In PCB design, layout is an important step to complete product design. The previous preparatory work is done for it. In the whole PCB, the design process of wiring is the most limited. The skill is the most detailed, and the workload is the largest. PCB layout includes single-sided wiring, double-sided wiring and multi-layer wiring.

There are also two ways of layout: automatic layout and interactive layout. Before automatic routing, you can use interactive routing to prepare the lines with stricter requirements. The edges of the input and output ports should avoid being adjacent to each other in parallel to avoid reflection interference.

If necessary, ground isolation should be added, and the wiring of two adjacent layers should be perpendicular to each other, and parasitic coupling is easy to occur in parallel.

The routing rate depends on a good layout. The routing rules can be preset, including the number of bending times, the number of via holes, and the number of steps. Generally, the exploratory warp routing is carried out first, and the short lines are quickly connected. Then the labyrinth routing is performed, and the wiring to be placed is firstly optimized for the global routing path. It can disconnect the already laid lines as needed, and try to re-route to improve the overall effect.

For the current high-density PCB design, the through hole is not suitable for it. It wastes a lot of valuable wiring channels, and saves many wiring channels to make the wiring process smoother and more convenient.

1. Power supply and ground wire

Even if the wiring in the entire PCB is completed well, the interference caused by the inconsiderate consideration of the power supply and ground wires will degrade the performance of the product. Sometimes it even affects the success rate of the product. Therefore, the layout should be taken seriously, and the noise interference generated by power supply and ground wire should be minimized to ensure product quality.

2. Common ground processing of digital circuit and analog circuit

Nowadays, many PCBs are no longer single-function circuits (digital or analog circuits), but are composed of a mixture of digital circuits and analog circuits. Therefore, it is necessary to consider the mutual interference between them when layout, especially the noise interference on the ground.

The digital circuit has high frequency. The sensitivity of the analog circuit is strong. The high frequency signal line should be 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 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. They are not connected to each other, but only at the interface between the PCB and the outside world (such as plugs, etc.). The digital ground and the analog ground are shorted a bit, please note that there is only one connection point. There are also non-common grounds on the PCB, which is determined by the system design.

3. Signal line on electrical (ground) layer

When talking about the layout of multi-layer printed boards, since there are not many lines left in the signal line layer, adding more layers will cause waste and increase the workload of production, and the cost will increase 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 is the second. It is best to preserve the integrity of the formation.

4. Connecting legs in large-area conductors

In large-area grounding, the commonly used components legs are connected. The treatment of the connecting legs needs to be considered comprehensively. In terms of electrical performance, it is better for the pads of the component legs to be fully connected to the copper surface. There are some unfavorable hidden dangers in the welding assembly of components such as: 1. Welding requires high-power heaters. 2. It is easy to cause virtual solder joints.

Therefore, taking into account the electrical performance and process requirements, a cross-shaped pad is made, which is called a heat shield, commonly known as a thermal, so that the possibility of virtual solder joints may be generated due to excessive cross-section heat dissipation during soldering.

5. Network system in layout

In many CAD systems, layout is determined according to the network system. If the grid is too dense, although the path has increased, the step is too small, and the data volume of the map field is too large. This inevitably has higher requirements for the storage space of the device. At the same time, it also affects the computing speed of computer electronic products.

Some paths are invalid, such as those occupied by the pads of component legs or by mounting holes and fixed holes. Too sparse a mesh 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.

The distance between the legs of standard components is 0.1 inches (2.54mm), so the basis of the grid system is generally set at 0.1 inches (2.54 mm) or an integer multiple of less than 0.1 inches, such as 0.05 inches, 0.025 inches, 0.02 inches etc.