Impedance Controlled PCB And Stack Design Issues

- Sep 11, 2017-

As PCB signal switching continues to grow, today's PCB designers need to understand and control the impedance of PCB traces. Corresponding to the modern digital circuit shorter signal transmission time and higher clock rate, the PCB trace is no longer a simple connection, but the transmission line.

In practical cases, it is necessary to control the trace impedance when the digital marginal velocity is higher than 1 ns or the analog frequency exceeds 300 MHz. One of the key parameters of the PCB trace is its characteristic impedance (ie, the ratio of voltage to current when the wave is transmitted along the signal transmission line). The characteristic impedance of the printed circuit board is an important indicator of the circuit board design. Especially in the PCB design of the high frequency circuit, it is necessary to consider whether the characteristic impedance of the wire and the characteristic impedance required by the device or signal are consistent and matched. This involves two concepts: impedance control and impedance matching, this paper focuses on impedance control and stack design issues.

Impedance control

Impedance control (eImpedance Controling), the circuit board will have a variety of signal transmission, in order to improve its transmission rate must improve its frequency, the line itself due to etching, stack thickness, wire width and other factors, will Causing the impedance to change and make the signal distorted. Therefore, in the high-speed circuit board conductor, the impedance value should be controlled within a certain range, known as the "impedance control."

The impedance of the PCB trace will be determined by its inductance and capacitive inductance, resistance and conductivity. The factors that affect the impedance of the PCB trace are: the width of the copper wire, the thickness of the copper wire, the dielectric constant of the medium, the thickness of the medium, the thickness of the pad, the path of the ground, and the alignment of the traces. The PCB impedance range is 25 to 120 ohms.

In practice, PCB transmission lines are usually composed of a conductive trace, one or more reference layers and insulating material. Traces and plates constitute the control impedance. PCBs will often use multilayer structures, and control impedance can also be built in a variety of ways. However, regardless of the method used, the impedance values will be determined by the physical properties of the physical structure and the insulating material: the width and thickness of the signal trace, the height of the core or prefilled material on both sides of the trace, the configuration of the trace and the lamina , The core and pre-filled material insulation constant, PCB transmission line is mainly in two forms: microstrip line and stripline.

1, when the differential traces in the middle signal layer alignment, the differential impedance control is more difficult, because the accuracy is not enough, that is to change the thickness of the dielectric layer on the differential impedance is not significant, only to change the spacing of the line on the differential impedance Larger. But when the alignment in the top or bottom, the differential impedance is better control, it is easy to meet the design requirements, through the actual calculation found that the important signal line is best to go to the surface, easy to impedance control, especially the clock signal differential pair.

2, in the PCB design, the first must be calculated by the impedance, the PCB stack parameters to determine, such as the thickness of each layer of copper, the thickness of the dielectric layer, etc., as well as differential alignment width and spacing need to be calculated in advance , These are the front-end simulation of the PCB to ensure that the important signal line impedance control to meet the design requirements.

3, in the actual impedance control, the general use of the media for the FR-4, the Er of about 4.2, the line thickness t on the impedance of the smaller, the actual adjustment can be the main H and W, W (design line width) Under normal circumstances Is determined by the designer, but in the design should take full account of the line width of the impedance of the coordination and the actual processing accuracy. Of course, the use of smaller W value after the effect of line thickness t can not be ignored. H (dielectric layer thickness) has the greatest impact on the impedance control. There are two cases of actual H: one is the core plate, and the thickness of H in the sheet provided by the material supplier is also made up of the above three kinds of prepregs. The combination of the process will inevitably consider the characteristics of three materials, and not any unconditional combination, so the thickness of the plate will have a certain provision, the formation of a corresponding list, and H also has a certain limit. Such as 0.17mm 1/1 of the core board is 2116 × 1,0.4mm 1/1 of the core board is 1080 × 2 +67628 × 1 and so on. The other is the thickness of the laminated portion of the multilayer board: the method is essentially the same as before but with the attention of the copper layer. If the inner layer is filled with a prepreg, the thickness loss of the prepreg is negligible when the number of the copper foil to be etched is small in the process of making the inner layer, and the filling of the resin in the prepreg is small. On the other hand, if the prepreg is filled with the prepreg, the thickness loss of the prepreg is large and difficult to estimate because the portions of the copper foil are etched away. Therefore, it was suggested that the signal layer at the inner layer require copper plating to reduce the thickness loss. (The above information comes from: P C B high-speed digital design of the impedance control (Southwest Electronic Telecommunications Technology Research Institute Chen Fei))

4, the characteristic impedance and the width of the transmission line is inversely proportional to the wider width, the lower the impedance, otherwise the impedance is higher.

5, in some board design requirements of the board thickness is limited, this time to achieve a better impedance control, the use of good stack design is critical. From the actual calculation can be drawn the following conclusions:

a. Each signal layer must have a reference plane adjacent to ensure its impedance and signal quality;

b. Each power plane must have a complete ground plane adjacent, making the power of the performance can be a better guarantee.

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