The impact of pcb board manufacturing on impedance control and solutions

2019-11-11 21:55:24

The circuit performance provided by the printed circuit board must be such that no reflection occurs during signal transmission, and the signal remains intact to reduce the transmission loss to match the impedance, so that a complete, reliable, accurate interference-free, noisy transmission signal can be obtained. In this paper, the problem of characteristic impedance control of surface microstrip line structure multilayer boards commonly used in practice is discussed.

Surface microstrip line and characteristic impedance

The surface microstrip line has a high characteristic impedance value and is widely used in practice. Its outer layer is the signal line surface for controlling the impedance, and it is separated from the adjacent reference plane by an insulating material, and the characteristic impedance is calculated. The formula is:

1, microstrip line (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 board material. This formula must be applied in the case where 0.1 < (W / H) < 2.0 and 1 < (Er) < 15.

2, stripline (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 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 thickness T of the wire, so that the characteristic impedance and the substrate material are known ( The relationship between copper clad laminates is very close, so the choice of substrate materials is very important in pcb design.

Dielectric constant of material and its influence

The dielectric constant of the material is determined by the manufacturer of the material at a frequency of 1 Mhz, and the same material produced by different manufacturers differs depending on the resin content. In this study, epoxy glass cloth is taken as an example to study the relationship between dielectric constant and frequency change. The dielectric constant decreases with increasing frequency. Therefore, 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 as the dielectric constant increases. Therefore, in order to obtain a high signal transmission speed, the dielectric constant of the material must be lowered, and at the same time, a high transmission speed is required. High characteristic resistance is used, and high characteristic resistance must use low dielectric constant material.

Wire width and thickness

The wire width is one of the main parameters affecting the characteristic impedance change. The figure shows the relationship between the impedance value and the wire width by taking the surface microstrip line as an example. When the wire width is changed by 0.025mm, the impedance value will be changed by 5-6 ohms. In actual production, if the signal line surface of the control impedance is 18μm copper foil, the allowable wire width variation tolerance is ±0.015mm. The tolerance of the control impedance is 35μm copper foil, and the allowable wire width variation tolerance is 0.025mm. It can be seen that the variation of the wire width allowed in the production will cause a great change in the impedance value. The width of the wire is determined by the designer. Determined by a variety of design requirements, it must meet the requirements of wire current carrying capacity and temperature rise, and obtain the desired impedance value.

This requires the producer to ensure that the line width meets the design requirements during production and that it is within tolerances to accommodate the impedance requirements. The wire thickness is also determined by 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 25 μm 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 the surface of the wire should be cleaned before the plating, the residue should not be stuck and the black oil should be repaired, and the copper should not be plated during the plating, so that the thickness variation of the local wire affects the characteristic impedance value. In addition, be careful during the brushing process, do not change the thickness of the wire, resulting in changes in the impedance value.

Effect of medium thickness H

It can be seen from the formula that the characteristic impedance is proportional to the natural logarithm of the thickness of the medium. Therefore, the thicker the dielectric thickness, the larger the impedance value, so the dielectric thickness is another major factor affecting the characteristic resistance. Since the wire width and the dielectric constant of the material have been determined prior to production, the wire thickness process requirements can also be used as a fixed value so controlling the laminate thickness (media thickness) is the primary means of controlling the characteristic impedance in production.

The relationship between the characteristic impedance value and the change in dielectric thickness. It can be seen from the figure that when the thickness of the medium changes by 0.025 mm, the corresponding change in the impedance value will be +5-8 ohms. In the actual production process, the allowable thickness variation of each layer will result in a very high impedance value. 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 figure during the production process. Determine the dielectric constant of the insulating material at the corresponding operating frequency, and then calculate the corresponding impedance value by using the formula, and then determine the corresponding dielectric thickness according to the wire width value and the calculated impedance value proposed by the user, and then according to the selected The thickness of the copper clad laminate and the copper foil determines the type and number of sheets of the prepreg.

The design of the microstrip line structure has a higher characteristic impedance value under the same dielectric thickness and material than the strip line design, and is generally 20 Ω to 40 Ω. Therefore, most of the high-frequency and high-speed digital signal transmission adopts the microstrip line structure design. At the same time, the characteristic impedance value will increase as the thickness of the medium increases. For high-frequency lines with strict control of characteristic impedance values, the error of the dielectric thickness of the copper-clad board should be strictly required. Generally speaking, the thickness of the medium does not change by more than 10%. For the multi-layer board, the medium thickness is still a processing factor. In particular, it is closely related to multilayer lamination processing and should therefore be closely controlled.

to sum up

In the actual production, the width and thickness of the wire, the dielectric constant of the insulating material and the thickness of the insulating medium will cause changes in the characteristic impedance value. In addition, the characteristic impedance value will be related to other production factors, in order to achieve the control of the characteristic impedance. The producer must understand the factors affecting the variation 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 variation within the allowable tolerance range to obtain the desired impedance value.

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