PCB Blog - How to deal with PCB board signal crossing the dividing line

In the process of PCB board design, the division of the power plane or the division of the ground plane will lead to the incomplete plane, so that when the signal is routed, its reference plane will appear from one power plane to another power plane. , this phenomenon is called signal cross-segmentation. Cross division may not matter for low-speed signals, but in high-speed digital signal systems, high-speed signals use 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 of the trace to be discontinuous; it will easily cause crosstalk between signals; it will cause reflection between signals; increase the current loop area, increase The loop inductance makes the output waveform easy to oscillate; increases the radiation interference to the space, and is easily affected by the space magnetic field; increases the possibility of magnetic field coupling with other circuits on the board; the high-frequency voltage drop on the loop inductance It forms a common mode radiation source and generates common mode radiation through an external cable. Therefore, PCB board routing should be as close as possible to a plane and avoid cross-splitting. These conditions are only allowed on low-speed signal lines if they must be split across or cannot be close to the power ground plane.

1. Handling of cross-segmentation in design

If cross-segmentation inevitably occurs in PCB board design, how to deal with it? In this case, the segmentation needs to be patched to provide a short return path for the signal. Common processing methods include adding patch capacitors and cross-line bridging.

2. Stiching Capacitor

Usually, a 0402 or 0603 packaged ceramic capacitor is placed at the signal cross section. The capacitance of the capacitor is 0.01uF or 0.1 uF. If space allows, you can add several more such capacitors. At the same time, try to ensure that the signal line is within the 200mil range of the sewing capacitor, and the smaller the distance, the better; and 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. The two colors are highlighted. two different networks:

3. Over-the-Line Bridging

It is common to "packet the ground" for the cross-segmented signal at the signal layer, and it may also include the signal lines of other networks. This "packet" line should be as thick as possible.

4. Multilayer wiring

High-speed signal wiring circuits often have high integration and high wiring density. The use of multi-layer boards 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, can make full use of the middle layer to set up shielding, can better achieve the nearest grounding, can effectively reduce the parasitic inductance, can effectively shorten the transmission length of the signal, and can greatly reduce the signal cross-interference, etc.

5. The less lead bends, the better

Lead bends between the pins of high-speed circuit devices should be as little as possible. The leads of the high-speed signal wiring circuit wiring are all straight lines and need to be turned. They can be turned with 45° broken lines or arcs. This requirement is only used to improve the fixing strength of the steel foil in low-frequency circuits. In high-speed circuits, meeting this requirement can reduce the external emission and mutual coupling of high-speed signals, and reduce the radiation and reflection of signals.

6. The shorter the leads, the better

The wiring between the pins of the high-speed signal wiring circuit device should be as short as possible. The longer the lead, the greater the distributed inductance and distributed capacitance, which will have a lot of influence on the passage of high-frequency signals in the system, and will also change the characteristic impedance of the circuit, causing the system to reflect and oscillate.

7. The less alternating between lead layers, the better

The less alternating between the lead layers between the pins of the high-speed circuit device, the better. The so-called "the less the interlayer alternation of the leads, the better" means that the fewer vias used in the component connection process, the better. According to the measurement, one via can bring about 0.5pf of distributed capacitance, which leads to a significant increase in the delay of the circuit. Reducing the number of vias can significantly improve the speed.

8. Beware of Parallel Cross Interference

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

9. Avoid branches and stumps

High-speed signal routing should try to avoid branches or stubs. Stumps have a large impact on impedance and can cause reflections and overshoots of the signal, so we should generally avoid stumps and branches in our design. Use daisy-chain wiring to reduce the impact on the signal.

10. The signal line should go as far as possible on the inner layer

High-frequency signal lines are prone to generate large electromagnetic radiation on the surface layer, and are also susceptible to interference from external electromagnetic radiation or factors. The high-frequency signal line is routed between the power supply and the ground line, and the radiation generated will be greatly reduced by the absorption of electromagnetic waves by the power supply and the bottom layer on PCB board.