In PCB design, wiring is an important step to complete product design. It can be said that the previous preparations are all done for it. PCB wiring includes single-sided wiring, double-sided wiring and multi-layer wiring. There are two ways of routing: automatic routing and interactive routing. Today, let professional engineers explain to you what are the PCB layout rules and techniques?
1. Handling of power supply and ground wire
The interference caused by improper handling of power supply and ground wire 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 wire should be taken seriously, and the noise interference generated by power supply and ground wire should be minimized to ensure the quality of the product. The specific method is as follows:
(1) Add a decoupling capacitor between the power supply and ground.
(2)) Try to widen the width of the power supply and ground wires. Their relationship is ground wire>power wire>signal wire. Usually the signal wire width is: 0.2~0.3mm, and the thinnest width can reach 0.05~0.07mm. Power wire 1.2~2.5mm.
(3) For the PCB of the digital circuit, a wide ground wire can be used to form a loop.
(4) Make a multi-layer board, the power supply and the ground wire each occupy one layer.
2. Co-location of digital circuits and analo circuits
There are many PCBs that are not single-function circuits, but are composed of a mixture of digital circuits and analo circuits. Therefore, it is necessary to consider the mutual interference between them when wiring, especially the noise interference on the ground.
The frequency of the digital circuit is high, and the sensitivity of the analo circuit is strong. For the ground wire, the PCB has only one node to the outside world, so the problem of digital and analo common ground must be dealt with inside the PCB; while the digital ground and the analo ground are actually separated inside the board, only the PCB is connected to the outside world. At the interface (such as plugs, etc.), there is only one connection point between the digital ground and the analo ground.
3. The signal line is laid on the electrical (ground) layer
When wiring a multilayer printed board, since the signal line layer is not finished and there is not much space left, adding more layers will cause waste. In order 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.
4. Treatment of connecting legs in large-area conductors
In large area grounding (electricity), the legs of commonly used components are connected to it. In terms of electrical performance, it is better for the pads of the component legs to be fully connected to the copper surface, but there are some hidden dangers to the soldering and assembly of the components:
① Welding requires a high-power heater;
② It is easy to cause virtual solder joints. Therefore, taking into account the electrical performance and process requirements, it is made into a cross-shaped pad, commonly known as a thermal pad. so,
5. The role of the network system in wiring
In many CAD systems, wiring is determined according to the network system. If the grid is too dense, although the number of channels increases, it will inevitably have higher requirements for the storage space of the device, and it will also have a great impact on the computing speed of electronic products. Too sparse a grid and too few channels have a great impact on the routing rate. Therefore, there must be a grid system with reasonable density to support the wiring.
6. Design rule checking (DRC)
After the wiring design is completed, it is necessary to carefully check whether the wiring design is reasonable. The general inspection includes the following aspects:
(1) Is the distance between lines and lines, lines and component pads, lines and through holes, component pads and through holes, and through holes and through holes reasonable?
(2) Is the width of the power line and the ground line appropriate? Is there a tight coupling between the power supply and the ground line? Is there any place in the PCB where the ground line can be widened?
(3) Do the analo and digital circuits have separate grounds?
(4) Will the graphics (such as icons and labels) added to the PCB later cause a signal short circuit?
(5) Is there a process line on the PCB? Does the solder mask meet the requirements of the production process? Is the size of the solder mask appropriate? Is the character mark pressed on the device pad, etc.? During soldering, virtual solder joints are generated due to excessive cross-section heat dissipation possibility is greatly reduced.
