China is in a good situation of economic construction as the center and reform and opening up. The annual growth rate of electronic industry will exceed 20% % The printed circuit board industry is dependent on the whole electronic industry, which will also rise with the trend, and will exceed 20% % The growth rate of China’s economy. The technological revolution and industrial structure changes in the world electronic 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 the metal wire in the electrical interconnection of electronic equipment, is not only the problem of current flow, but also the function of signal transmission line. That is to say, the electrical test of PCB for high-frequency signal and high-speed digital signal transmission should not only measure whether the circuit on-off and short circuit meet the requirements, but also measure whether the characteristic impedance value is within the specified qualified range. Only when these two directions are qualified can the circuit board meet the requirements.

The circuit performance provided by the printed circuit board must be able to make the signal transmission process without reflection phenomenon, keep the signal intact, reduce the transmission loss and play the role of matching impedance, so as to get a complete, reliable, accurate and noise free transmission signal. In this paper, the problem of characteristic impedance control of multilayer plate with surface microstrip line structure is discussed.

one . Surface microstrip line and its characteristic impedance

The characteristic impedance of surface microstrip line is high and widely used in practice. Its outer layer is the signal line plane which controls the impedance. It is separated from the adjacent reference plane by insulating material

a . Microstrip line ( microstrip )

Z ={ eighty-seven /[ sqrt ( Er + one . forty-one )]} ln [ five . 98H /( 0 . 8W + T )] Where W is the line width, t is the thickness of copper sheet, h is the distance from the line to the reference plane, and Er is the dielectric constant of PCB material ( dielectric constant )。 This formula must be in 0 . one <( W / H )< two . 0 and 1 <( Er )< 15.

b . Stripline ( stripline )

Z =[ sixty / sqrt ( Er )] ln { 4H /[ 0 . sixty-seven π( 0 . 8W + T )]} Where h is the distance between the two reference planes, and the routing is located in the middle of the two reference planes. This formula must be in 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, (4) copper thickness T. therefore, the characteristic impedance is closely related to the substrate material (copper clad plate), so the selection of substrate material is very important in PCB design.

two . Dielectric constant of materials and its influence

The dielectric constant of the material is determined by the manufacturer of the material at the frequency of 1MHz. The same material produced by different manufacturers is different because of its resin content. Taking epoxy glass cloth as an example, this paper studies the relationship between dielectric constant and frequency change. Dielectric constant decreases with the increase of frequency, so in practical application, the dielectric constant of material should be determined according to working frequency. Generally, the average value can meet the requirements. The transmission speed of signal in dielectric material will decrease with the increase of dielectric constant, Therefore, in order to obtain high signal transmission speed, we must reduce the dielectric constant of the material, and at the same time, we must use high characteristic resistance to obtain high transmission speed.

three . Influence of wire width and thickness

Wire width is one of the main parameters that affect the change of characteristic impedance. Taking the surface microstrip line as an example, the relationship between impedance value and wire width is illustrated. It can be seen from the figure that when the wire width changes 0 . 025mm will cause the corresponding change of impedance value – 6 ohm, while in actual production, if 18 ohm is used to control the impedance of the signal line μ M copper foil, the allowable tolerance of wire width variation is ± 0 . 015mm, if the variation tolerance of control impedance is 35 μ The tolerance of wire width variation is 0 . It can be seen that the change of conductor width allowed in production will lead to great change of impedance value. The width of conductor is determined by the designer according to various design requirements. It should not only meet the requirements of conductor ampacity and temperature rise, but also get the expected impedance value. This requires that the producer should ensure that the line width meets the design requirements in production, and make it change within the tolerance range to adapt to the impedance requirements. The conductor thickness is also determined according to the current carrying capacity required by the conductor and the allowable temperature rise. In production, in order to meet the use requirements, the average thickness of the coating is 25 μ m. 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 conductor should be kept clean, and there should be no residue and oil black on the repair plate. As a result, the copper is not plated during electroplating, and the thickness of the local conductor changes, which affects the characteristic impedance value. In addition, in the process of brushing the board, be careful not to change the thickness of the wire, resulting in changes in the impedance value.

four . Influence of medium thickness h

It can be seen from the formula that the characteristic impedance is directly proportional to the natural logarithm of the dielectric thickness. Therefore, the thicker the dielectric thickness is, the greater the impedance value is. Therefore, the dielectric thickness is another major factor affecting the characteristic resistance value. Because the wire width and the dielectric constant of the material have been determined before the 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 the production. The relationship between the characteristic impedance value and the change of dielectric thickness can be obtained from the figure. It can be seen from the figure that when the medium thickness changes 0 . 025mm, the impedance value will change accordingly + five – In the actual production process, the allowable thickness change of each layer will lead to a great change in the impedance value. In actual production, different types of prepreg are selected as the insulating medium, and the thickness of the insulating medium is determined according to the number of prepreg. Take the surface microstrip line as an example: in the production process, you can refer to the figure. Determine the dielectric constant of the insulating material under the corresponding working frequency, and then use the formula to calculate the corresponding impedance value. According to the user’s wire width value and calculated impedance value, find out the corresponding dielectric thickness through the diagram, and then determine the type and number of semi cured sheet according to the thickness of copper clad laminate and copper foil.

It can be seen from the figure that the design of microstrip line structure has higher characteristic impedance value than that of stripline structure under the same dielectric thickness and material, which is generally 20 times larger Ω- forty Ω。 Therefore, microstrip structure is mostly used for high-frequency and high-speed digital signal transmission. At the same time, the characteristic impedance will increase with the increase of dielectric thickness. Therefore, for high-frequency lines with strictly controlled characteristic impedance value, strict requirements should be put forward for the error of dielectric thickness of CCL. Generally speaking, the variation of dielectric thickness should not exceed 10% % For multi-layer plate, the thickness of medium is also a processing factor, especially closely related to multi-layer lamination processing, so it should also be strictly controlled.

five . Conclusion

In the 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 change of the characteristic impedance value. In addition, the characteristic impedance value will be related to other production factors. Therefore, in order to control the characteristic impedance, the manufacturer must understand the factors that affect the change of the characteristic impedance value and master the actual production conditions, According to the requirements of the designer, the process parameters are adjusted within the allowable tolerance to get the desired impedance value.