Electronic Design - What wiring rules should the manufactured PCB follow?

PCB wiring is a road that connects equipment and power-on signals, just like building a road that connects the city with traffic. In PCB design, wiring is an important step in product design, and it is also the most complex and qualified step. Even some experienced engineers have a headache for wiring. These are some common rules of PCB wiring, whether you are a white man or a production line engineer, you should be proficient.

General rules for PCB wiring

1: Direction wire control rules The wires at the input and output should avoid adjacent parallel connection. When wiring the PCB, adjacent layers use orthogonal structure wiring to avoid using different signal lines in the same direction as the adjacent layers to reduce unnecessary inter-layer interference. When PCB wiring is subject to structural constraints (such as certain backplanes), it is difficult to avoid parallel wiring, especially when the signal rate is high, the ground plane isolation of each wiring layer should be considered, and the ground line of each signal line isolation.

2: Circuit open loop inspection rules In PCB wiring, in order to avoid the "antenna effect" of wiring and reduce unnecessary interference radiation and reception, usually one end is not allowed to appear in the form of wiring, otherwise it may bring unpredictable results . Circuit board design

3: Line length control rules Short-term rules. In the design, the wiring length should be as short as possible to reduce the interference caused by long lines. Especially for some important signal lines, such as clock lines, the oscillator must be close to the equipment . In the case of driving multiple devices, the network topology should be determined according to the specific situation.

4: Impedance matching inspection rules The wiring width of the same network should be the same. Variations in the line width will cause uneven line characteristics, slow resistance, and reflection when the transmission speed is high. This situation should be avoided in the design. Under certain conditions, such as plug-in leads and similarly structured BGA packages, the line width may not be avoided, and the effective length of the central inconsistent part should be minimized.

5: Chamfering rules In PCB wiring, wiring is inevitable, and when the line appears in the right corner, additional parasitic capacitance and parasitic inductance will be generated on the corner. It should be avoided to avoid the corners of the line angle in the form of right angles and right angles, so as to avoid unnecessary radiation, and the processing performance of the right angle and right angle forms is not good. The angle of all lines must be greater than or equal to 135 degrees. If a straight line really needs a right angle, two ways to improve can be taken: one is to change the 90-degree angle into two 45-degree angles, and the other is to use rounded corners. The rounded corners are the best, with a 45-degree angle, which can be used at a frequency of 10 GHz. For a 45-degree angle line, the angle length is best satisfied with L.3W.

6: Equipment decoupling rules Add necessary decoupling capacitors on the printed board to filter out interference signals on the power supply and stabilize the power signal. On a multilayer board, the location of the decoupling capacitor is usually not very high, but the layout of the double-layer board, the layout of the half-plug capacitor and the wiring method of the power supply will directly affect the stability of the entire system, and sometimes even related to the success or failure of the design. . In the design of double-layer boards, filter wave capacitors should usually be used to filter the current before using the equipment. In high-speed PCB design, the correct use of decoupling capacitors is related to the stability of the entire circuit board.

7: 3W rule In order to reduce the crosstalk between lines, the line spacing should be large enough. When the center of the line spacing is not less than 3 times the line width, 70% of the electric field can be maintained without interfering with each other, which is called the 3W rule. In order to achieve 98% of the electric field does not interfere with each other, a 10W interval can be used.

8: Ground loop rule The minimum rule of the loop, that is, the smaller the loop area formed by the signal line and its loop, the smaller the loop area, the smaller the external radiation, and the smaller the external interference.

9: Shielding protection In fact, the corresponding ground loop rules are also to minimize the signal loop area, in some of the more important signals, such as clock signals, synchronization signals; and so on. It should also consider how to effectively combine the shield grounding with the actual ground plane.