# Influence of PCB processing on impedance control and solutions

my country is in a good situation with economic construction as the center and reform and opening up. The annual growth rate of the electronics industry will exceed 20%. The printed circuit board industry will also follow the trend and the growth rate will exceed 20%. The technological revolution and industrial structure changes in the world electronics industry are bringing new opportunities and challenges to the development of printed circuits. With the development of miniaturization, digitization, high frequency and multi-function of electronic equipment, printed circuit, as a metal wire in the electrical interconnection of electronic equipment, is not only a problem of current flow, but also a signal transmission line. 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 not only necessary to measure whether the circuit on-off and short-circuit meet the requirements, but also whether the characteristic impedance value is within the specified qualified range. Only when these two directions are qualified, the circuit board meets the requirements.

The circuit performance provided by the printed circuit board must be able to prevent reflection during signal transmission, maintain the integrity of the signal, reduce transmission loss and play the role of matching impedance, so as to obtain a complete, reliable, accurate, interference-free, noise-free transmission signal. This paper discusses the problem of characteristic impedance control of the surface microstrip line structure multilayer board commonly used in practice.

### 1. Surface microstrip line and characteristic impedance

The characteristic impedance value of the surface microstrip line is high and widely used in practice. Its outer layer is the signal line surface with controlled impedance, and it is separated from the adjacent reference surface by insulating materials. The calculation of the characteristic impedance The formula is:

a. Microstrip
Z={87/[sqrt(Er+1.41)]}ln[5.98H/(0.8W+T)] Among them, W is the line width, T is the copper thickness of the trace, H is the distance from the trace to the reference plane, and 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 traces are located on the two reference planes in the middle. This formula can only 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) the dielectric constant Er, (2) the thickness H of the medium, (3) the width W of the wire, and (4) the thickness T of the copper wire, so it can be known that the characteristic impedance is related to the substrate material ( The relationship between copper clad plate) 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 measured at a frequency of 1Mhz, and the same material produced by different manufacturers is different due to its different resin content. This study takes epoxy glass cloth as an example to study the relationship between dielectric constant and frequency. The dielectric constant decreases with the increase of frequency, so in practical applications, the dielectric constant of the material should be determined according to the operating frequency. 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, in order to obtain a high signal transmission speed, the dielectric constant of the material must be reduced. 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 affecting the characteristic impedance change. The figure takes 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 accordingly by 5-6 ohms. In actual production, if 18μm copper foil is used for the signal wire surface of the control impedance, the allowable wire width variation tolerance It is ±0.015mm. If the variation tolerance of the control impedance is 35μm copper foil, the allowable variation tolerance of the wire width is 0.025mm. It can be seen that the variation of the wire width allowed in the production will lead to a great change in the impedance value. The width of the wire is determined by the designer according to various design requirements. It must not only meet the requirements of the current carrying capacity and temperature rise of the wire, but also obtain the desired impedance value. This requires the manufacturer to ensure that the line width meets the design requirements and changes 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 coating 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 coating. It should be noted that the surface of the wire should be clean before electroplating, and there should be no residues and black trimming oil. As a result, the copper is not plated during electroplating, so that the thickness of the local wire changes and affects the characteristic impedance value. In addition, care must be taken in the process of brushing the board, so as not to change the thickness of the wire and cause the impedance value to change.

### 4. Influence of dielectric thickness H

It can be seen from the formula that the characteristic impedance is proportional to the natural logarithm of the dielectric thickness, so 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 means to control the characteristic impedance in production. From the figure, the relationship between the characteristic impedance value and the change of the dielectric thickness can be obtained. It can be seen from the figure that when the thickness of the medium changes by 0.025mm, the corresponding change in the impedance value will be +5-8 ohms, and in the actual production process, the allowable change in the thickness of each layer of the laminate will cause the impedance value to change significantly. 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: you can refer to the diagram 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 calculated impedance value proposed by the user, find the corresponding dielectric thickness through the figure, and then according to the selected The thickness of the copper clad laminate and copper foil determines the type and number of sheets of the prepreg.

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, which is generally 20Ω-40Ω larger. 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 with the increase of the dielectric thickness. 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 change of the dielectric thickness does not exceed 10%. For multi-layer boards, the dielectric thickness is still a process. Factors, especially those closely related to the multi-layer lamination process, should also be closely controlled.

### 5 Conclusion

In actual production, a slight change in the width, thickness of the wire, the dielectric constant of the insulating material and the thickness of the insulating medium will cause the characteristic impedance value to change, and the characteristic impedance value will also be related to other production factors. The control manufacturer must understand the factors affecting the change of the characteristic impedance value, master the actual production conditions, and adjust the various process parameters according to the requirements of the designer to make the change within the allowable tolerance range to obtain the desired impedance value.