Introduction:
With the development of the society and the progress of science and technology, electronic products are on the increase, the kinds of content is becoming more and more complex, the structure of electronic products and equipment has also been gradually optimized, PCB design also gradually show the characteristics of multi-level and high density, PCB design in all aspects of the interference problem also gradually attention and arouse people's gaze. The EMC (electromagnetic compatibility) design currently used can play an important role in promoting the normal operation and stable work of printed circuit board circuits, so that these circuits will not interfere with each other, and the external radiation and conduction emission of PCB can be effectively.
Types of Interference in PCB Design:
PCB interference mainly has three aspects, the first is the interference problem of the layout, this kind of problem is mainly due to the PCB components in the unreasonable placement of the PCB. Secondly, the interference problem of PCB layers mainly refers to the noise interference caused by the unscientific setting of printed circuit board. Finally, there is the interference problem of routing, which is mainly due to the printed circuit board signal line and the line distance between the power line and the ground line or line width setting unreasonable.
Countermeasures to Restrain Interference Problems of PCB:
Countermeasures to Restrain the Interference Problem of the Layout:
The layout of printed circuit board first needs to reasonably set the circuit position of each functional module in the printed circuit board according to the signal flow, and at the same time, try to ensure that the signal direction is consistent. In the position layout of printed circuit board, its center is generally set as the core component of module circuit. It is necessary to shorten the wiring between various components of printed circuit board, especially the lead between high-frequency components. The second is the integrated thermosensitive elements and chips and other parts, which shall be handled at a relatively long distance from the heater elements. In combination with the specific position of components on the PCB board, the position of connectors should be reasonably determined. Generally, connectors should be placed on the same side of the PCB board to avoid the situation that cables are drawn from both sides of the printed circuit board, so as to reduce the radiation of common mode current. Sensitive components should not be too close and input and output components should be kept at sufficient distance from each other. Finally, it is necessary to ensure that the position of the driver and connector in the printed circuit board is close to each other to avoid the phenomenon of signal routing over a long distance.
Countermeasures to Restrain the Interference Problem of the PCB Layers:
In order to solve the problem of PCB layer interference, PCB design information, signal density, power supply and GND demand should be considered, so as to determine the reasonable power supply design and routing layer number. Whether the layering of PCB is reasonable will directly affect the transient voltage and electromagnetic shielding effect of GND layer and power layer. In general, if it is a multi-layer plate, the signal layer should be designed to be close to the power layer or GND layer, and the power layer and GND layer should be adjacent and the spacing should be as short as possible. For multi-layer PCBs, the important signal lines are generally spread over the lower safety ground lines to prevent crosstalk between the signal lines and reduce the area of the signal loop. You can do the same for double-layer PCBs, or you can make sure that the GND layer is large enough on the projection surface of the important signal. It is worth noting that this kind of printed circuit boards design does not apply to digital or analog circuits, which mainly because there is no reference plane, and this can lead to parallel alignment and radiation enhancement. If PCB design budget is sufficient, multilayer PCB can be used. At this point in the design should pay attention to the following points.
1. Signal lines with high sensitivity or strong radiation, such as CLK or high sensitivity signal lines, are generally set in the signal layer adjacent to GND layer or between two GND layers. The purpose of this is to reduce the area of the signal loop, weaken the radiation intensity, and improve the anti-interference of PCB.
2. High frequency signal lines are generally not set at the top or bottom, so as to reduce their external conduction radiation.
3. The power supply layer shall be designed to indent 5-20H from the interior of the adjacent layer so as to reduce the edge radiation.
Countermeasures to Restrain the Interference Problem of the PCB Routing:
When designing PCB, in order to reduce the interference of routing, the following principles should be followed:
1. Avoid routing at 90゜, usually 135゜ or arc wiring instead.
2. The purpose of setting the width rather than the length of the wire is to reduce the resistance of the wire.
3. In order to reduce parallel crosstalk, parallel wiring of input wire and output wire is generally avoided, or GND line is interspersed between them.
4. Minimize the wire routing circuit, especially when there is a current through, the purpose is to reduce the external radiation of the current loop.
5. Set up key signals, including shunt isolation and protection route signals.
6. Routing in the same layer, physical isolation of adjacent lines (especially signal lines) should be ensured to prevent crosstalk and noise coupling.
7. The width of the power line should be as large as possible. Generally, the minimum line width should be 25mil to reduce the loop resistance, and the wiring direction should be consistent with the signal routing.
8. When designing PCB with more than three layers, an independent power supply layer should be set to ensure the independent power supply of each functional module as far as possible.
The above mentioned are the basic principles that should be followed when PCB routing. In addition, detailed routing in different situations should be carried out according to the specific situation of PCB and the implementation function.
Conclusion:
At present, PCB design tends to be complicated and multi-layered, and data signals tend to be high-frequency, which cannot be limited to the realization of functions. Various interference factors should be considered in the design stage. Scientific layering, reasonable layout and wiring, pure signal quality and stable and reliable signal transmission process are very important for PCB quality.
