What is the printed circuit board wiring like?

What is the printed circuit board wiring like?

1.Wire

(1) width

的 The minimum width of the printed wire is mainly determined by the adhesion strength between the wire and the insulating substrate and the value of the current flowing through them. The printed wires can be as wide as possible, especially the power and ground wires, and as wide as the board allows, even if the area is tight, it is generally not less than 1mm. In particular, even if the ground wire is not allowed to be widened locally, it should be widened where it is allowed to reduce the resistance of the entire ground wire system. For wires longer than 80mm, even if the working current is not large, it should be widened to reduce the effect of the 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. In particular, the gate of the field effect transistor, the base of the triode and the high-frequency circuit should pay attention to the short wiring.

(3) Pitch

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 pitch. In order to facilitate operation and production, the spacing should be as wide as possible, and the minimum spacing should be selected to be at least suitable for the applied voltage. This voltage includes operating voltage, additional ripple voltage, overvoltage, and peak voltages due to other reasons. When there is mains voltage in the circuit, the spacing should be wider for safety reasons.

(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 cause imbalance in line impedance. Therefore, it is best to keep the width of the path constant. In wiring, it is best to avoid the use of right angles and acute angles. Generally, the corners should be greater than 90 °. The inner edges of a right-angle path can generate a concentrated electric field that generates noise coupled to adjacent paths. The 45 ° path is better than the right-angle and acute-angle paths. When the two wires meet at an acute angle, change the acute angle to a circle.

2, aperture and land size

The diameter of the component mounting hole should match the diameter of the lead of the component better, so that the diameter of the mounting hole is slightly larger than the diameter of the component lead (0.15 to 0.3) mm. Generally, the DIL package pins and most small components use 0.8mm apertures, and the pad diameter is about 2mm. For large-aperture pads, in order to obtain better adhesion, the ratio of pad diameter to aperture is about 2 for epoxy glass substrates and (2.5 to 3) for phenol paperboard substrates.

Vias are generally used in multi-layer PCBs. The 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. For 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 the number of vias per signal line is the same. And the number of vias should be reduced as much as possible. If necessary, a printed wire protection ring or protection wire should be set to prevent oscillation and improve circuit performance.