Experience exchange on PCB layout and design

For electronic products, PCB design is a necessary design process from the electrical schematic diagram to a specific product. The rationality of its design is closely related to product production and product quality. For many people who are just engaged in electronic design, they have less experience in this area. Although they have learned PCB design software, they often have problems of one kind or another. The engineer recommended by Xiao Bian has been engaged in PCB design for many years. He will share his experience in PCB layout with you, hoping to play a role in attracting talents.

The general order of placing components on PCB:

1. Place components in fixed positions closely matching with the structure, such as power sockets, indicator lights, switches, connectors, etc. After placing these components, lock them with the LOCK function of the software, so that they will not be moved by mistake in the future;

2. Place special components and large components on the circuit, such as heating components, transformers, ICs, etc;

3. Place the widget.

Distance between components and PCB edge:

If possible, all components and parts shall be placed within 3mm from the edge of the PCB board or at least greater than the thickness of the PCB board. This is because they should be provided for the guide rail slot when mass production assembly line plug-ins and wave soldering are used. At the same time, in order to prevent the edge part from being damaged due to contour processing, if there are too many components on the PCB board and it is necessary to exceed the 3mm range, 3mm auxiliary edge can be added to the edge of the PCB board, and the auxiliary edge can be opened with a V-shaped slot, It can be broken by hand during production.

Isolation between high and low voltage:

Many PCB boards have high voltage circuits and low voltage circuits at the same time. The components and parts of the high voltage circuit should be separated from the low voltage part. The isolation distance is related to the withstand voltage to be borne. Generally, the distance on the PCB board at 2000kV should be 2mm, which should be increased in proportion. For example, if the withstand voltage test of 3000V is to be borne, the distance between the high and low voltage lines should be more than 3.5mm. In many cases, to avoid creepage, Slots are also made between high and low voltage on the PCB.

PCB routing:

The layout of printed wires shall be as short as possible, especially in high-frequency circuits; The corners of printed wires should be rounded, and right angles or sharp corners will affect the electrical performance in the case of high frequency circuits and high wiring density; When wiring the two panels, the conductors on both sides should be perpendicular, oblique, or bent to avoid parallel, so as to reduce parasitic coupling; The printed wires used as the input and output of the circuit shall not be adjacent and parallel as much as possible to avoid feedback. It is better to add grounding wires between these wires.

Width of printed wire:

The wire width should meet the electrical performance requirements and be convenient for production. Its minimum value depends on the current size, but should not be less than 0.2mm. In high-density and high-precision printed circuit, the wire width and spacing can generally be taken as 0.3mm; The temperature rise of the conductor width should also be considered under the condition of large current. The single panel experiment shows that when the copper foil thickness is 50 μ m. When the conductor is 1~1.5mm wide and the current is 2A, the temperature rise is very small. Therefore, the conductor with a width of 1~1.5mm may meet the design requirements without causing temperature rise.

The common ground wire of the printed wire should be as thick as possible. If possible, use lines larger than 2-3 mm, which is particularly important in circuits with microprocessors. Because when the local line is too thin, the noise tolerance will be degraded due to the change of the current flowing, the change of the ground potential, and the unstable level of the microprocessor timing signal; The principles of 10 － 10 and 12 － 12 can be applied to the wiring between the IC pins of DIP package, that is, when two pins pass through two wires, the pad diameter can be set to 50 mil, and the line width and line spacing can be set to 10 mil. When only one wire passes between two pins, the pad diameter can be set to 64 mil, and the line width and line spacing can be set to 12 mil.

Spacing of printed wires:

The spacing between adjacent conductors must meet the electrical safety requirements, and in order to facilitate operation and production, the spacing should also be as wide as possible. The minimum spacing shall at least be suitable for the withstand voltage. This voltage generally includes working voltage, additional fluctuating voltage and peak voltage caused by other reasons.

If the relevant technical conditions allow a certain degree of metal residues between conductors, their spacing will be reduced. Therefore, the designer should take this factor into account when considering the voltage. When the wiring density is low, the spacing of signal lines can be appropriately increased, and the signal lines with high and low level differences should be as short as possible and the spacing should be increased.

Shielding and grounding of printed wire:

The common ground wire of the printed wire shall be arranged at the edge of the printed circuit board as far as possible. Copper foil should be reserved as much as possible on the PCB as ground wire, so that the shielding effect is better than a long ground wire, the transmission line characteristics and shielding effect will be improved, and the distributed capacitance will be reduced.

The common ground wire of the printed wire should preferably form a loop or network. This is because when there are many integrated circuits on the same board, especially the components that consume more power, the ground potential difference is generated due to the restrictions on the graphics, which will reduce the noise tolerance. When the circuit is made, the ground potential difference is reduced.

In addition, the figure of grounding and power supply should be parallel to the flow direction of data as much as possible, which is the secret of enhancing the ability to suppress noise; Several layers of multilayer PCB can be used as shielding layers. The power supply layer and ground wire layer can be regarded as shielding layers. Generally, the ground wire layer and power supply layer are designed in the inner layer of multilayer PCB, and the signal wire is designed in the inner layer and outer layer.