EMC study notes (12) processing across partitions and slots

1. Generation of slotting

Slots are a common structure in PCB design. The "cross-partition problem" we often say is actually a slotting problem. The generation of slotting can be summarized into the following two situations.

1.1 Slots caused by power/ground plane splitting

When there are many different power sources or grounds on the PCB, it is generally impossible to allocate a complete plane for each power network and ground network. The common practice is to split power or ground on one or more planes . Slots are formed between different divisions on the same plane.

1.2 Through holes are too dense to form slots

Vias include pads and vias. When vias pass through ground or power planes without electrical connection to them, some space (i.e. spacer) around the vias is required for electrical isolation: but when the vias are too close together, the spacer will overlap to form a groove. This is also known as the hot pad problem. The figure below is an example of a densely formed slot through holes.

Figure 1 shows a schematic diagram of a densely formed slot through holes. At present, our board widely uses 2mm connectors (or other high-density connectors). When the connector passes through the power supply or ground plane, in order to complete effective electrical isolation or safety isolation, the pins of the connector and the power supply or ground plane must be There is an isolation ring for isolation: when the radius of the isolation ring is greater than 1mm (half the spacing between pins), a slot will be formed.

2. The effect of slotting on the EMC performance of PCB board

Slots will have a certain impact on the EMC performance of the PCB board. This effect can be negative or positive.

2.1 Surface current distribution of high-speed signal and low-speed signal

At low speeds, current flows along the path of least resistance. Figure 2 shows the situation where the return signal returns from the ground plane to the source when the low-speed current flows from A to B. At this time, the surface current distribution is wide.

At high speeds, the effect of inductance on the signal return path will outweigh the effect of resistance. High-speed return signals will flow along the path of least impedance. Figure 3 shows a typical return path for high-speed signals.

At this time, the distribution of the surface current is very narrow, and the return signal is concentrated below the signal line in a bundle.

2.2 The concept of subdivision

When there are incompatible circuits on the PCB, it is necessary to perform "ground division" processing, that is, to set the ground wires separately according to different power supply voltages, digital and analog signals, high-speed and low-speed signals, and high-current and low-current signals.
The role of ground division can be easily understood from the distribution of high-speed signal and low-speed signal return given above: ground division can prevent the superposition of return signals of incompatible circuits and prevent impedance coupling of common ground lines.
There are two points to note:

First, the concept of ground division is different from the "signal crossing the slot on the power plane or ground plane" that will be discussed below. The ground division is only to set the ground wires (or planes) according to different types of signals;

Second, the ground division does not completely isolate the various grounds without any electrical connection. After the ground division, the various grounds will be connected at appropriate positions to ensure the electrical continuity of the entire formation.

2.3 The problem of the signal crossing the slot on the power plane or the ground plane

Regardless of high-speed signals or low-speed signals, they should not cross the split routing. Routing across splits can present a number of serious problems, including:

Increase the area of ​​the current loop, increase the loop inductance, and make the output waveform easy to oscillate:

Increase the radiation interference to space, and at the same time be susceptible to the influence of space magnetic field;

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

The high-frequency voltage drop across the loop inductance constitutes a source of common-mode radiation and generates common-mode radiation through the external cables.

Figure 4 below shows a schematic diagram of crosstalk caused by high-frequency signals caused by a ground trough.

For high-speed signal lines that require strict impedance control and are routed according to the stripline model, the stripline model will be damaged due to the slotting of the upper plane or the lower plane or the upper and lower planes, resulting in discontinuity of impedance and serious damage. Signal integrity issues.

3. Treatment of slotting

The processing of slotting should follow the following principles.

3.1 For high-speed signal lines that require strict impedance control, their tracks are strictly prohibited from crossing split lines

Cross-split traces create impedance discontinuities that can cause serious signal integrity issues.

3.2 When there are incompatible circuits on the PCB board, the ground should be divided

The ground division should not cause cross-split routing of high-speed signal lines, and try not to cause cross-split routing of low-speed signal lines.

3.3 When routing across slots is unavoidable, bridges should be used

When the signal line cannot avoid the situation of crossing the slot, it should be effectively bridged to connect the ground plane in the direction along the signal path. Figure 5 below shows a schematic diagram of the bridge.

3.4 Connectors (external) should not be placed on formation gaps

In Figure 6(a), if there is a large potential difference between point A and point B on the formation, it is possible to generate common-mode radiation through the external cable. Therefore, it should be changed to the installation method in Figure 6(b), so that there will be no potential difference between points A and B.

3.5 Handling of High Density Connectors

When high-density connectors (such as the widely used 2mm connectors) pass through the power and ground planes, if the radius of the isolation ring is too large, as shown in Figure 1, slots will be formed. When designing a PCB, unless there are special requirements (such as strict safety distance requirements for individual signals), it should generally be ensured that the ground network surrounds each pin, and the ground network can also be evenly arranged when the pins are arranged. Ensure the continuity of the ground plane and prevent slotting. The schematic diagram is shown in Figure 7.

3.6 Handling of Cross-Stationary Segmentation

For the IO signals that go out of the subrack or cabinet through cables, when shielding and filtering, it is required to have a "clean ground that is not polluted by internal noise. Without this "quiet ground", the filtering of high-frequency signals It has almost no effect. The "quiet ground" can be a metal frame or a protective ground. The "quiet ground" is not connected to the logic ground inside the board. Figure 8 shows a simple schematic diagram of the "quiet ground".

It can be seen from Figure 8 that after the "quiet ground" is set on the PCB, the I/O signal line will inevitably be divided across the "quiet ground" and requires certain processing.
For differential signal lines, crossing "quiet ground" traces can effectively suppress common-mode noise without any processing:

For ordinary signal lines, the return path of the signal must be provided. When wiring the PCB, the GND network led by the interface device should be treated as an ordinary signal line. Figure 9 shows a simple example of wiring for an RS-232 serial port.