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What about printed circuit board wiring?

2020/05/13 16:46
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1. Wire
(1) Width
The minimum width of the printed wire is mainly determined by the adhesion strength between the wire and the insulating substrate and the current value flowing through them. The printed conductor can be as wide as possible, especially the power cord and ground wire, as wide as possible under the conditions of the board surface, even if the area is tight, it is generally not less than 1mm. Especially the ground wire, even if it is not allowed to be widened locally, it should be widened where possible to reduce the resistance of the entire ground wire system. For wires with a length of more than 80mm, even if the working current is not large, it should be widened to reduce the influence of the wire voltage drop on the circuit.
(2) Length
To minimize the length of the wiring, the shorter the wiring, the less interference and crosstalk, and the lower its parasitic reactance and less radiation. Especially the gate of the field effect transistor, the base of the transistor and the high-frequency loop should pay attention to the short wiring.
(3) Spacing
The distance between adjacent wires should meet the requirements of electrical safety. Crosstalk and voltage breakdown are the main electrical characteristics that affect the wiring distance. In order to facilitate operation and production, the pitch should be as wide as possible, and the minimum pitch should be selected at least suitable for the applied voltage. This voltage includes working voltage, additional fluctuating voltage, overvoltage and peak voltage due to other reasons. When there is a mains voltage in the circuit, the spacing should be wider for safety.
(4) Path
The width of the signal path should be constant from drive to load. Changing the path width changes the path impedance (resistance, inductance, and capacitance), which can cause reflections and unbalanced line impedance. Therefore, it is best to keep the width of the path unchanged. In the wiring, it is best to avoid the use of right angles and acute angles, generally the corners should be greater than 90 °. The edges inside a right-angle path can generate a concentrated electric field that generates noise coupled to adjacent paths. A 45 ° path is better than a right-angle and acute-angle path. When the two wires meet and connect at an acute angle, the acute angle should be changed to round.
2. Aperture and pad size
The diameter of the component mounting hole should match the diameter of the component lead better, so that the diameter of the mounting hole is slightly larger than the component lead diameter (0.15 ~ 0.3) mm. Usually DIL package pins and most small components use 0.8mm aperture, and the pad diameter is about 2mm. In order to obtain better adhesion for large aperture pads, the ratio of the diameter of the pad to the aperture is about 2 for the epoxy glass substrate, and (2.5 to 3) for the phenol paperboard substrate.
Vias are generally used in multilayer PCBs. Its minimum usable diameter is related to the thickness of the substrate. Usually, the ratio of the thickness of the substrate to the diameter of the via is 6: 1. At high-speed signals, the via generates a path of (1 to 4) nH inductance and (0.3 to 0.8) pF capacitance. Therefore, when laying high-speed signal channels, vias should be kept to an absolute minimum. For high-speed parallel lines (such as address and data lines), if layer changes are inevitable, you should ensure that each signal line has the same number of vias. And the number of vias should be reduced as much as possible. If necessary, a printed wire protection ring or protection wire should be provided to prevent oscillation and improve circuit performance.