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China is in a good situation of reform and opening up centered on economic construction. The annual growth rate of the electronics industry will exceed 20%, and the dependence of the printed circuit board industry on the entire electronics industry will exceed 20%. The technological revolution of the world's electronics industry and changes in the industrial structure have brought new opportunities and challenges to the development of printed circuits. With the development of miniaturization, digitization, high frequency and multi-functionalization of electronic equipment, printed circuits, as the metal wires in the electrical interconnection of electronic equipment, are not only a current problem, but also a signal transmission line. That is to say, to conduct electrical testing of high-frequency signal and high-speed digital signal transmission on the PCB, it is necessary not only to measure whether the on-off and short-circuit of the circuit meets the requirements, but also to measure whether the characteristic impedance value is within the specified qualified range. Only if these two directions are qualified, the circuit board can meet the requirements.
The circuit performance provided by the printed circuit board must be able to ensure that there is no reflection in the signal transmission process, maintain the integrity of the signal, reduce the transmission loss, and play the role of matching impedance, so as to obtain a complete, reliable, accurate, and noise-free transmission signal. This article discusses the characteristic impedance control of the surface microstrip structure multilayer board.
1. Surface microstrip line and characteristic impedance
The surface microstrip line has high characteristic impedance and is widely used in practice. Its outer layer is the signal line plane that controls the impedance. It is separated from the adjacent datum plane by insulating material
A. Microstrip
Z={87/[sqrt(ER+1.41)]}ln[5.98h/(0.8W+T)], where W is the line width, T is the thickness of the copper sheet, h is the distance from the line to the reference plane, and ER is The dielectric constant of the PCB material. This formula can only be applied when 0.1<(w/h)<2.0 and 1<(ER)<15.
B strip line
Z=[60/Sqt(ER)] Ln {4H/(0.67π(0.8W+T)}}, where H is the distance between the two reference planes, and the line is in the middle of the two reference planes. This formula only Suitable for w/h<0.35 and T/h<0.25
It can be seen from the formula that the main factors affecting the characteristic impedance are (1) the dielectric constant Er, (2) the dielectric thickness h, (3) the line width W, and (4) the copper thickness T. Therefore, the characteristic impedance is closely related to the substrate material (copper clad laminate), so the choice of substrate material is very important in PCB design.
2. The dielectric constant of the material and its influence
The dielectric constant of the material is determined by the material manufacturer, and the frequency is 1MHz. The same material produced by different manufacturers differs due to its different resin content. Taking epoxy glass cloth as an example, the relationship between dielectric constant and frequency change was studied. The dielectric constant decreases as the frequency increases, so in practical applications, the dielectric constant of the material should be determined according to the operating frequency. Under normal circumstances, the average value can meet the requirements. The transmission speed of the signal in the dielectric material will decrease with the increase of the dielectric constant. Therefore, in order to obtain a higher signal transmission speed, the dielectric constant of the material must be reduced, and the high characteristic resistance must be used to obtain higher transmission. Speed.
3. The influence of wire width and thickness
The line width is one of the main parameters that affect the characteristic impedance change. Taking the surface microstrip line as an example, the relationship between the impedance value and the line width is explained. It can be seen from the figure that when the wire width changes by 0.025mm, the impedance value will change by 5-6 ohms. In actual production, if 18 ohms is used to control the impedance μ of the signal line plane, the tolerance of conductor width change is ±0.015mm, if the tolerance of impedance change is 35μ, it can be seen that the allowable change of conductor width in production will lead to impedance Great change in value. The wire width is determined by the designer according to various design requirements. It is necessary to meet the current-carrying capacity and temperature rise requirements of the wire, but also to achieve the expected impedance value. This requires the manufacturer to ensure that the line width meets the design requirements during production and changes it within the tolerance range to meet the impedance requirements. The thickness of the wire is also determined according to the required current carrying capacity of the wire and the allowable temperature rise. In production, in order to meet the requirements of use, the average thickness of the coating is 25μm. The thickness of the wire is equal to the thickness of the copper foil plus the thickness of the coating. It should be noted that before electroplating, the surface of the wire should be kept clean, and there should be no residue and oil black on the repair board. As a result, during the electroplating process, copper is not electroplated, and the thickness of the local conductor changes, thereby affecting the characteristic impedance value. In addition, in the process of brushing the board, be careful not to change the thickness of the wire to cause changes in the impedance value.
4. The influence of medium thickness H
It can be seen from the formula that the characteristic impedance is proportional to the natural logarithm of the dielectric thickness. Therefore, the thicker the dielectric, the greater the impedance value. Therefore, the thickness of the dielectric is another major factor affecting the characteristic resistance value. Since the line width and dielectric constant of the material have been determined before production, the line thickness process requirements can also be used as a fixed value, so controlling the thickness of the laminate (dielectric thickness) is the main means to control the characteristic impedance in production. The relationship between the characteristic impedance value and the change in the thickness of the medium can be obtained from the figure. It can be seen from the figure that when the thickness of the medium changes by 0.025mm, the impedance value changes by +5-8 ohms. In the actual production process, the change in the allowable thickness of each layer will cause a great change in the impedance value. In actual production, different types of prepregs are selected as the insulating medium, and the thickness of the insulating medium is determined according to the number of prepregs. Take the surface microstrip line as an example: in the production process, you can refer to the diagram to determine the dielectric constant of the insulating material at the corresponding operating frequency, and then use the formula to calculate the corresponding impedance value. According to the user's line width value and the calculated impedance value, find the corresponding medium thickness through the chart, and then determine the type and quantity of the prepreg according to the thickness of the copper clad laminate and copper foil.
It can be seen from the figure that when the dielectric thickness and material are the same, the design of the microstrip line structure has a higher characteristic impedance value than the strip line structure, generally 20Ω-40Ω. Therefore, the microstrip structure is mostly used for high-frequency and high-speed digital signal transmission. At the same time, the characteristic impedance increases as the thickness of the medium increases. Therefore, for high-frequency circuits with strictly controlled characteristic impedance values, strict requirements are placed on the dielectric thickness error of the copper clad laminate. Generally speaking, the thickness of the medium does not change more than 10%. For multi-layer boards, the thickness of the media is still a processing factor, especially closely related to the multi-layer lamination process, so it must be strictly controlled.
5. Conclusion
In actual production, a slight change in the width and thickness of the wire, the dielectric constant of the insulating material, and the thickness of the insulating medium will cause the characteristic impedance to change. In addition, the characteristic impedance value is also related to other production factors. Therefore, in order to control the characteristic impedance, the PCB manufacturermust understand the factors affecting the change of the characteristic impedance value according to the requirements of the designer, master the actual production conditions, and adjust the process parameters within the allowable tolerance range to obtain the required impedance value.
