Wiring is the most time-consuming and labor-intensive aspect of PCB design, and it also tests the technical level of PCB engineers the most. As an aspiring PCB design engineer, how can you improve your wiring level? According to experience, in order to achieve fast and efficient wiring and make your PCB wiring look high-end, you need to do the following 5 points:
1. Follow PCB wiring rules
This is the most basic requirement and foundation for PCB designers. PCB wiring should generally follow the following rules:
a) The number of printed wire wiring layers is determined as needed. The wiring occupied channel ratio should generally be above 50%;
b) According to the process conditions and wiring density, rationally select the wire width and wire spacing, strive to achieve uniform wiring within the layer, and similar wiring density in each layer. If necessary, auxiliary non-functional connection pads or printed wires should be added to the missing line area;
c) Two adjacent layers of wires should be laid out in oblique or curved lines perpendicular to each other to reduce parasitic capacitance;
d) Printed wire routing should be as short as possible, especially for high-frequency signals and highly sensitive signal lines; for important signal lines such as clocks, equal delay wiring should also be considered if necessary;
e) When multiple power supply (layers) or ground (layers) are laid out on the same layer, the separation distance should not be less than 1mm;
f) For large-area conductive patterns larger than 5×5mm2, windows should be partially opened;
g) Thermal isolation design should be carried out between the large-area graphics of the power layer and ground layer and their connection pads;
h) The special requirements of other circuits should comply with relevant regulations.
2. Handling of power supply and ground wires
Even if the wiring in the entire PCB board is completed well, interference caused by insufficient consideration of power supply and ground wires will degrade the performance of the product and sometimes even affect the success rate of the product. Therefore, the wiring of power supply and ground wires must be taken seriously to minimize the noise interference generated by power supply and ground wires to ensure the quality of the product.
Every engineer who is engaged in the design of electronic products understands the cause of noise between the ground wire and the power wire. Now we only describe the reduced noise suppression: the well-known is to add a noise between the power supply and the ground wire. Lotus root capacitor.
Try to widen the width of the power supply and ground wires. It is best to make the ground wire wider than the power wire. Their relationship is: ground wire>power wire>signal wire. Usually the signal wire width is: 0.2~0.3mm, and the finest width can be up to 0.05 ~0.07mm, the power cord is 1.2~2.5mm.
For digital circuit PCBs, wide ground wires can be used to form a loop, that is, to form a ground network. The ground of analog circuits cannot be used in this way. A large area of copper layer is used as a ground wire, and the unused ground wires on the printed board are not used. All places are connected to the ground and used as ground wires. Or it can be made into a multi-layer board, with power supply and ground wires occupying one layer each.
3. Common ground processing of digital circuits and analog circuits
Nowadays, many PCBs are no longer single-functional circuits, but are composed of a mixture of digital circuits and analog circuits. Therefore, it is necessary to consider the problem of mutual interference between them when wiring, especially the noise interference on the ground line.
The frequency of digital circuits is high, and the sensitivity of analog circuits is strong. For signal lines, high-frequency signal lines should be as far away from sensitive analog circuit devices as possible. For ground lines, the entire PCB has only one node to the outside world, so it must be The problem of digital and analog common ground is dealt with inside the PCB, but the digital ground and analog ground are actually separated inside the board. They are not connected to each other, but are only at the interface where the PCB connects to the outside world (such as plugs, etc.).
The digital ground is a little shorted to the analog ground, please note that there is only one connection point. There are also different grounds on the PCB, which is determined by the system design.
4. The signal lines are laid on the power layer or ground layer
When wiring multi-layer printed boards, there are not many unfinished lines left on the signal line layer. Adding more layers will cause waste and increase the workload of production, and the cost will also increase accordingly. To solve this contradiction, you can consider wiring on the power layer or ground layer.
The power layer should be considered first, followed by the ground layer. Because it is best to preserve the integrity of the formation.
5. Design Rule Check (DRC)
After the wiring design is completed, it is necessary to carefully check whether the wiring design complies with the rules set by the designer. It is also necessary to confirm whether the rules set meet the needs of the printed board production process. General inspections include the following aspects:
a) Whether the distance between wires and wires, wires and component pads, wires and through holes, component pads and through holes, and through holes and through holes is reasonable and meets production requirements.
b) Whether the width of the power cord and ground wire is appropriate. Is the power supply and ground wire tightly coupled? Is there any place in the PCB where the ground wire can be widened?
c) Whether the best measures have been taken for key signal lines, such as the shortest length, adding protective lines, and clearly separating input lines and output lines.
d) Whether the analog circuit and digital circuit parts have independent ground wires. Whether graphics (such as icons, labels) added to the PCB later will cause signal short circuits.
e) Modify some unideal line shapes.
f) Whether there are process lines on the PCB. Whether the solder resist meets the requirements of the production process, whether the solder resist size is appropriate, and whether the character mark is pressed on the device pad to avoid affecting the quality of the electrical assembly.
g) Whether the edge of the outer frame of the power supply ground layer in the multilayer board is reduced. If the copper foil of the power supply ground layer is exposed outside the board, it is easy to cause a short circuit.
