Wiring is the most important process in the entire PCB design, which will directly affect the performance of the PCB board. When engineers are doing PCB wiring, they must not only follow some principles, but also meet the process requirements of power lines, pads, vias, etc.
Process requirements for PCB wiring
1. Line
Under normal circumstances, the signal line width is 0.3mm (12mil), and the power line width is 0.77mm (30mil) or 1.27mm (50mil); the distance between lines and between lines and pads is greater than or equal to 0.33mm (13mil). In actual applications, when conditions permit, consider increasing the distance;
When the wiring density is high, it can be considered (but not recommended) to use two lines between IC pins, the line width is 0.254mm (10mil), and the line spacing is not less than 0.254mm (10mil). In special cases, when the device pins are dense and the width is narrow, the line width and line spacing can be appropriately reduced.
2. Pads
The basic requirements for pads (PADs) and transition holes (VIAs) are: the diameter of the pad should be larger than the diameter of the hole by 0.6mm; for example, general pin-type resistors, capacitors and integrated circuits use pad/hole sizes of 1.6mm/0.8mm (63mil/32mil), and sockets, pins and diodes 1N4007 use 1.8mm/1.0mm (71mil/39mil). In practical applications, it should be determined according to the size of the actual components. If conditions permit, the pad size can be appropriately increased;
The component mounting hole diameter designed on the PCB board should be about 0.2 to 0.4mm larger than the actual size of the component pin.
3. Vias
Generally 1.27mm/0.7mm (50mil/28mil);
When the wiring density is high, the via size can be appropriately reduced, but it should not be too small. You can consider using 1.0mm/0.6mm (40mil/24mil).
4. Spacing requirements for pads, lines, and vias
PAD and VIA: ≥ 0.3mm (12mil)
PAD and PAD: ≥ 0.3mm (12mil)
PAD and TRACK: ≥ 0.3mm (12mil)
TRACK and TRACK: ≥ 0.3mm (12mil)
When the density is high:
PAD and VIA: ≥ 0.254mm (10mil)
PAD and PAD: ≥ 0.254mm (10mil)
PAD and TRACK: ≥ 0.254mm (10mil)
TRACK and TRACK: ≥ 0.254mm (10mil)
Appendix: Basic principles of PCB wiring
1) In general, the power line and ground line should be wired first to ensure the electrical performance of the circuit board. As far as conditions allow, try to widen the width of the power and ground wires. It is best that the ground wire is wider than the power wire. The relationship between them is: ground wire > power wire > signal wire. Usually the signal wire width is: 0.2~0.3mm, the thinnest width can reach 0.05~0.07mm, and the power wire is generally 1.2~2.5mm. For the PCB of digital circuits, a wide ground wire can be used to form a loop, that is, to form a ground network for use (the ground of analog circuits cannot be used in this way)
2) Pre-wire the wires with stricter requirements (such as high-frequency wires). The edge wires of the input and output ends should avoid adjacent parallel to avoid reflection interference. If necessary, ground wire isolation should be added. The wiring of two adjacent layers should be perpendicular to each other. Parallel wiring is prone to parasitic coupling.
3) The oscillator shell is grounded, and the clock line should be as short as possible and cannot be led everywhere. Under the clock oscillator circuit and the special high-speed logic circuit part, the ground area should be increased, and other signal lines should not be used to make the surrounding electric field approach zero;
4) Use 45° folded line wiring as much as possible, and do not use 90° folded line to reduce the radiation of high-frequency signals; (high requirements should also use double arc lines)
5) No signal line should form a loop. If it is unavoidable, the loop should be as small as possible; the vias of the signal line should be as few as possible;
6) The key lines should be as short and thick as possible, and protective ground should be added on both sides.
7) When transmitting sensitive signals and noise field band signals through flat cables, they should be led out in the way of “ground wire-signal-ground wire”.
8) Test points should be reserved for key signals to facilitate production and maintenance and testing
9) After the schematic wiring is completed, the wiring should be optimized; at the same time, after the preliminary network inspection and DRC inspection are correct, the ground wire is filled in the unwired area, and a large area of copper layer is used as the ground wire. On the printed circuit board, all unused areas are connected to the ground as ground wires. Or it can be made into a multi-layer board, with the power supply and ground wire occupying one layer each.