In PCB design, wiring is an important step to complete product design, it can be said that the previous preparation work is done for it, in the entire PCB, the design process of wiring is limited to the highest, the most detailed skills, the largest workload. PCB wiring has single-sided wiring, double-sided wiring and multi-layer wiring. There are also two ways of wiring: automatic wiring and interactive wiring, before automatic wiring, you can use interactive wiring in advance to the more strict requirements of the line, the input and output side lines should avoid parallel adjacent, so as to avoid reflection interference. Ground line isolation should be added if necessary, and the wiring of the two adjacent layers should be vertical to each other, and parallel is easy to produce parasitic coupling.
The distribution flux of automatic wiring depends on a good layout, and the wiring rules can be pre-set, including the number of bending lines, the number of through holes, the number of steps, etc. Generally, first explore the cloth warp line, quickly connect the short line, and then labyrinth wiring, first to cloth the wire for global wiring path optimization, it can disconnect the cloth line according to need. And try to re-wire to improve the overall effect.
For the current high density PCB design has felt through the hole is not suitable, it wastes a lot of valuable wiring channels, in order to solve this contradiction, there are blind holes and buried holes technology, it not only completes the role of the through hole, but also saves many wiring channels to make the wiring process more convenient, more smooth, more perfect, PCB board design process is a complex and simple process, in order to master it well, but also need the majority of electronic engineering designers to experience, in order to get the true meaning of it.
1, power supply, ground processing
Even if the wiring in the entire PCB board is completed well, the interference caused by the inconsiderate power supply and ground wire will reduce the performance of the product, and sometimes even affect the success rate of the product. Therefore, the wiring of electricity and ground wires should be taken seriously, and the noise interference caused by electricity and ground wires should be reduced to a minimum to ensure the quality of products.
For every engineer engaged in the design of electronic products, it is understood that the cause of the noise between the ground and the power line is generated, and only the reduced noise suppression is described:
It is well known that the decoupling capacitor is added between the power supply and the ground wire.
Try to widen the width of the power supply and ground wire, it is best that the ground wire is wider than the power line, their relationship is: ground wire > power line > signal line, usually the signal line width is: 0.2 ~ 0.3mm, the maximum width can reach 0.05 ~ 0.07mm, the power line is 1.2 ~ 2.5mm
The PCB of the digital circuit can be composed of a wide ground wire loop, that is, a ground network to be used (the ground of the analog circuit cannot be used in this way).
A large area of copper layer is used as ground wire, and the places that are not used on the printed board are connected with the ground as ground wire. Or make a multi-layer board, the power supply, the ground line occupy a layer.
2. Common ground processing of digital circuit and analog circuit
There are now many PCBS that are no longer single-function circuits (digital or analog circuits), but are composed of a mixture of digital and analog circuits. Therefore, when wiring, it is necessary to consider the interference between them, especially the noise interference on the ground wire.
The frequency of the digital circuit is high, the sensitivity of the analog circuit is strong, for the signal line, the high-frequency signal line is as far away from the sensitive analog circuit device as possible, for the ground line, the whole PCB has only one node to the outside world, so it must be handled in the PCB internal number, mode common ground problem, The digital and analog inside the board are actually separated from each other, only at the interface between the PCB and the outside world (such as plugs, etc.). There is a bit of a short connection between digital and analog, but note that there is only one connection point. There are also common on the PCB, which is determined by the system design.
3, the signal line is distributed on the electrical (ground) layer
In the multi-layer printed board wiring, because there is not much left in the signal line layer, more layers will cause waste and will increase a certain amount of work for production, and the cost will increase accordingly, in order to solve this contradiction, you can consider wiring on the electrical (ground) layer. The power layer should be considered first, followed by the formation. Because it is better to preserve the integrity of the formation.
4. Processing of connecting legs in large area conductors
In a large area of grounding (electrical), the legs of commonly used components are connected to them, and the treatment of the connecting legs needs comprehensive consideration. As far as electrical performance is concerned, the welding pad of the component legs is fully connected with the copper surface, but there are some bad hidden dangers in the welding assembly of the components, such as:
① welding requires high-power heaters.
② Easy to cause virtual solder joints. Therefore, taking into account the electrical performance and process needs, make a cross welding pad, called heat shield, commonly known as Thermal pad, in this way, the possibility of virtual solder joints caused by excessive heat dissipation in the welding section is greatly reduced. The treatment of the electrical (ground) leg of the multilayer board is the same.
5, the role of the network system in wiring
In many CAD systems, the wiring is determined by the network system. The grid is too dense, although the path has increased, the step is too small, and the amount of data in the graph field is too large, which inevitably has higher requirements for the storage space of the device, and also has a great impact on the computing speed of the object computer electronic products. Some paths are not valid, such as those occupied by the pads of the component legs or by mounting holes or fixing holes. Too sparse grid and too few channels have great influence on the distribution flux. Therefore, it is necessary to have a reasonable grid system to support the wiring.