z0 whether conform to within + / - 10%?
Solution: according to the formula
Z0 = 87 + 1.41 ln5.98 H/r/W (0.8 + T)
Substitution: line W0 = 100 microns wide, W1 = 80 microns, W2 = 120 microns, line T = 20 microns thick, medium thickness H = 100 microns, is: Z01 / Z02 = 1.20
So, Z0 just plus or minus 10%, cannot achieve the plus or minus 10%.
To achieve the characteristic impedance Z0 < plus or minus 10%, wire width deviation must be narrowed, far less than + / - 20%.
Similarly, to control Z0 5% or less, wire width tolerance must control plus or minus 10% or less.
Therefore, we would not be difficult to understand why, teflon PCB, PCB, and some FR - 4 request line width + / - 0.02 mm, its reason is to control the characteristic impedance Z0 values.
Five, the characteristic impedance control PCB process control
1, film production management and inspection
Constant temperature and humidity room (21 + 2 ° C, 55 + / - 5%), dust; Line width compensation process.
2, the makeup design
Makeup edge can't be too narrow, uniform coating, electroplating and false cathode, dispersed electric current;
Design the whole edge test Z0 prototype (coupon).
3, etching
Strict process parameters, reducing the lateral erosion, the first inspection;
To reduce the line residual copper, copper slag, copper pieces;
Check the line width, control within the scope of the required (plus or minus 10% or + / - 0.02 mm).
4, AOI inspection
Lining board must find wire gap, protruding mouth, to 2 GHZ high-speed signal, namely make the 0.05 mm gap, also must be scrapped; Control the inner line width, and defects is the key.
5, laminated
Vacuum laminating machine, reduce the pressure to reduce flow glue, try to keep more amount of resin, as resin affect epsilon r, resin to save more, epsilon r will be lower. Control layer thickness tolerance. Because the thickness is not uniform, suggests that the medium thickness change, affects Z0.
6, choose good base material
Strictly according to customer requirements of plate type. Model of fault, epsilon r wrong, thickness, PCB manufacturing process for all, as scrap. Because Z0 affected by epsilon r.
7, resistance welding
Panel Z0 value of the resistance welding can make the signal lines to reduce Ω 1 ~ 3, theoretically resistance welding thickness shoulds not be too thick, in fact is not significantly affected. Copper wire surface contact is the air (epsilon r = 1), so the measured Z0 value is higher. But in the resistance measurement Z0 will decline after welding Ω 1 ~ 3, the reason was that the resistance welding epsilon r is 4.0, much higher than air.
8, bibulous rate is
Finished products profes sional suction to avoid as far as possible, because the water epsilon r = 75, of Z0 will get to a big drop and the instability of the effect.
Six, the summary
A multilayer signal transmission line characteristic impedance Z0, the current control scope Often is: 50 Ω plus or minus 10%, 75 Ω plus or minus 10%, or 28 Ω plus or minus 10%.
Control range, must consider four factors:
(1) the signal line width W;
(2) signal wire thickness T,
(3) medium thickness H;
(4) dielectric constant epsilon r.
Affect the largest is the medium thickness, the second is the dielectric constant, wire width, the smallest is the thickness of the conductor. After the selected base material, the epsilon r changed little, H change is small, T is easy to control, and it is difficult to control in the plus or minus 10% line width W, and line width problem and a conductor on the pinhole, gap, depression and other issues. In a sense, the control Z0, the most effective method of the most important is to control the line width adjustment.
KINGFORD PCB ELECTRONICS CO., LTD
Source from: http://www.kingfordpcb.com
E-mail: sales@kingfordpcb.com
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