PCB Tech - Summary of PCB wiring experience how much do you know

Wiring is the most important process in the entire PCB design. This will directly affect the performance of the PCB board. In the PCB design process, there are generally three divisions of wiring: first, wiring, which is the most basic requirement for PCB design. If the lines are not connected and there are flying lines 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, so that it can achieve the best electrical performance. Then comes aesthetics. If your wiring is properly routed, there is nothing that affects the performance of the electrical appliance, but at first glance it is messy and colorful and 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, otherwise it will be the end of the day.

1. The wiring is mainly carried out according to the following principles:

1. . Under normal circumstances, the power line and ground line should be wired first to ensure the electrical performance of the circuit board. Within the scope permitted by conditions, try to widen the width of the power and ground lines, preferably the ground line is wider than the power line, their relationship is: ground line>power line>signal line, usually signal line width: 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 the lines with strict requirements (such as high-frequency lines) in advance, and the edge lines of the input end and the output end should be avoided adjacent to parallel to avoid reflection interference. If necessary, ground wire should be added for isolation, and the wiring of two adjacent layers should be perpendicular to each other. Parasitic coupling is easy to occur in parallel.

3. . The housing of the oscillator is grounded, and the clock line should be as short as possible, and it should not be drawn everywhere. The area of the ground under the clock oscillation circuit and the special high-speed logic circuit should be enlarged, and other signal lines should not be used to make the surrounding electric field approach zero;

4. . Use 45o polyline wiring as much as possible, not 90o polyline to reduce the radiation of high-frequency signals; (highly demanding lines should also use double-curved lines)

5. . Any signal line should not form a loop. If it is unavoidable, the loop should be as small as possible; the signal line’s 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. . Key signals should be reserved for testing points to facilitate production and maintenance testing.

9. . After the schematic wiring is completed, the wiring should be optimized; at the same time, after the preliminary network inspection and DRC check are correct, the unwiring area is filled with ground wire, and a large area of copper layer is used as the ground wire. All places used are connected to the ground as a ground wire. Or it can be made into a multi-layer board, and the power supply and ground wire occupy one layer each.

--PCB wiring process requirements

1. . String

In general, the signal line width is 0.3mm (12mil), the power line width is 0.77mm (30mil) or 1.27mm (50mil); the distance between the line and the line and the pad is greater than or equal to 0.33mm (13mil) ), in actual applications, increase 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 (PAD)

The basic requirements for pads (PAD) and transition holes (VIA) are: 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, and the pad size can be appropriately increased when conditions permit;

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 (VIA)

Generally 1.27mm/0.7mm (50mil/28mil);

When the wiring density is high, the size of the via can be appropriately reduced, but it should not be too small, and 1.0mm/0.6mm (40mil/24mil) can be considered.

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)

PAD and TRACK: ≥ 0.254mm (10mil)

TRACK and TRACK: ≥ 0.254mm (10mil)

2. Wiring optimization and silk screen printing. "There is no best, only better"! No matter how you deliberately design, wait for you to finish drawing, and then take a look, you will still think that many places can be modified. The general design experience is: the time to optimize the wiring is twice the time of the first wiring. After feeling that there is nothing to modify, you can lay the copper (Place->polygon Plane). The copper is generally grounded (note the separation of analog ground and digital ground), and power supply may also be required for multilayer boards. When it comes to silk screen printing, be careful not to be blocked by the device or removed by vias and pads. At the same time, the design should face the component surface, and the words on the bottom layer should be mirrored so as not to confuse the layer.

3.: Network and DRC inspection and structure inspection. First of all, on the premise that the circuit schematic design is correct, the generated PCB network file and the schematic network file are physically connected to the network check (NETCHECK), and the design is revised in time according to the output file result to ensure the wiring The correctness of the connection relationship;

After the network check is passed correctly, the PCB design is checked for DRC, and the design is revised in time according to the output file results to ensure the electrical performance of the PCB wiring. Finally, it is necessary to further check and confirm the mechanical installation structure of the PCB.

4: Plate making. Prior to this, it is best to have a review process.

PCB design is a thought-provoking job. Whoever is thoughtful and experienced should just design a board. Therefore, you must be extremely careful when designing, fully consider various factors (for example, easy maintenance and inspection are not considered by many people), keep improving, and you will be able to design a good board.