Cross-division design of PCB

During circuit design, there are more and more power supplies and multiple locations on a PCB, such as 48 V, 12 V-
Common types of power supplies such as 12 V, 5 V,-5 V, 3.3 V, 2.5 V, 1.8v1.v, agnd (analog location) dgnd (Digital location), pgnd (Protected Area) the ground plane required for different functions, some IC clearly requires the single point grounding, and the required power supply, ground plane hollowing. In order to ensure high reliability of these locations and power supplies, assigning each power supply and ground layer, that is, a plane, will inevitably lead to an increase in the Board stack, and the cost of making the Board will increase significantly. As mentioned above, the production cost of the circuit board is proportional to the number of layers. In order to save costs and ensure the reliability of the circuit board, engineers will design two or more PCB power supplies on the same plane according to the characteristics of the circuit board during PCB design, as a result, the power supply and ground plane are incomplete, that is, the ground (power supply) layer is separated.

I. Cross-division design of PCB

When the PCB goes through the ground (power supply) layer, the signal integrity is greatly affected, and the EMI and EMC characteristics of the circuit are also changed. This is the cross-division problem.These problems are often overlooked by electronic engineers. There are two main sources for cross-segmentation:

1 division of power supply/ground plane

As shown in Figure 4-12, the same layer (strata) is separated by a simulated ground and a numerical ground. When the PCB goes through this split band, the cross-split problem occurs.

2 split caused by improper pin definitions of intensive over-hole or intensive plug-in (crimping) Devices

If the distribution of dense, over-hole or intensive plug-in (crimping) devices is unreasonable when the PIN is defined, and the distance between the pins is very small, it may cause a narrow isolation band in the formation or definition layer. If there is a line through this split, it will lead to cross-split.

Let's take a look at the cabling shown in 4-13.

On the surface, these cables are impeccable: neat and beautiful. They are also the preferred wiring method for electronic or circuit engineers. Let's take a look at the power supply/ground plane in the corresponding area of these passing holes, and we will find that the power supply and formation segmentation problems are generated.

In Figure 4-13, a long split band is generated on the power supply/ground plane because the distance between the passing holes is too close, as shown in Figure 4-14. If a strip passes through the split band, the problem of Cross-segmentation also arises. This type of problem mainly occurs when the bus is wiring in the circuit and must be noticed by engineers.

Similarly, when the through hole (including the pad and pass through the hole) passes through the ground/power plane, if the distance between the through holes is too close, the above problem also occurs. These problems mainly occur in intensive plug-ins (crimping)
The device pin definition is not easy to appear at the time. Therefore, when defining the pin signals of intensive plug-in (crimping) devices, we must consider not only the number of signal types, but also the signal bus arrangement to avoid
Face-to-Face splitting brings about cross-segmentation issues.

Ii. hazards of cross-split cabling of PCB boards

Main dangers of cross-segmentation include:

(1) lead to inconsistent impedance of cabling;

(2) It is easy to cause crosstalk between signals;

(3) possible signal reflection;

(4) increase the current loop area, increase the loop inductance, and make the output waveform oscillate easily;

(5) Increase the radiation interference to the space and be susceptible to the influence of the space magnetic field;

(6) increase the possibility of magnetic field coupling with other circuits on the Board;

(7) high-frequency voltage drops on the loop inductor constitute a common-mode radiation source, and common-mode radiation is produced through an external cable.

In order to visually describe the harm of the Cross-split strip to the circuit, we use Figure 4-15 to show a slot that causes high-frequency signals to produce crosstalk.

For high-speed signal lines that require strict impedance control and line-by-line model, the above-plane, lower-plane, or upper-and lower-plane slotted damage to the line-by-line model, resulting in discontinuous impedance, this causes serious signal distortion, reducing the signal reliability.

In order to visually describe the influence of cross-split cabling on electromagnetic interference, the author makes an experimental comparison. From the comparison results, we can easily see the effect of cross-segmentation.

Two cables are laid on a PCB. The two cables share the same excitation source and load. They all go on the same wiring layer and have a length of mil. The only difference is that one is separated across planes, another reference plane is complete. By switching, one of the networks is powered on when the external conditions are identical. The test results in the half-wave darkroom are as follows:

From Figure 4-16 and Figure 4-17, we can clearly see that the signal cross-division area will increase the radiation by 8db-10db. The specific increase in the radiation intensity depends on the large loop area of the reflux path caused by the cross-division.
Small, also related to the surrounding electromagnetic interference environment. If a clock line is arranged across cables near the external interface, the electromagnetic interference intensity may exceed 20 dB. It can be seen that the cross-split wiring will cause great electromagnetic interference, not only interfere with itself, but also its electromagnetic radiation will interfere with other circuits or systems around it. Therefore, we must pay attention when wiring and try to avoid cross-line splitting.

Iii. How to avoid cross-division of PCB

Cross-split cabling brings great harm to the circuit. In actual circuit, we should avoid cross-split as much as possible. Pay attention to the following points:

(1) do not strip through the split band. when the line is laid, consider the division of the ground and power supply to enable the Division of the power supply, you can also increase the number of layers of the circuit to avoid the problem of cross-division.

(2) When wiring, the power supply is designed after the wiring signal line. When dividing the power supply or ground, pay attention to the ground and power supply, which signals will be affected, which signal lines will be split across lines, which can be avoided, pay attention to proper adjustment and division of power supply.

(3) the pass-through design and the split hole should not be too dense, resulting in the isolation band of the ground and power supply plane.

(4) When the plug-in device or plug-in is defined, the distribution should be reasonable, and whether the impact on the ground and power supply plane should be fully considered, resulting in EMI enhancement.