In PCB wiring, improper layout of ground and power lines will cause interference in the system, resulting in decreased line performance. Only by properly handling the ground and power lines and minimizing the interference noise they generate can the quality of PCB wiring be guaranteed.
Ground and power line design rules
The design of ground (layer) and power lines (layer) of digital circuits should generally follow the following principles:
a) The ground line generally uses wide printed lines to form a dense ground network. It is recommended to connect all unused copper surfaces to the ground. If necessary, a ground plane should be used;
b) The power line should be close to the ground line. For ECL and TTL circuits, it is recommended to use wide printed lines to form a dense power network. If necessary, a power plane should be used;
c) Digital circuits (especially TIL circuits) and sensitive analog circuits should strictly avoid using common ground and power lines; TTL circuits and ECL circuits should generally have their own independent ground and power lines.
The PCB wiring rules for ground and power lines are as follows:
1. Add decoupling capacitors between the power supply and ground lines.
2. Try to widen the width of the power line and ground line, preferably the ground line is wider than the power line.
3. The PCB of the digital circuit can use a wide ground conductor to form a loop, that is, to form a ground network for use. The ground of the analog circuit cannot be used in this way.
4. Use a large area of copper layer as the ground line, connect all the unused places on the printed board to the ground as the ground line, or make a multi-layer board, with the power supply and ground lines occupying one layer each.
Checking the rules
a) After the wiring design is completed, it is necessary to carefully check whether the wiring design conforms to the rules set by the designer, and also to confirm whether the rules set conform to the requirements of the printed board production process. Generally, the following aspects are checked.
b) Whether the distance between lines, lines and component pads, lines and through holes, component pads and through holes, and through holes and through holes is reasonable and meets the production requirements.
c) Whether the width of the power line and the ground line is appropriate, whether the power and ground lines are tightly coupled (low wave impedance), and whether there is still room in the PCB to widen the ground line.
d) Whether the best measures have been taken for the key signal lines, such as the shortest length, adding protection lines, and the input and output lines are clearly separated.
e) Whether the analog circuit and the digital circuit parts have their own independent ground lines.
f) Whether the graphics added to the PCB later (such as icons, annotations) will cause signal short circuits.
g) Modify some ideal line shapes.
h) Whether there are process lines on the PCB, whether the solder mask meets the requirements of the production process, whether the solder mask size is appropriate, whether the character logo is pressed on the device pad, etc.
i) Whether the outer frame of the power ground layer in the multilayer board is reduced, such as the copper foil of the power ground layer exposed outside the board is likely to cause a short circuit.
Extended Q&A:
1. When designing the PCB, what will happen if the positions of the power line and the ground line are not matched or well matched?
The positions of the power line and the ground line are well matched, which can reduce the possibility of electromagnetic interference (EMl). If the power line and ground line are not properly matched, a system loop will be designed and noise is likely to be generated. An example of PCB design with improper power line and ground line matching is shown in the figure. On this circuit board, different routes are used to lay the power line and ground line. Due to this inappropriate matching, the electronic components and lines of the circuit board are more likely to be affected by electromagnetic interference (EMI).
2. Recently, I learned about PCB design. For high-speed multi-layer PCBs, what is the appropriate setting for the line width of the power line, ground line and signal line? What are the commonly used settings? Can you give an example? For example, how to set it when the operating frequency is 300Mhz?
Answer: The 300MHz signal must be simulated by impedance to calculate the line width and the distance between the line and the ground; the power line needs to determine the line width according to the current; the ground is generally not used as a “line” when it is a mixed signal PCB, but the entire plane is used, so as to ensure that the loop resistance is minimized and there is a complete plane under the signal line.