PCB Tech - Location of wiring and components of the printed circuit board

1. The usual order of placing components on the printed 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.

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 printed circuit boards. The components of the high voltage circuit part and the low voltage part should be separated from each other. 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 above 3.5mm. In many cases, it is to avoid Creepage, also slotting between the high and low voltage on the printed circuit board.

2. The wiring of the printed circuit board:

The layout of the printed wires should be as short as possible, especially in high-frequency circuits; the bends of the printed wires should be rounded, and the right or sharp corners will affect the electrical in the case of high-frequency PCB circuits and high wiring density. Performance; when wiring two panels, 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.

The width of the printed wire: the width of the wire should meet the electrical performance requirements and be convenient for production. Its minimum value is determined by the current size, but the minimum should not be less than 0.2mm. In high-density, high-precision printing In the circuit, the wire width and spacing can generally be 0.3mm; the wire width should also consider its temperature rise 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 to 1.5mm, and the passing current is 2A, The temperature rise is very small. Therefore, generally choosing 1~1.5mm width wire 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 a wire larger than 2~3mm. This is particularly important in circuits with microprocessors, because when the ground wire is too thin, due to changes in the current flowing, the ground potential changes, and the level of the microprocessor timing signal is unstable, which will degrade the noise margin; The 10-10 and 12-12 principles can be applied to the wiring between the IC pins of the DIP package, that is, when two wires pass between the two pins, the pad diameter can be set to 50mil, and the line width and line spacing are both 10mil. 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 12 mils.

3. the spacing 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.

4. The shielding and grounding of printed wires

The common ground wire of the printed wire should be arranged on the edge of the printed 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, generally the ground layer and power layer are designed on the inner layer of the multi-layer PCB, and the signal wires are designed on the inner and outer layers.