Analysis of Basic PCB Layout Principles

1.First, we will place the components that have structural requirements, paying attention to the placement position of pin 1 of the connector according to the imported structure.

2.During layout, attention should be paid to the height restrictions in the structure.

3.If you want the layout to be aesthetically pleasing, generally position components based on their outer frame or central coordinate (center-aligned).

4.The overall layout should consider heat dissipation.

5.During layout, it is necessary to consider the evaluation of wiring channels and the space required for equal lengths.

6.During layout, it is necessary to consider the power flow direction and evaluate the power channel.

7.High-speed, medium-speed, and low-speed circuits should be separated.

8.High current, high voltage, and strong radiation components should be kept away from low current, low voltage, and sensitive components.

9.Analog, digital, power, and protection circuits should be separated.

10.Interface protection devices should be placed as close to the interface as possible.

11.The sequence of interface protection devices is required:

• The typical sequence for power surge protection devices is: varistor, fuse, suppression diode, EMI filter, inductor or common mode inductor. For any missing components in the schematic, layout them sequentially.
• The typical sequence for interface signal protection devices is: ESD (TVS diode), isolation transformer, common mode inductor, capacitor, resistor. For any missing components in the schematic, layout them sequentially, strictly according to the schematic order (ability to judge the correctness of the schematic is required) in a “linear” layout.

12.Level shifting chips (such as RS232) should be placed close to the connector (such as serial port).

13.Devices that are susceptible to ESD interference, such as NMOS and CMOS devices, should be kept away from areas prone to ESD interference (such as the edges of the PCB).

14.Clock device layout:

• Crystals, oscillators, and clock distributors should be placed as close as possible to the relevant IC devices;
• The filter for the clock circuit (preferably use a “∏” type filter) should be close to the power input pin of the clock circuit;
• Check if there is a 22-ohm resistor in series with the output of the oscillator and clock distributor;
• Check if the unused output pins of the clock distributor are grounded through a resistor;
• The layout of crystals, oscillators, and clock distributors should be kept away from high-power components, heat sinks, and other heat-generating devices;
• Check if the distance from the crystal oscillator to the board edge and interface devices is greater than 1 inch.

15.Switching power supplies should be kept away from AD/DA converters, analog devices, sensitive devices, and clock devices.

16.Switching power supply layout should be compact, with input/output separated, and layout should be strictly according to the schematic requirements, without randomly placing the capacitors of the switching power supply.

17.Capacitors and filter devices:

• Capacitors must be placed close to the power pins, and the smaller the capacitance value, the closer it should be to the power pins;
• EMI filters should be close to the power input of the chip;
• In principle, each power pin should have a 0.1uF small capacitor, and each integrated circuit should have one or more 10uF large capacitors, which can be adjusted according to the specific situation.

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