PCB Board Design Precautions -2

Fourth: Wiring

Wiring is the most important process in the entire PCB design. This will directly affect the performance of the PCB board. In the process of PCB design, there are generally three divisions of wiring.

First, the layout is the most basic requirement for PCB design. If the wires are not connected and there are flying wires everywhere, it will be a substandard board, and it can be said that you have not yet started.

The second is the satisfaction of electrical performance. This is a measure of whether a printed circuit board is qualified. This is after the deployment, carefully adjust the wiring to achieve the best electrical performance.

Then comes aesthetics. If your wiring is properly routed, there is nothing to affect the performance of the electrical appliance, but at first glance, it is messy, plus colorful, then no matter how good your electrical performance is, it is still a piece of rubbish in the eyes of others. This brings great inconvenience to testing and maintenance. The wiring should be neat and uniform, not crisscross and out of order. All of these must be achieved while ensuring the performance of electrical appliances and meeting other individual requirements.
The wiring is mainly carried out according to the following principles:

① Generally, the power wire and ground wire should be wired first to ensure the electrical performance of the circuit board. As far as the conditions allow, widen the width of the power and ground wires as much as possible, preferably the ground wire is wider than the power wire, their relationship is ground wire> power wire> signal wire, usually the signal wire width is 0.2 ~ 0.3mm. The smallest width can reach 0.05～0.07mm, and the power cord is generally 1.2～2.5mm. For the PCB of the digital circuit, a wide ground wire can be used to form a loop, that is, to form a ground net to use (the ground of the analog circuit cannot be used in this way).

②. Wire lines with strict requirements (such as high-frequency lines) in advance. The edges of the input end and the output end should be avoided adjacent to parallel to avoid reflection interference. If necessary, ground isolation should be added. The wiring of two adjacent layers should be perpendicular to each other. Parasitic coupling is likely to occur in parallel.

③ The shell of the oscillator shall be grounded, and the clock line should be as short as possible, and it should not be drawn everywhere. Below the clock oscillation circuit, the special high-speed logic circuit part should increase the area of the ground, and should not take other signal lines to make the surrounding electric field approach zero.

④ Use 45 degree polyline wiring as much as possible, and 90 degree polyline should not be used to reduce the radiation of high-frequency signals. (Highly demanding lines should also use double-curved lines.)

⑤ Do not form a loop on any signal line. If it is unavoidable, the loop should be as small as possible. Signal line vias should be as few as possible.

⑥ The key line should be as short and thick as possible, and protective ground should be added on both sides.

⑦ When transmitting sensitive signals and noisy 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 testing.

⑨ After the schematic diagram wiring is completed, the wiring should be optimized. At the same time, after the preliminary network inspection and DRC check are correct, fill the unwiring area with ground wire, use a large area of copper layer as the ground wire, and use it on the printed board. The unused places are connected to the ground as a ground wire. Or it can be made into a multilayer board, and the power supply and ground wires occupy one layer each.

Requirements:

① Line

In general, the signal line width is 0.3mm (12mil), and the power line width is 0.77mm (30mil) or 1.27mm (50mil). The distance between the line and line and the pad is greater than or equal to 0.33mm (13mil) ), in actual applications, consider increasing the distance when conditions permit.

When the wiring density is high, you can consider (but not recommend) 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 Pad

The basic requirements for pads (PAD) and transition holes (VIA) are that the diameter of the disk is greater than the diameter of the hole by 0.6mm. For example, general-purpose pin resistors, capacitors, and integrated circuits, etc., use a disk/hole size of 1.6mm/0.8 mm (63mil/32mil). Sockets, pins and diodes 1N4007, etc., adopt 1.8mm/1.0mm (71mil/39mil). In actual applications, it should be determined according to the size of the actual component. If conditions permit, the pad size can be appropriately increased.

The component mounting aperture designed on the PCB board should be about 0.2～0.4mm larger than the actual size of the component pin.

③ Via

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, consider using 1.0mm/0.6mm (40mil/24mil).

④. Pitch 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 higher:

PAD and VIA: ≥ 0.254mm(10mil)

PAD and PAD: ≥ 0.254mm(10mil)