Microwave Tech - The influence of high-frequency PCB circuit board processing on impedance control and its solutions

Our country is in a favorable situation with economic construction as the center and reform and opening up. The annual growth rate of the electronic industry will exceed 20%. The technological revolution and industrial structural changes in the world's electronics industry are bringing 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 interconnections of electronic equipment, are not only a question of whether current flows or not, but also serve as signal transmission lines. effect. That is to say, for the electrical test of the PCB used for the transmission of high-frequency signals and high-speed digital signals, it is necessary to measure whether the circuit continuity and short-circuit meet the requiremen ts, but also whether the characteristic impedance value is within the specified qualified range. Only if both directions are qualified, the circuit board meets the requirements.

The circuit performance provided by the printed circuit board must be able to prevent reflections during signal transmission, keep the signal intact, reduce transmission loss, and play the role of matching impedance, so that a complete, reliable, accurate, interference-free, and noise-free transmission signal can be obtained. This article discusses the characteristic impedance control of the surface microstrip structure multilayer board commonly used in practice.

1.Surface microstrip line and characteristic impedance
The characteristic impedance of the surface microstrip line is relatively high and is widely used in practice. Its outer layer is the signal line surface that controls the impedance. It is separated from the adjacent reference plane with an insulating material. The calculation of the characteristic impedance The formula is:

a. Microstrip

Z={87/[sqrt(Er+1.41)]}ln[5.98H/(0.8W+T)] where W is the line width, T is the copper thickness of the trace, and H is the trace to the reference plane Distance, Er is the dielectric constant of the PCB material. This formula must be applied when 0.1<(W/H)<2.0 and 1<(Er)<15.

b. stripline

Z=[60/sqrt(Er)]ln{4H/[0.67π(0.8W+T)]} where H is the distance between the two reference planes, and the trace is located in the middle of the two reference planes. This formula must be applied when 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) dielectric constant Er, (2) dielectric thickness H, (3) wire width W, and (4) wire copper thickness T. Therefore, the characteristic impedance and the substrate material ( The relationship between copper clad board) is very close, 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 manufacturer of the material at a frequency of 1Mhz. The same material produced by different manufacturers is different due to the different resin content. This study takes epoxy glass cloth as an example to study the relationship between the dielectric constant and the frequency change. The dielectric constant decreases with the increase of frequency. Therefore, the dielectric constant of the material should be determined according to the operating frequency in practical applications. Generally, the average value can be used to meet the requirements. The transmission speed of the signal in the dielectric material will decrease with the increase of the dielectric constant. Therefore, to obtain a high signal transmission speed, the dielectric constant of the material must be reduced, and at the same time, it is necessary to obtain a high transmission speed. Use high characteristic resistance value, and high characteristic resistance value must choose low dielectric constant material.

3. The influence of wire width and thickness

The wire width is one of the main parameters that affect the characteristic impedance change. The figure uses the surface microstrip line as an example to illustrate the relationship between the impedance value and the wire width. 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 18μm copper foil is used to control the impedance of the signal line surface, the allowable wire width variation tolerance It is ±0.015mm. If the tolerance of the control impedance change is 35μm copper foil, the allowable wire width variation tolerance is 0.025mm. It can be seen that the allowable wire width change in production will cause a great change in the impedance value. The width is determined by the designer according to a variety of design requirements. It must not only meet the requirements of wire carrying capacity and temperature rise, but also obtain the desired 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 conductor and the allowable temperature rise. In order to meet the requirements of use in production, the thickness of the plating layer is generally 25μm on average, and the thickness of the wire is equal to the thickness of the copper foil plus the thickness of the plating layer. It should be noted that before electroplating, the surface of the wire must be clean, and there should be no residues and repairing oil black, which will cause the copper to not be plated during electroplating, which will change the thickness of the local wire and affect the characteristic impedance value. In addition, you must be careful in the process of brushing, do not change the thickness of the wire and cause the impedance value to change.

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, it can be seen that the thicker the dielectric thickness, the greater the impedance value, so the dielectric thickness is another main factor affecting the characteristic resistance value. Because the wire width and the dielectric constant of the material have been determined before production, the wire thickness process requirements can also be used as a fixed value, so controlling the laminate thickness (dielectric thickness) is the main method to control the characteristic impedance in production. From the figure, the relationship between the characteristic impedance value and the change in dielectric thickness can be drawn. It can be seen from the figure that when the thickness of the medium changes by 0.025mm, it will cause a corresponding change in the impedance value of +5-8 ohms. In the actual production process, the allowable change in the thickness of each layer will cause the impedance value to change greatly. Big change. 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: refer to the picture during the production process. Determine the dielectric constant of the insulating material at the corresponding operating frequency, and then use the formula to calculate the corresponding impedance value, and then according to the wire width value and the impedance value proposed by the user, find the corresponding dielectric thickness through the graph, and then according to the selected The thickness of copper clad laminate and copper foil determines the type and number of prepregs.

It can be seen from the figure that the design of the microstrip line structure has a higher characteristic impedance value than the stripline design under the same dielectric thickness and material, generally 20Ω-40Ω. Therefore, the microstrip line structure design is mostly used for high-frequency and high-speed digital signal transmission. At the same time, the characteristic impedance value will increase as the thickness of the medium increases. Therefore, for high-frequency circuits with strictly controlled characteristic impedance values, strict requirements should be placed on the error of the dielectric thickness of the copper clad laminate. Generally speaking, the dielectric thickness of the copper clad laminate does not change more than 10%. For the multilayer board, the dielectric thickness is still a process. Factors, especially closely related to multi-layer lamination processing, should also be closely 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 will also be related to other production factors, so in order to achieve the characteristic impedance Control producers must understand the factors that affect the characteristic impedance value change, master the actual production conditions, and adjust each process parameter according to the requirements of the designer to make the change within the allowable tolerance range to obtain the desired impedance value.