How to perform PCB wiring in a personable manner_PCB wiring rules sharing

In the process of PCB design, wiring can be roughly divided into three realms: The first is distribution, which is also the most basic requirement of PCB design. If the circuit is blocked, then the basic function of the board is not available, it is a waste board, let alone other things. The second is performance, which is a measure of the quality of a printed circuit board. After the wiring is completed, it is necessary to consider how to achieve the best performance and avoid various interferences. The third is beautiful, smooth lines and excellent performance, but if the picture is beautiful but you dare not look at it, you should consider how to beautify the chaotic lines. The wiring is neat and uniform, which can also bring great convenience for future testing and maintenance. This is also the basic quality that a senior engineer should possess.

Wiring is far from being as simple as imagined, and it is not just a simple task, but if wiring is done as an art, there will be different gains. There are pictures and the truth, let us feel the difference in wiring between novices and masters.

So how to wire elegantly? The wiring method can be very different, in order to avoid the parallel of the input end and the output end during the wiring, the reflection interference and the parasitic coupling of two adjacent wiring layers parallel to each other will affect the stability of the line, even when the interference is serious. The circuit board cannot work at all. There are many factors that need to be considered when wiring. Now I will give you the rules of PCB wiring.

3W rule

Here 3W is the line-to-line distance to keep 3 times the line width. You said that 3H is fine too. But here H refers to the line width. Not the media thickness. In order to reduce the crosstalk between the lines, the line spacing should be large enough. If the line center distance is not less than 3 times the line width, 70% of the electric field between the lines can be kept without mutual interference, which is called the 3W rule. If you want to achieve 98% of the electric field without mutual interference, you can use the 10W rule.

20H rule

The adoption of the "20H rule" means to ensure that the edge of the power plane is at least 20 times smaller than the edge of the 0V plane, which is equivalent to the spacing between the two planes. It means the distance between the power supply layer and the ground layer is reduced by 20H, and of course it is also to suppress the edge radiation effect. Electromagnetic interference is radiated outward at the edge of the board. Shrink the power plane so that the electric field is only conducted within the ground plane. Taking a H (the thickness of the medium between the power supply and the ground) as a unit, if the shrinkage is 20H, 70% of the electric field can be confined to the edge of the ground layer; if the shrinkage is 100H, 98% of the electric field can be confined.

Five-five rule

The rules for selecting the number of layers of the printed board, that is, if the clock frequency reaches 5MHz or the pulse rise time is less than 5ns, the PCB must be a multi-layer board. This is a general rule. Sometimes due to cost and other factors, a double-layer board is used When constructing, in this case, it is best to use one side of the printed board as a complete ground plane.

Power and ground rules

The interference between the power supply and the bottom line directly affects the performance of the product, so the wiring of the power supply and the ground line should strive to minimize the noise interference generated to ensure the quality of the product. Every engineer who is engaged in the design of electronic products understands the cause of the noise between the ground wire and the power wire, and now only the reduced noise suppression is expressed: It is well known that the power supply and the ground wire are added. Coupling capacitor. 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, usually the signal wire width is: 0.2~0.3mm, the smallest width can be reached 0.05~0.07mm, the power cord is 1.2~2.5 mm. For the PCB of the digital circuit, a wide ground wire can be used to form a loop, that is, to form a ground net to use (the ground of the analog circuit cannot be used in this way). Use a large area of copper layer as the ground wire Use, connect the unused places on the printed board with the ground as a ground wire. Or it can be made into a multilayer board, and the power supply and ground wires occupy one layer each.

The signal line is laid in the electric (ground) layer rule

In the multi-layer printed board wiring, because 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 the production workload, 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 second. Because it is best to preserve the integrity of the formation.

Common ground rules for digital and analog circuits

Nowadays, PCBs are mostly composed of digital circuits and analog circuits, and mutual interference issues should be considered when wiring, especially 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 components. 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 are actually separated inside the board and they are not connected to each other, only at the interface (such as a plug, etc.) connecting the PCB to the outside world. There is a short connection between the digital ground and the analog ground. 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.

Treatment rules for connecting legs in large area conductors

In large-area grounding (electricity), the legs of commonly used components are connected to it. The treatment of the connecting legs needs to be considered comprehensively. In terms of electrical performance, it is better to connect the pads of the component legs to the copper surface. There are some undesirable hidden dangers in the welding assembly of components, such as: ① Welding requires high-power heaters. ②It is easy to cause virtual solder joints. Therefore, both electrical performance and process requirements are made into cross-patterned pads, called heat shields, commonly known as thermal pads (Thermal), so that virtual solder joints may be generated due to excessive cross-section heat during soldering. Sex is greatly reduced. The processing of the power (ground) leg of the multilayer board is the same.

Network system rules

The grid grounding system is an effective method to reduce trace inductance and provide a return path for RF current. The grid is too dense, although the path has increased, but the step is too small, and the data volume of the field is too large. This will inevitably have higher requirements for the storage space of the device, and also the computing speed of the computer-type electronic products. Great influence. Some paths are invalid, such as those occupied by the pads of the component legs, or by mounting holes and fixed holes. Too sparse grids and too few channels have a great impact on the distribution rate. Therefore, there must be a dense and reasonable grid system 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 to 0.1 inches (2.54 mm) or less than an integral multiple of 0.1 inches, such as: 0.05 inches, 0.025 inches, 0.02 Inches etc.