Other factors that affect the characteristic impedance of the printed circuit board (below)

4 wire width control significance

4.1 The key to wire width control

The key to the control of the wire width is how to achieve the Z0 value or the control value Z0 of the OEM design within the range of change through the management and control of the whole process of PCB production and processing. Due to the proper choice of substrate material and the completion of the PCB design, the three parameters of dielectric constant, dielectric width, and wire width are basically fixed. Although wire width and dielectric thickness may be affected by PCB production and processing, mechanical polishing and micro-etching in the humidification process may make the copper foil layer thinner to facilitate the improvement of Z0, and the manufacture of buried blind via interconnects and outer layers The hole patterning and electroplating of the pattern can make the thickening of the copper foil layer unfavorable to Z0, so care should be taken to control it. However, the width of the wire is entirely produced by PCB production. At the same time, the manufacture of fine wires for the transmission of high-frequency signals and high-speed digital signals is still a key technology for today's high-density interconnect PCBs. The essence of fine wire manufacturing is fundamentally the control and management of fine wires. Therefore, as a signal transmission line application PCB manufacturing, the manufacturing of the wire width should be treated as a key issue.

4.2 Meaning of wire width control

High-frequency signals and high-speed digital (logic) signals are transmitted from the driving element and sent to the receiving element via the PCB signal transmission line. This is a signal transmission process. In this signal transmission process, if the PCB's signal transmission process, the PCB's signal transmission line's characteristic impedance value Z and the two components of the "electronic impedance" exactly match (in fact, the impedance of the receiving component is greater than the impedance of the drive component When it is only reasonable, the energy of the transmitted signal is completely transmitted. This situation is ideal. If the transmission line Z0 of the PCB does not match and there is a variation deviation or the variation deviation is too large, problems such as reflection, loss, attenuation, or time delay will occur during signal transmission. In severe cases, it may even cause complete "distortion" and fail to receive the original true signal.

Therefore, high-frequency signals and high-speed digital signals must be completely transmitted in the PCB transmission line, and it must be done that the characteristic impedance value Z0 at any point on the PCB transmission line should be equal, which means that in the PCB transmission line, The cross-sectional area (including no defect and ideal) in any one place must be the same. However, it is impossible to completely complete the actual production and processing of PCB transmission lines. Therefore, the control of the transmission line in the PCB, after the substrate material is determined, is essentially a control problem of the consistency of the cross-sectional area of ​​the transmission line in the double-sided board; in the multilayer board, the consistency of the cross-sectional area of ​​the transmission line is essentially The control of the uniformity of the medium thickness is mainly due to the consistency and integrity of the cross-sectional area of ​​the transmission line. Due to too many processing procedures and process parameters (especially dynamic process parameters) involved in the processing of the PCB transmission line, even full automation of production and processing is difficult to achieve. Therefore, one can only control the entire cross-sectional area of ​​the PCB transmission line for production and processing within the specified range. Therefore, the Z0 of the PCB transmission line can only be controlled within the design specification value according to the application object.

Traditionally, PCB wire width deviations have been allowed to be ±20%, which is already satisfactory for PCB wires (wire lengths less than one-seventh of the signal wavelength) for conventional electronic products that are not transmission lines. But for the signal transmission line with Z0 control requirements, PCB wire width deviation ± 20% can not meet the requirements, therefore, the error at this time has generally exceeded ± 10%, and Z0 error will also be thin with the media thickness and bias Big.

From the above theoretical calculations, such understanding and conclusion can be drawn. The traditional line width error precision control regulations are not applicable to the requirements of the transmission line. The error accuracy of the transmission line width must be determined according to the characteristics of the transmission line transmission signal. If the transmission line of the high frequency signal is transmitted, its precision control must be much stricter to achieve a smaller Z0 deviation value. These requirements can be calculated based on the formula (1) and the known medium thickness, wire thickness and Z0 bias value to calculate the wire's accuracy (error) control size.

From automation and instrumentation

Tian Li Cao Anzhao
(Anhui University of Engineering Science and Technology Department of Electrical Engineering