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1. The wiring must have a reasonable direction: such as input/output, AC/DC, strong/weak signal, high frequency/low frequency, high voltage/low voltage, etc..., their directions should be linear (or separated), not mutually blend. Its purpose is to prevent mutual interference. The best direction is to follow a straight line, but it is generally not easy to achieve. Avoid circular routing. For DC, small signal, low voltage PCB design requirements can be lower. The edges of the input end and the output end should be avoided adjacent to parallel to avoid reflection interference. If necessary, ground wire should be added for isolation, and the wiring of two adjacent layers should be perpendicular to each other. Parasitic coupling is easy to occur in parallel.
2. Choose a good grounding point: Generally, a common ground is required, and the digital ground and the analog ground are connected at the power input capacitor.
3. Arrange power filter/decoupling capacitors reasonably: Arrange these capacitors as close as possible to these components, and they will be useless if they are too far away. The decoupling capacitor of the chip device is best placed on the belly of the device on the other side of the board. The power and ground must pass the capacitor first, and then enter the chip.
4. Lines are exquisite: wide lines should never be thin if possible; high-voltage and high-frequency lines should be round and smooth, without sharp chamfers, and corners should not be at right angles. Generally, an angle of 135 degrees is used. The ground wire should be as wide as possible, and it is best to use a large area of copper, which can greatly improve the problem of grounding points. In the design, the line holes should be reduced as much as possible, and the parallel line density should be reduced.
5. Widen the width of the power and ground wires as much as possible, preferably the ground wire is wider than the power wire, their relationship is: ground wire>power wire>signal wire.
6. The common ground processing of digital circuit and analog circuit, many PCBs are no longer a single functional circuit (digital or analog circuit), but are composed of a mixture of digital and analog circuits. Therefore, it is necessary to consider the mutual interference between them when PCB wiring, especially the noise interference on the ground wire.
The frequency of the digital circuit is high, and the sensitivity of the analog circuit is strong. For the signal line, the high-frequency signal line should be as far away as possible from the sensitive analog circuit device. For the ground line, the whole PCB has only one node to the outside world, so The problem of digital and analog common ground must be dealt with inside the PCB, and the digital ground and analog ground inside the board are actually separated and they are not connected to each other, but at the interface (such as plugs, etc.) connecting the PCB to the outside world. The digital ground and the analog ground are short-circuited a bit.
7. When the signal line is laid on the electrical (ground) layer and the multilayer printed board is laid out, there are not many wires left in the signal line layer that have not been laid out. Adding more layers will cause waste and increase production. The workload and cost have also increased accordingly. To solve this contradiction, you can consider wiring on the electrical (ground) layer. The power layer should be considered first, and the ground layer second. Because it is best to preserve the integrity of the formation.
8. For the processing of key signals, key signals such as clock lines should be grounded to avoid interference. At the same time, a solder joint is made on the side of the crystal oscillator device to ground the crystal oscillator shell.
9. Design Rule Check (DRC)
After the wiring design is completed, it is necessary to carefully check whether the wiring design meets the rules set by the designer, and at the same time, it is also necessary to confirm whether the rules set meet the requirements of the printed board production process. The general inspection has the following aspects:
Whether the distance between the line and the line, the line and the component pad, the line and the through hole, the component pad and the through hole, the through hole and the through hole is reasonable, and whether it meets the production requirements.
Is the width of the power line and the ground line appropriate, and is there a tight coupling between the power line and the ground line (low wave impedance)? Is there any place in the PCB where the ground wire can be widened?
Whether the best measures have been taken for the key signal lines, such as the shortest length, the protection line is added, and the input line and output line are clearly separated.
Is there a separate ground wire for the analog circuit and digital circuit part?
Whether the graphics (such as icons, annotations) added to the PCB will cause signal short circuit.
Modify some undesirable linear shapes.
Is there a process line on the PCB? Whether the PCB solder mask meets the requirements of the production process, whether the solder mask size is appropriate, and whether the character logo is pressed on the device pad, so as not to affect the quality of the electrical equipment.
Whether the outer frame edge of the power ground layer in the multilayer board is reduced. For example, the copper foil of the power ground layer is exposed to the outside of the board and it is easy to cause a short circuit.
10. Regarding EMC:
A. Choose devices with slower signal slopes as much as possible to reduce the high frequency components generated by the signal.
B. Pay attention to the placement of high-frequency components, not too close to the external connector.
C. Pay attention to the impedance matching of high-speed signals, the wiring layer and its return current path to reduce high-frequency reflection and radiation.
D. Place sufficient and appropriate decoupling capacitors on the power supply pins of each device to alleviate the noise on the power plane and ground plane. Pay special attention to whether the frequency response and temperature characteristics of the capacitor meet the design requirements.
E The power layer is 20H shrinking from the ground layer, and H is the distance between the power layer and the ground layer.
