PCB layout experience

- Nov 17, 2018-

Distance of components from edge of PCB board:

Possible all the components are placed within 3 mm to the edge of the PCB board or at least is greater than the thickness of the PCB, this is because when mass production assembly line plug-in and wave soldering, to provide for the use of guide groove, and at the same time in order to prevent the defect caused by edge contour machining parts, if too much on the PCB components, forced to beyond 3 mm, can add 3 mm at the edge of the PCB and auxiliary, auxiliary edge V trough, with the hand break off when in production.

Isolation between high and low pressure:

On many PCB have high voltage and low voltage circuit, at the same time of the components of high voltage circuit with low voltage part to open space, distance between related to bear pressure, is often the case in the 2000 kv distance on the PCB to 2 mm, on top of this ratio is also increased, for example, if you want to bear the pressure test of 3000 v, is the distance between the high and low voltage lines should be above 3.5 mm, in many cases to avoid creepage, also on the PCB board slot between high and low pressure.

Routing of PCB board:

The layout of printed conductors should be as short as possible, especially in high frequency circuits;The corner of printed conductor should be rounded, while the right Angle or sharp Angle will affect the electrical performance in the case of high frequency circuit and high wiring density.When the two panels are wired, the conductors on both sides should be perpendicular, oblique, or bent to run the wires, so as to avoid being parallel to each other, so as to reduce parasitic coupling;Printed conductors used as input and output of the circuit should be kept as far as possible from being adjacent and parallel to prevent feedback. It is best to put a ground wire between these conductors.

Width of printed conductor:

The width of the conductor shall be suitable for meeting the electrical performance requirements and convenient production. The minimum value of the conductor shall be determined by the current to be borne, but the minimum value shall not be less than 0.2mm.In high-density and high-precision printed lines, the width and spacing of wires are generally 0.3mm.Under the condition of large current, the wire width should also be considered with its temperature rise. The single-panel experiment shows that when the thickness of copper foil is 50 oxidation m, the wire width is 1 ~ 1.5mm, and the passing current is 2A, the temperature rise is very small. Therefore, generally, the wire with a width of 1 ~ 1.5mm can meet the design requirements without causing temperature rise.

The common ground of printed wires should be as thick as possible, using lines larger than 2 ~ 3mm if possible, especially in circuits with microprocessors.Because when the local line is too thin, due to the change of the current flowing through, the ground potential changes, the microprocessor timing signal level is not stable, will make the noise tolerance degradation;The principle of 10-10 and 12-12 can be applied to line between IC feet in DIP package, that is, when two wires are passed between the two feet, the diameter of the pad can be set to 50mil, the width and distance of the wire can be set to 10mil, and when only one wire is passed between the two feet, the diameter of the pad can be set to 64mil, the width and distance of the wire can be set to 12mil.

Spacing of printed wires:

The spacing between adjacent conductors must meet the electrical safety requirements and should be as wide as possible for ease of operation and production.Minimum spacing should be at least adequate to withstand voltage.This voltage generally includes the operating voltage, the additional fluctuation voltage and the peak voltage caused by other reasons.

If the technical conditions permit some degree of metal residue between the wires, the spacing between the wires will be reduced.Therefore, the designer should take this factor into consideration when considering the voltage.When the density of wiring is low, the spacing of signal lines can be appropriately increased, and the gap between 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 of printed wires shall be arranged as far as possible on the edge of printed circuit board.As much as possible, copper foil should be retained as the ground wire on the PCB board. In this way, the shielding effect is better than a long ground wire. The transmission line characteristics and shielding effect will be improved, and the distributed capacitance can be reduced.

The common ground line of printed conductor is better to form a loop or network. This is because when there are many integrated circuits on the same plate, especially the components that consume a lot of electricity, the ground potential difference is generated due to the restriction on the graph, which leads to the reduction of noise tolerance. When the circuit is formed, the ground potential difference is reduced.

In addition, the graph of ground and power source should be as parallel as possible to the flow direction of data, which is the secret of suppressing noise enhancement.The multi-layer PCB board can adopt several layers as shielding layer, the power supply layer and ground wire layer can be considered as shielding layer. Generally the ground wire layer and power supply layer are designed in the inner layer of the multi-layer PCB board, and the signal wire is designed in the inner layer and outer layer.