PCB Tech - PCB printed circuit board layout experience

For electronic products, printed circuit board design is a design process necessary for it to change from an 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 For people, they have less experience in this area. Although they have learned printed circuit board design software, the printed circuit boards they designed often have problems of this kind, and there are few articles in this area in many electronic publications.

PCB layout:

The usual order of placing the components on the PCB circuit board:

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

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

Place small devices. The distance between the components and the edge of the board: if possible, all components should be placed within 3mm from the edge of the board or at least greater than the board thickness. This is because the assembly line plug-in and wave soldering in mass production must be provided to the guide groove In order to prevent the defect of the edge part due to the shape processing, if there are too many components on the printed circuit board, if it is necessary to exceed the 3mm range, you can add a 3mm auxiliary edge to the edge of the board, and the auxiliary edge shall be V-shaped The groove can be broken by hand during production.

Isolation between high and low voltage: There are both high voltage and low voltage circuits on many PCB circuit boards. The components of the high voltage circuit part should be separated from the low voltage part. The isolation distance is related to the withstand voltage to be withstood. Normally, At 2000kV, the distance between the board should be 2mm, and the distance should be increased in proportion to this. For example, if you want to withstand the 3000V withstand voltage test, the distance between the high and low voltage lines should be more than 3.5mm. In many cases, to avoid climbing Electricity is also slotted between the high and low voltage on the printed circuit board.

The wiring of the printed circuit board:

The layout of printed wires should be as short as possible, especially in high-frequency circuits; the bends of printed wires should be rounded, and right or sharp corners will affect electrical performance in high-frequency circuits and high wiring density. ; When the two panels are wired, the wires on both sides should be perpendicular, oblique, or bent to avoid parallel to each other to reduce parasitic coupling; printed wires used as the input and output of the circuit should be avoided as far as possible. In order to avoid feedback, it is best to add a ground wire between these wires.

Width of printed wire:

The width of the wire should be such that it can meet the electrical performance requirements and is convenient for production. Its minimum

The value is determined by the magnitude of the current, but the minimum should not be less than 0.2mm. In high-density PCB and high-precision PCB printed circuits, the wire width and spacing can generally be 0.3mm; the wire width should also consider its temperature in the case of large currents. Single-panel experiments show that when the thickness of the copper foil is 50μm, the wire width is 1～1.5mm, and the passing current is 2A, the temperature rise is very small. Therefore, the general selection of 1～1.5mm width wire may meet the design requirements without causing temperature. The common ground wire of the printed wire should be as thick as possible. If possible, use a line larger than 2~3mm. This is particularly important in circuits with microprocessors, because when the ground wire is too thin, the Changes in current, changes in ground potential, and unstable levels of microprocessor timing signals will degrade the noise margin; the principles of 10-10 and 12-12 can be applied to the wiring between the IC pins of the DIP package. When two wires pass between the legs, the pad diameter can be set to 50 mils, and the line width and line spacing are both 10 mils. When only one wire passes between the two legs, the pad diameter can be set to 64 mils, and the line width and line spacing are both It is 12mil.

Pitch of printed wires:

The distance between adjacent wires must be able to meet electrical safety requirements, and in order to facilitate operation and production, the distance should be as wide as possible. The minimum distance must be at least 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 residue between the wires, the spacing will be reduced. Therefore, the designer should take this factor into consideration when considering the voltage. When the wiring density is low, the spacing of the signal lines can be appropriately increased, and the signal lines with high and low levels should be as short as possible and the spacing should be increased.

Shielding and grounding of printed wires:

The common ground wire of the printed wire should be arranged on the edge of the PCB circuit board as far as possible. Keep as much copper foil as the ground wire on the printed circuit board. The shielding effect obtained in this way is better than that of a long ground wire. The transmission line characteristics and shielding effect will be improved, and the distributed capacitance will be reduced. . The common ground of the printed conductors is best to form a loop or a mesh. This is because when there are many integrated circuits on the same board, especially when there are more power-consuming components, the ground potential difference is generated due to the limitation of the pattern., Resulting in the reduction of noise tolerance, when it is made into a loop, the ground potential difference is reduced. In addition, the graphics of grounding and power supply should be as parallel as possible to the direction of data flow. This is the secret of enhancing the ability to suppress noise; multi-layer printed circuit boards can adopt several layers as shielding layers, and the power layer and ground layer are both visible. For the shielding layer, the ground layer and power layer are generally designed on the inner layer of the multi-layer PCB circuit board, and the signal wires are designed on the inner and outer layers.