PCB News - PCB design process and steps

The general PCB design process is as follows: preliminary preparation -> PCB structure design -> PCB layout -> wiring -> wiring optimization and silk printing -> network and DRC inspection and structure inspection -> plate making.

First: preliminary preparation. This includes preparing component libraries and schematics. "If you want to do well, you must first sharpen your tools." To make a good board, in addition to designing the principles, you must also draw well. Before proceeding with PCB design, we must first prepare the component library of the schematic SCH and the component library of the PCB. The component library can use Peotel's own library, but it is generally difficult to find a suitable one. It is best to make your own component library based on the standard size data of the selected device. In principle, do the PCB component library first, and then do the SCH component library. The PCB component library requirements are high, which directly affects the installation of the board; the SCH component library requirements are relatively loose, as long as you pay attention to the definition of the pin attributes and the corresponding relationship with the PCB components. PS: Pay attention to the hidden pins in the standard library. After that is the design of the schematic diagram, and when it is done, it is ready to start the PCB design.

Second: PCB structure design. In this step, draw the PCB surface in the PCB design environment according to the determined PCB board size and various mechanical positioning, and place the required connectors, buttons/switches, screw holes, assembly holes, etc. according to the positioning requirements. And fully consider and determine the wiring area and non-wiring area (such as how much area around the screw hole belongs to the non-wiring area).

Third: PCB layout. To put it bluntly, the layout is to put devices on the board. At this time, if all the preparations mentioned above are done, you can generate the netlist (Design->Create Netlist) on the schematic diagram, and then import the netlist (Design->Load Nets) on the PCB diagram. You can see the whole stack of devices crashing, and there are flying wires between the pins to indicate the connection. Then you can lay out the device. The general layout is carried out according to the following principles:
1. . According to the reasonable division of electrical performance, it is generally divided into: digital circuit area (that is, afraid of interference and interference), analog circuit area (fear of interference), power drive area (interference source);
2. . Circuits that complete the same function should be placed as close as possible, and each component should be adjusted to ensure the most concise connection; at the same time, adjust the relative position between the functional blocks to make the connection between the functional blocks the most concise;
3. . For high-quality components, the installation location and installation strength should be considered; heating components should be placed separately from temperature-sensitive components, and thermal convection measures should be considered when necessary;
4. . The I/O drive device is as close as possible to the edge of the printed board and to the lead-out connector;
5. . The clock generator (such as crystal oscillator or clock oscillator) should be as close as possible to the device that uses the clock;
6. . Between the power input pin of each integrated circuit and the ground, a decoupling capacitor (generally a monolithic capacitor with good high-frequency performance is used); when the board space is dense, one can also be added around several integrated circuits Tantalum capacitors.
7. . A discharge diode should be added to the relay coil (1N4148 is sufficient);
8. . The layout requirements should be balanced, dense and orderly, not top-heavy or heavy

--Special attention is needed. When placing components, the actual size of the components (occupied area and height) and the relative position between the components must be considered to ensure the electrical performance of the circuit board and the feasibility of production and installation At the same time as convenience, the placement of the components should be appropriately modified under the premise of ensuring that the above principles can be reflected to make them neat and beautiful. For example, the same components should be placed neatly and in the same direction.
This step is related to the overall image of the board and the difficulty of wiring in the next step, so a little effort must be taken into consideration. When laying out, you can make preliminary wiring and fully consider the places that are not sure about it.

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 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 deployed, there is nothing to affect 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. 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 range allowed 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. Under the clock oscillation circuit, the area of 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, and 90º polyline should not be used to reduce the radiation of high-frequency signals; (the lines with high requirements 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; 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 multilayer board, and the power supply and ground wires occupy one layer each.

5.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 practical 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 the disk/hole size 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 (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)
PAD and TRACK: ≥ 0.254mm (10mil)
TRACK and TRACK: ≥ 0.254mm (10mil)

6: 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, face up to the component surface when designing, and the words on the bottom layer should be mirrored to avoid confusion.

7: 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;
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 board.

8: 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, many people don't consider it), and keep improving, you will be able to design a good board.