PCB Blog - How to design PCB board wiring?

PCB board wiring is the process of laying a path for connecting various devices with power on signals. PCB board wiring is the process of laying a path for connecting various devices with power on signals.

PCB board wiring 

In PCB design, wiring is an important step in completing product design. It can be said that the previous preparation work is done for it. In the entire PCB design, the wiring design process has the highest limit, the most detailed skills, and the largest workload. PCB board wiring includes single-sided wiring, double-sided wiring, and multi-layer wiring.

There are also two ways of wiring: automatic wiring and interactive wiring. Before automatic routing. It is possible to use interactive pre-wiring for lines with strict requirements, and the wiring between the input and output ends should avoid adjacent parallelism to avoid reflection interference. If necessary, ground wires should be added for isolation. The wiring of adjacent layers should be perpendicular to each other, while parallel layers are prone to parasitic coupling.

The wiring rate of automatic wiring depends on a good layout, and the routing rules can be pre-set, including the number of bends in the wiring, the number of through holes, and the number of steps. Generally, exploratory wiring is carried out first to quickly connect short lines, and then maze wiring is carried out. The wiring to be laid is first optimized for global wiring paths, which can disconnect the laid lines as needed.

PCB board wiring rules

1. The distance between SMD devices should be greater than.

2. The distance between the outer side of the SMD device pad and the outer edge of adjacent THD components should be greater than 2mm.

3. Ground circuit rules

The minimum rule of the loop is that the area of the loop formed by the signal line and its loop should be as small as possible. The smaller the loop area, the less external radiation, and the smaller the interference received from the outside. In response to this rule, when segmenting the ground plane, it is necessary to consider the distribution of the ground plane and important signal lines to prevent problems caused by ground plane slots and other factors; In the design of double-layer boards, while leaving sufficient space for the power supply, the remaining part should be filled with reference ground and some necessary holes should be added to effectively connect the double-sided ground signals. For some key signals, ground wire isolation should be used as much as possible. For some high-frequency designs, special consideration should be given to the ground plane signal circuit problem, and it is recommended to use multi-layer boards.

4. CrossTalk control refers to the mutual interference caused by long parallel wiring between different networks on a PCB, mainly due to the distributed capacitance and inductance between parallel lines. The main measures to overcome crosstalk are to increase the spacing of parallel wiring and follow the 3W rule; Insert a grounded isolation wire between parallel lines. Reduce the distance between the wiring layer and the ground plane.

5. Shielding protection

The corresponding ground circuit rules are actually aimed at minimizing the circuit area of the signal as much as possible, and are commonly seen in some important signals, such as clock signals and synchronization signals; For signals that are particularly important and have high frequencies, a copper shaft cable shielding structure design should be considered, which means that the wires on and off the line are separated by ground wires on the left and right sides, and it is also necessary to consider how to effectively combine the shielding ground with the actual ground plane.

6. Direction control rules for wiring

The routing direction of adjacent layers is in an orthogonal structure. Avoid running different signal lines in the same direction in adjacent layers to reduce unnecessary inter-layer interference; When it is difficult to avoid this situation due to board structure limitations (such as certain backplanes), especially when the signal rate is high, it should be considered to isolate each wiring layer with a ground plane and each signal line with a ground signal line.

7. Open loop inspection rules for wiring

Generally, dangling lines with one end floating are not allowed, mainly to avoid "antenna effects" and reduce unnecessary interference radiation and reception, otherwise unpredictable results may occur.

8. Impedance matching check rules

The wiring width of the same network should be consistent. The change in the line width will cause an uneven characteristic impedance of the line. When the transmission speed is high, reflection will occur. This situation should be avoided as far as possible in the design. Under certain conditions, such as connector leads and BGA packaged leads with similar structures, it may not be possible to avoid changes in line width, and the effective length of inconsistent parts in the middle should be minimized as much as possible.

9. Wiring closed-loop inspection rules

Prevent signal lines from forming self-loops between different layers. This type of problem is prone to occur in the design of multi-layer boards, and self-circulation will cause radiation interference.

10. Control rules for branch length of wiring

Try to control the length of the branches as much as possible, and the general requirement is Tdelay<=Trise/20.

11. Resonance rules for wiring

Mainly for high-frequency signal design, the wiring length should not be an integer multiple of its wavelength to avoid resonance phenomenon.

12. Line length control rules

The short line rule is that when designing, the wiring length should be as short as possible to reduce interference caused by excessively long wiring. Especially for important signal lines, such as clock lines, it is important to place their oscillators very close to the device. For driving multiple devices, the decision on which network topology to use should be based on the specific situation.

13. Integrity rules for power and ground layers

For areas with dense conductive holes, attention should be paid to avoiding interconnections between the holes in the excavated area of the power supply and the formation, forming a division of the plane layer, thereby damaging the integrity of the plane layer and leading to an increase in the circuit area of the signal line in the formation.

14. Rules for overlapping power and ground layers

Different power layers should avoid overlapping in space. The main purpose is to reduce interference between different power sources, especially for some power sources with significant voltage differences. The overlapping problem of power supply planes must be avoided, and if it is difficult to avoid, intermediate isolation layers can be considered.

PCB board wiring skills and precautions

1. Precautions for wiring between the power supply and ground wire

1) Coupling capacitance shall be added between the power supply and the ground. Make sure that the power supply is connected to the pin of the chip after passing through the decoupling capacitor. (The decoupling capacitor generally has two functions: one is to provide an instantaneous current of the chip, and the other is to remove power noise).

2) Try to widen the power and ground wires as much as possible, preferably with the ground wire wider than the power line and the power line wider than the signal line.

3) A large area of copper layer can be used as a ground wire, connecting unused areas on the printed circuit board to the ground for use as a ground wire. Or it can be made into a multi-layer board, with one layer for the power supply and one layer for the ground wire.

2. Processing when mixing digital and analog circuits

Nowadays, many PCBs are no longer single-function circuits but are composed of a mixture of digital and analog circuits. Therefore, when wiring, it is necessary to consider the issue of mutual interference between them, especially the noise interference on the ground wire. Due to the high frequency of digital circuits and the strong sensitivity of analog circuits, high-frequency signal lines should be as far away from sensitive analog circuit components as possible. However, for the entire PCB, the PCB can only have one external node, so it is necessary to handle the problem of digital and analog signals sharing ground inside the PCB. However, within the circuit board, the ground of the digital circuit and the ground of the analog circuit are actually separate, Only at the connection between the PCB and the outside world. There is a short circuit between the ground of the digital circuit and the ground of the analog circuit. Please note that there is only one connection point, and there are also cases where there is no common ground on the PCB, which is determined by the system design.

3. Treatment of Line Corners

Usually, there will be thickness changes at the corners of the line, but when the diameter of the line changes, some reflection phenomena will occur. Corners have the worst effect on the thickness variation of lines, with a right angle being the worst, a 45-degree angle being the better, and a rounded corner being the best. However, rounded corners are more cumbersome to handle in PCB design, so it is generally determined based on the sensitivity of the signal. Generally, a 45-degree angle is sufficient for signals, and rounded corners are only used for particularly sensitive lines.

Good PCB board wiring can handle some practical issues that are not fully considered in schematic design, such as adjusting the component layout, handling wire thickness, spacing, and routing to meet production standards.