PCB Design Tips

Preliminary preparation—PCB structure design—PCB layout—wiring—wiring optimization and silk screen printing—network and DRC inspection and structural inspection—plate making.

1. Preliminary preparations include the preparation of component libraries and schematic diagrams
“If a worker wants to be good at his work, he must first sharpen his tools.” To make a good board, in addition to designing the principles well, it is also necessary to draw well. Before PCB design, the component library of the schematic diagram SCH and the component library of the PCB must be prepared first. The component library can use the library that comes with peotel, but it is difficult to find a suitable one in general. It is best to make the component library by yourself according to the standard size data of the selected device. In principle, make the PCB component library first, and then the SCH component library.
PCB component library requirements are relatively high, which directly affects board installation; SCH component library requirements are relatively loose, as long as the pin attributes and the corresponding relationship with PCB components are well defined. PS: Note 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.

2. PCB structure design
     In this step, draw the PCB surface in the PCB design environment according to the determined circuit 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).

3. PCB layout
To put it bluntly, layout is to place components on the board. At this time, if the preparatory work mentioned above is done well, the netlist (Design-“Create Netlist) can be generated on the schematic diagram, and then the netlist (Design-“Load Nets) can be imported on the PCB diagram. I saw all the components piled up, and there were flying wires between the pins to indicate the connection. Then you can place the device. The general layout is carried out according to the following principles:
a) According to the reasonable division of electrical performance, it is generally divided into: digital circuit area (that is, fear of interference and interference), analog circuit area (fear of interference), and power drive area (interference source);
b) Circuits that complete the same function should be placed as close as possible, and each component should be adjusted to ensure the simplest connection; at the same time, the relative position of each functional block should be adjusted to make the connection between the functional blocks the simplest;
c) For components with large mass, the installation location and installation strength should be considered; heating components should be placed separately from temperature sensitive components, and heat convection measures should also be considered if necessary;
d) The I/O drive device should be as close to the edge of the printed board as possible, and close to the lead-out connector;
e) The clock generator (such as: crystal oscillator or clock oscillator) should be as close as possible to the device that uses the clock;
f) 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 circuit board space is dense, it can also be placed around several integrated circuits Add a tantalum capacitor.
g) A discharge diode (1N4148 is enough) should be added to the relay coil;
h) The layout requirements should be balanced, dense and orderly, not top-heavy or heavy–special attention should be paid, when placing components, the actual size of the components (occupied area and height), the distance between the components and the size of the components must be considered. In order to ensure the electrical performance of the circuit board and the feasibility and convenience of production and installation, at the same time, on the premise of ensuring that the above principles can be reflected, the placement of the devices should be properly modified to make them neat and beautiful. Arrange neatly and in the same direction, and cannot be placed in a “patchwork” manner. This step is related to the overall image of the board and the difficulty of the next step of wiring, so it takes a lot of effort to consider. When laying out, you can make preliminary wiring for places that are not sure, and give full consideration.

4. Wiring
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, the wiring generally has such three levels of division: the first is the routing, which is the most basic requirement for PCB design. If the lines are not connected and there are flying wires everywhere, it will be a substandard board, and it can be said that it has not been started yet. The second is the satisfaction of electrical performance. This is the standard to measure whether a printed circuit board is qualified. This is after Bouton, carefully adjust the wiring so that it can achieve the best electrical performance. Then there is beauty. If your wiring is clear, there is nothing that affects the performance of the electrical appliances, but at a glance, it is messy, colorful and colorful, then no matter how good your electrical appliances are, it is still a piece of garbage in the eyes of others. This brings great inconvenience to testing and maintenance. The wiring should be neat and uniform, not criss-cross and disorderly. All of these must be realized under the condition of ensuring the performance of electrical appliances and meeting other individual requirements, otherwise it will be a waste of money.

Five main principles when wiring
Generally, the power and ground wires should be routed first to ensure the electrical performance of the circuit board. As far as conditions permit, try to widen the width of the power supply and ground wires. It is best that the ground wires are wider than the power wires. Their relationship is: ground wire>power wire>signal wire, and the signal wire width is usually: 0.2~0.3mm , the thinnest 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 network (the ground of the analog circuit cannot be used in this way).
1. Pre-wire the lines with relatively strict requirements (such as high-frequency lines), and the side lines of the input end and the output end should not be adjacent to each other in parallel to avoid reflection interference. If necessary, ground isolation should be added, and the wiring of two adjacent layers should be perpendicular to each other, and parasitic coupling is easy to occur in parallel.
2. The shell of the oscillator is grounded, and the clock line should be as short as possible, and it cannot be lead everywhere. Under the clock oscillation circuit and the special high-speed logic circuit, the area of the ground should be increased, and other signal lines should not be used to make the surrounding electric field approach zero;
3. Try to use 45o broken line wiring instead of 90o broken line to reduce the radiation of high-frequency signals;
4. Do not form a loop on any signal line. If it is unavoidable, the loop should be as small as possible; the vias of the signal line should be as few as possible;
5. The key line should be as short and thick as possible, and protective ground should be added on both sides.
6. When transmitting sensitive signals and noise field signals through flat cables, use the “ground wire-signal-ground wire” method to lead out.
7. Test points should be reserved for key signals to facilitate production and maintenance testing. After the wiring of the schematic diagram is completed, the wiring should be optimized;
8. At the same time, after the preliminary network inspection and DRC inspection are correct, fill the unwired area with ground wires, use a large area of copper layer as the ground wire, and connect the unused places to the ground on the printed board Used as a ground wire. Or make a multi-layer board, the power supply and the ground wire each occupy one layer

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