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PCB Tech

PCB Tech

PCB Tech

PCB Tech

10 useful rules for PCB routing
2021-10-07
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Author:Downs

PCB wiring, that is, laying the roads for energizing signals to connect various devices, which is like connecting various cities to traffic by repairing roads. In PCB design, wiring is an important step to complete product design. It is also the most detailed and most restrictive step. Even some experienced engineers have a headache for wiring. The following are some common rules of PCB wiring, whether you are a novice or an engineer already in the industry, you should master it.

Common rules for PCB wiring

1. Direction control rules for routing

The wires of the input and output terminals should try to avoid being adjacent and parallel. In the PCB layout, the routing directions of adjacent layers are orthogonal to avoid different signal lines in the same direction on adjacent layers to reduce unnecessary inter-layer interference. When PCB wiring is restricted by the structure (such as some backplanes), it is difficult to avoid parallel wiring, especially when the signal rate is high, consider using a ground plane to isolate each wiring layer, and use a ground wire to isolate each signal line. A schematic diagram of the routing direction of adjacent layers is shown in the figure below.

2. Open-loop inspection rules for wiring

In the PCB wiring, in order to avoid the "antenna effect" caused by wiring and reduce unnecessary interference radiation and reception, it is generally not allowed to have a floating wiring form at one end, otherwise it may bring unpredictable results.

pcb board

3. Line length control rules

That is to say, the short-line rule. When designing, you should try to keep the wiring length as short as possible to reduce interference problems caused by excessively long wiring. Especially for some important signal lines, such as clock lines, be sure to place their oscillators very close to the device. The place. In the case of driving multiple devices, the network topology should be decided according to the specific situation.

4. Impedance matching inspection rules

The wiring width of the same network should be kept consistent. Variations in line width will cause uneven line characteristic impedance, and reflections will occur when the transmission speed is high. This situation should be avoided as much as possible in the design. Under certain conditions, such as the similar structure of the lead wire of the connector and the lead wire of the BGA package, the change of the line width may not be avoided, and the effective length of the middle inconsistent part should be minimized.

5. Chamfering rules

When PCB wiring, it is inevitable that the trace will bend. When the trace has a right-angle corner, additional parasitic capacitance and parasitic inductance will be generated at the corner. The corner of the trace should be designed to be sharp and right-angled to avoid occurrence Unnecessary radiation, and the process performance of acute and right-angle forms is not good. It is required that the angle between all lines and the line should be greater than or equal to 135°. In the case that the wiring does require a right-angle corner, two improvement methods can be adopted: one One is to change the 90° corner into two 45° corners; the other is to use rounded corners. The rounded corner method is the best, and the 45° corner can be used at 10GHz frequency. For the 45° corner wiring, the corner length is the longest Good to satisfy L≥3W.

6. Device decoupling rules

A. Add necessary decoupling capacitors on the printed board to filter out interference signals on the power supply and stabilize the power signal. In multi-layer boards, the position of decoupling capacitors is generally not very demanding, but for double-layer boards, the layout of the decoupling capacitors and the wiring of the power supply will directly affect the stability of the entire system, and sometimes even the design. Success or failure.

B. In the double-layer board design, the current should generally be filtered by the filter capacitor before being used by the device.

C. In high-speed circuit design, whether decoupling capacitors can be used correctly is related to the stability of the entire board.

7, 3W rule

In order to reduce crosstalk between lines, the line spacing should be large enough. When the line center spacing is not less than 3 times the line width, 70% of the electric field can be maintained without mutual interference, which is called the 3W rule. If you want to achieve 98% of the electric field without interfering with each other, you can use a spacing of 10W.

8. Ground circuit rules

The minimum loop rule is that the loop area formed by the signal line and its loop should be as small as possible. The smaller the loop area, the less the external radiation and the less interference from the outside world.

9. Shielding protection

Corresponding to the ground loop rules, in fact, it is also to minimize the signal loop area, which is more common in some more important signals, such as clock signals and synchronization signals; for some particularly important and high-frequency signals, copper cables should be considered Shielding structure design, that is, to isolate the left, right, and left lines of the wiring, and also consider how to effectively combine the shielding ground with the actual ground plane.

10. Resonance rules of wiring

The resonant rules of PCB board routing are mainly for high-frequency signal design, that is, the length of the wiring must not be an integer multiple of its wavelength to avoid resonance.