Summary of PCB layout and wiring rules

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Summary of PCB layout and wiring rules [Copy link]

, 68, 68)]10 rules for component layout:

, 68, 68)]2. Refer to the principle block diagram in the layout and arrange the main components according to the main signal flow of the single board.

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3. The arrangement of components should be convenient for debugging and maintenance. In other words, large components should not be placed around small components, and there should be enough space around components and devices that need to be debugged. 4. For circuits with the same structure, try to adopt the "symmetrical" standard layout as much as possible;

68)]5. Optimize the layout according to the standards of uniform distribution, balanced center of gravity, and beautiful layout;

6. The same type of plug-in components should be placed in the same direction in the X or Y direction. The same type of polar discrete components should also strive to be consistent in the X or Y direction to facilitate production and inspection. 7. Heat-generating components should generally be evenly distributed to facilitate the heat dissipation of the board and the entire machine. Temperature-sensitive devices other than temperature detection components should be kept away from components that generate large amounts of heat. :

8. The layout should try to meet the following requirements: the total connection line should be as short as possible, and the key signal line should be the shortest; high voltage and high current signals should be completely separated from small current and low voltage weak signals; analog signals should be separated from digital signals; high frequency signals should be separated from low frequency signals; and the spacing between high frequency components should be sufficient. 9. The layout of the decoupling capacitor should be as close to the power pin of the IC as possible, and the loop between it and the power supply and ground should be as short as possible.

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10. When laying out components, consider placing devices using the same power supply together as much as possible to facilitate future power supply separation.

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2. Right-angle routing

Right-angle routing is generally a situation that is required to be avoided as much as possible in PCB routing, and it has almost become one of the standards for measuring the quality of routing. So how much impact will right-angle routing have on signal transmission? In principle, right-angle routing will change the line width of the transmission line, causing impedance discontinuity. In fact, not only right-angle routing, but also corner and sharp-angle routing may cause impedance changes.

The influence of right-angle routing on the signal is mainly reflected in three aspects:

68)]First, the corner can be equivalent to a capacitive load on the transmission line, slowing down the rise time;

Second, impedance discontinuity can cause signal reflection;

The third is the EMI generated by the right-angle tip.

[color=rg b(68, 68, 68)]3. Differential wiring

Differential signals are increasingly used in high-speed circuit design, and the most critical signals in the circuit often have to be designed with differential structures. Definition: In layman's terms, the driver sends two equal and opposite signals, and the receiver determines whether the logic state is "0" or "1" by comparing the difference between the two voltages. The pair of traces that carry differential signals is called differential traces.

Compared with ordinary single-ended signal routing, the most obvious advantages of differential signals are reflected in the following three aspects:

a. Strong anti-interference ability, because the coupling between the two differential traces is very good, when there is external noise interference, it is almost coupled to the two lines at the same time, and the receiving end only cares about the difference between the two signals, so the external common-mode noise can be completely offset. b. Can effectively suppress EMI. For the same reason, because the two signals have opposite polarities, the electromagnetic fields radiated by them can cancel each other out. The tighter the coupling, the less electromagnetic energy discharged to the outside. c. Accurate timing positioning. Since the switching change of the differential signal is located at the intersection of the two signals, unlike the ordinary single-ended signal that relies on high and low threshold voltages for judgment, it is less affected by the process and temperature, which can reduce the timing error and is more suitable for circuits with low amplitude signals. The currently popular LVDS (low voltage differential signaling) refers to this small amplitude differential signal technology.

For PCB engineers, the most important thing is how to ensure that the advantages of differential routing can be fully utilized in actual routing. Perhaps anyone who has been exposed to Layout will understand the general requirements of differential routing, which is "equal length and equal distance".

Equal length is to ensure that the two differential signals always maintain opposite polarity and reduce the common mode component; equal distance is mainly to ensure that the differential impedance of the two is consistent and reduce reflection. The "principle of keeping as close as possible" is sometimes also one of the requirements for differential routing.

4. Snake line:

The serpentine line is a type of routing method often used in Layout. Its main purpose is to adjust the delay and meet the system timing design requirements. Designers must first have this understanding: the serpentine line will damage the signal quality and change the transmission delay, so it should be avoided as much as possible during wiring. However, in actual design, in order to ensure that the signal has enough hold time or reduce the time offset between the same group of signals, it is often necessary to deliberately wind the line.

Note:

Paired differential signal lines are usually routed in parallel, and vias should be used as little as possible. When vias must be used, both lines should be drilled together to achieve impedance matching.

A group of buses with the same attributes should be routed side by side as much as possible and as equal in length as possible. The vias leading from the SMD pads should be as far away from the pads as possible.

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