PCB Tech - How does a circuit board factory design an anti-interference PCB board

The basic task of PCB anti-interference design is that the system or device neither malfunctions or loses function due to external electromagnetic interference, nor sends excessive noise interference to the outside world, so as not to affect the normal operation of other systems or devices. Therefore, improving the anti-interference ability of the system is also an important part of the system design.

Summary of circuit anti-jamming design principles:
1. The design of the power cord
Choose the right power supply;
Try to widen the power cord;
Ensure that the power cord, bottom line direction and data transmission direction are consistent;
Use anti-interference components;
Add a decoupling capacitor (10～100µF) to the power inlet.

2. Design of the ground wire
Separate analog ground and digital ground;
Try to use single point grounding;
Try to widen the ground wire;
Connect the sensitive circuit to a stable ground reference source;
Partition design of the PCB board to separate the high-bandwidth noise circuit from the low-frequency circuit;
Minimize the area of the ground loop (the path formed by returning all devices to the power ground after being grounded is called the "ground loop").

3. Configuration of components
Do not have too long parallel signal lines;
Ensure that the clock generator, crystal oscillator and clock input of the CPU are as close as possible to the PCB, while keeping away from other low-frequency components;
The components should be configured around the core components, and the lead length should be minimized;
Partition layout of PCB board;
Consider the position and direction of the PCB board in the chassis;
Shorten the leads between high-frequency components.

4. Configuration of decoupling capacitors
Add a charge and discharge capacitor (10uF) for every 10 integrated circuits;
Leaded capacitors are used for low frequencies, and chip capacitors are used for high frequencies;
A 0.1µF ceramic capacitor shall be arranged for each integrated chip;
The anti-noise ability is weak, and high-frequency decoupling capacitors should be added to the devices with large power changes when shutting down;
Do not share vias between capacitors;
Decoupling capacitor leads should not be too long.

5. Principles of reducing noise and electromagnetic interference
Try to use a 45° fold line instead of a 90° fold line (to minimize the external emission and coupling of high-frequency signals);
Use series resistance to reduce the jump rate of the circuit signal edge;
The shell of the quartz crystal oscillator should be grounded;
Don’t float the circuits that are not in use;
When the clock is perpendicular to the IO line, the interference is small;
Try to make the electromotive force around the clock tend to zero;
The IO drive circuit is as close as possible to the edge of the PCB;
Any signal should not form a loop;
For high-frequency boards, the distributed inductance of the capacitor cannot be ignored, nor can the distributed capacitance of the inductor;
Usually the power line and AC line should be on a different board from the signal line as much as possible.

6. Other design principles
The unused pins of CMOS should be connected to the ground or power supply (usually directly connected to the ground) through a resistor;
Use RC circuit to absorb the discharge current of relays and other original components;
Adding about 10kΩ pull-up resistor on the bus is helpful for anti-interference;
Using full decoding has better anti-interference;
The components do not need pins to connect to the power supply through a 10k resistor;
The bus should be as short as possible and try to keep the same length;
The wiring between the two layers should be as vertical as possible;
Avoid sensitive components with heating components;
The front side is routed horizontally, and the reverse side is routed longitudinally. As long as space permits, the thicker the wiring, the better (only ground wire and power wire);
To have a good ground line, try to route the line from the front side, and use the back side as a ground line;
Keep a sufficient distance, such as the input and output of the filter, the input and output of the optocoupler, the AC power line and the weak signal line, etc.;
Long line plus low-pass filter. The trace should be as short as possible, and the long line that has to be taken should be inserted in a reasonable position with a C, RC, or LC low-pass filter;
Except for the ground wire, do not use thick wires if you can use thin wires.

7. Wiring width and current
Generally, the width should not be less than 0.2.mm (8mil);
On high-density and high-precision PCBs, the spacing and line width are generally 0.3mm (12mil);
When the thickness of the copper foil is about 50um, the wire width is 1～1.5mm (60mil) = 2A;
The common area is generally 80mil, and more attention should be paid to applications with microprocessors.

8. Power cord
The power cord should be as short as possible, in a straight line, preferably in a tree shape, not a loop.

9. Layout
First, consider the PCB size. When the PCB size is too large, the printed lines will be long, the impedance will increase, the anti-noise ability will decrease, and the cost will also increase; if the PCB size is too small, the heat dissipation will not be good, and adjacent lines will be easily disturbed.
After determining the PCB size, determine the location of the special components. Finally, according to the functional units of the circuit, all the components of the circuit are laid out.
The following principles should be observed when determining the location of special components:
Try to shorten the wiring between high-frequency components as much as possible, try to reduce their distribution parameters and mutual electromagnetic interference. Components that are susceptible to interference should not be too close to each other, and input and output components should be kept as far away as possible.
There may be a high potential difference between some components or wires, and the distance between them should be increased to avoid accidental short circuits caused by discharge. The components with high voltage should be arranged as far as possible in places that are not easily reachable by hands during debugging.
Components weighing more than 15g should be fixed with brackets and then welded. Those components that are large, heavy, and generate a lot of heat should not be mounted on the printed circuit board, but should be mounted on the chassis bottom plate of the whole machine, and the heat dissipation problem should be considered. Thermal components should be far away from heating components.
For the layout of adjustable components such as potentiometers, adjustable inductors, variable capacitors, and micro switches, the structural requirements of the whole machine should be considered. If it is adjusted inside the machine, it should be placed on the printed circuit board where it is convenient for adjustment; if it is adjusted outside the machine, its position should match the position of the adjustment knob on the chassis panel.
The position occupied by the positioning hole of the printed board and the fixed bracket should be reserved. When laying out all the components of the circuit according to the functional units of the circuit, the following principles must be met:
Arrange the position of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal circulation, and the signal is kept in the same direction as possible.
Take the core component of each functional circuit as the center and lay out around it. The components should be evenly, neatly and compactly arranged on the PCB. Minimize and shorten the leads and connections between the components.
For circuits operating at high frequencies, the distributed parameters between components must be considered. Generally, the circuit should be arranged in parallel as much as possible. In this way, it is not only beautiful, but also easy to install and weld. It is easy to mass produce.
The components located at the edge of the circuit board are generally not less than 2mm away from the edge of the circuit board. The best shape of the circuit board is rectangular. The aspect ratio is 3:2 to 4:3. The size of the circuit board is greater than 200x1.

The above is how the circuit board factory designs the anti-interference PCB board. Ipcb also provides PCB manufacturers, PCB manufacturing technology and so on.