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PCB Tech

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PCB Tech

PCB Tech

How to deal with PCB signal crossing the dividing line?
2020-09-11
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Author:Dag

In the PCB design process, the division of the power plane or the ground plane will lead to the incomplete plane. When the signal is routed, its reference plane will cross from one power supply plane to another. This phenomenon is called signal cross segmentation.

 PCB signal crossing the dividing line.jpg

Schematic diagram of cross segmentation phenomenon

In high-speed digital signal system, high-speed signal takes the reference plane as the return path, which is the return path. When the reference plane is incomplete, the following adverse effects will occur:

It will cause the impedance discontinuity of the wiring;

It is easy to cause crosstalk between signals;

It will cause reflection between signals;

Increasing the loop area and inductance of current makes the output waveform easy to oscillate;

At the same time, it is easy to be affected by the space magnetic field;

Increase the possibility of magnetic field coupling with other circuits on the board;

The high frequency voltage drop on the loop inductance forms the common mode radiation source, and generates the common mode radiation through the external cable.

Therefore, PCB wiring should be as close to a plane as possible and avoid cross segmentation. These conditions are only allowed in low-speed signal lines if they must cross the division or cannot be close to the ground plane of the power supply.

Treatment of span segmentation in design

How to deal with the inevitable cross segmentation in PCB design? In this case, it is necessary to sew the segmentation to provide a short return path for the signal. The common processing methods are adding the sewing capacitor and bridging across the line.

, stitching capacitor

Usually, a 0402 or 0603 packaged ceramic capacitor is placed at the signal cross section. The capacitance value of the capacitor is 0.01uF or 0.1uF. If space permits, several more such capacitors can be added.

At the same time, try to ensure that the signal line is within the range of 200mil of the sewing capacitor, and the smaller the distance, the better; while the network at both ends of the capacitor corresponds to the network of the reference plane through which the signal passes. See the network connected at both ends of the capacitor in the figure below, two different networks with bright colors:

Cross line bridging

It is common to "packet ground" the signals across the segmentation in the signal layer, or the signal lines of other networks may be included. This "packet ground" line should be as thick as possible. For this processing method, refer to the following figure.

High speed signal wiring skills

Multi layer wiring

The high-speed signal wiring circuit is usually highly integrated and has high wiring density. The use of multi-layer board is not only necessary for wiring, but also an effective means to reduce interference.

Reasonable selection of the number of layers can greatly reduce the size of the printed board, make full use of the intermediate layer to set the shielding, better realize the nearby grounding, effectively reduce the parasitic inductance, effectively shorten the transmission length of the signal, and greatly reduce the cross interference between signals.

The less bending of lead wire, the better

The less the lead bend between the pins of high speed circuit, the better.

The lead wire of high-speed signal wiring circuit adopts full straight line, which needs turning, and can be turned by 45 ° broken line or arc. This requirement is only used to improve the fixation strength of steel foil in low-frequency circuit.

In the high-speed circuit, meeting this requirement can reduce the external emission and coupling of high-speed signals, and reduce the radiation and reflection of signals.

The shorter the lead, the better

The shorter the lead between the device pins of high-speed signal wiring circuit, the better.

The longer the lead, the greater the distributed inductance and capacitance, which will have a lot of influence on the passing of high frequency signal of the system, and also change the characteristic impedance of the circuit, resulting in reflection and oscillation of the system.

The less interlaminar alternation, the better

The less alternation of lead layers between pins of high-speed circuit, the better.

The so-called "the less interlaminar alternation of leads, the better", means that the fewer vias used in the process of component connection, the better.

It is measured that a via can bring about 0.5pf distributed capacitance, which leads to a significant increase in circuit delay. Reducing the number of vias can significantly improve the speed.

Pay attention to parallel cross interference

In high-speed signal wiring, attention should be paid to the "cross interference" caused by the close parallel routing of signal lines. If parallel distribution cannot be avoided, a large area of "ground" can be arranged on the opposite side of the parallel signal line to greatly reduce the interference.

Avoid branches and stumps

High speed signal wiring should avoid branching or stubs.

Stump has a great influence on impedance, which can lead to signal reflection and overshoot, so we should avoid stump and branch in design.

Using chrysanthemum chain wiring will reduce the impact on the signal.

The signal line should be in the inner layer as far as possible

High frequency signal line is easy to produce large electromagnetic radiation when it goes on the surface, and it is also easy to be interfered by external electromagnetic radiation or factors.

If the high frequency signal line is laid between the power supply and the ground wire, the radiation generated will be greatly reduced through the absorption of electromagnetic wave by the power supply and the bottom layer.