How does PCB design handle signals across power ?

In the PCB design process, the division of the power plane or the division of the ground plane will lead to the incompleteness of the plane, so that when the signal is routed, its reference plane will appear from one power plane to another power plane, which is a phenomenon we call signal cross-division.

Cross-segmentation may not be relevant for low-speed signals, but in high-speed digital signal systems, high-speed signals take the reference plane as the return path, that is, the return path. When the reference plane is incomplete, the following adverse effects will occur:

1.Will cause the impedance discontinuity of the cable;

2. Easy to cause crosstalk between signals; 

3. It will cause reflection between signals; 

4. Increase the loop area of the current, increase the loop inductance, so that the output waveform is easy to oscillate; 

5.Increase the radiation interference to space, while susceptible to the influence of space magnetic field; 

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

7. The high frequency voltage drop on the loop inductor constitutes a common mode radiation source and generates common mode radiation through the external cable.

Therefore, PCB wiring should be as close to a plane as possible and avoid cross-segmentation. If it is necessary to cross the division or cannot be near the ground plane of the power supply, these conditions are only allowed in the low-speed signal line.

If cross-segmentation is inevitable in PCB design, how to deal with it? In this case, it is necessary to patch the segmentation to provide a short return path for the signal, and the common processing methods include adding sewing capacitors and crossing Bridges.

1. Stiching Capacitor

Usually, a 0402 or 0603 ceramic capacitor is placed at the signal cross-segment, and the capacitance value of the capacitor is 0.01uF or 0.1uF, if space allows, several more such capacitors can be added.

At the same time, try to ensure that the signal line is within the range of 200mil sewing capacitance, and the smaller the distance, the better; The network at both ends of the capacitor corresponds to the network of the reference plane through which the signal passes.

2.Cross-cable bridge

It is common to "ground process" the signal across the division at the signal layer, and it may also be the signal line of other networks, and this "ground" line is as thick as possible.

3.multi-layer wiring

High speed signal wiring circuit often has high integration and high wiring density, the use of multilayer board is not only necessary for wiring, but also an effective means to reduce interference.

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

4. The less the lead bends, the better

The less lead bending between pins of high-speed circuit devices, the better.

The wiring lead of the high-speed signal wiring circuit uses a straight line, which needs to be turned, and can be used as a 45° fold line or an arc turn, which is only used to improve the fixed strength of the steel foil in the low-frequency circuit. In high-speed circuits, meeting this requirement can reduce the transmission of high-speed signals and the coupling between each other, and reduce the radiation and reflection of signals.

5. The shorter the lead, the better

The shorter the lead between the pins of the high-speed signal routing circuit device, the better. The longer the lead, the larger the value of the distributed inductance and distributed capacitance, which has a lot of influence on the passing of the high-frequency signal of the system, and also changes the characteristic impedance of the circuit, resulting in reflection and oscillation of the system.

6. The less alternations between lead layers, the better

The less interlayer alternations between pins of high-speed circuit devices, the better. The so-called "the less interlayer alternations of the leads, the better" means that the fewer holes used in the connection process of the components, the better.

It has been measured that a pass hole can bring about 0.5pf of distributed capacitance, resulting in a significant increase in the delay of the circuit, and reducing the number of pass holes can significantly improve the speed.

7. pay attention to parallel cross interference

High-speed signal wiring should pay attention to the "cross interference" introduced by the signal line in the short distance parallel line, if the 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.

8. Avoid branches and stumps

High-speed signal wiring should try to avoid branching or forming a Stub. Stumps have a great impact on impedance and can cause signal reflection and overshoot, so we should usually avoid stumps and branches in the design.Using Daisy chain wiring, the impact on the signal will be reduced. 

9. the signal line as far as possible to the inner layer

High frequency signal lines walking on the surface are easy to produce large electromagnetic radiation, and are also easy to be interfered by external electromagnetic radiation or factors. The high-frequency signal line is routed between the power supply and the ground wire, and the radiation generated will be reduced a lot through the absorption of electromagnetic waves by the power supply and the bottom.

kingford Electronics is a professional PCB design company engaged in electronic product circuit board design (layout layout design), mainly undertake multi-layer, high-density PCB design and circuit board design proofing business. With an average of more than 10 years of work experience in PCB design team, can skillfully use the market mainstream PCB design software, professional and efficient communication to ensure PCB design progress, to help you seize the market opportunity one step earlier!