PCB Technical - PCB board design

PCB board design
The basic design process of a general PCB board is as follows:

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

1. Preliminary preparation includes preparing component libraries and schematic diagrams

"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 the PCB board design, we must first prepare the component library of the schematic SCH and the component library of the PCB board. 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, make the component library for PCB proofing first, and then do the component library for SCH.

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 board 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.

2. The structure design of the PCB manufacturer. 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).

Third, the layout of the PCB board

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. I saw that the devices were all piled up, and there were 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 heat 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, it can also be around several integrated circuits Add a tantalum capacitor.

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-headed-special attention is needed. When placing components, the actual size of the components (occupied area and height) and between components must be considered. In order to ensure the electrical performance of the circuit board and the feasibility and convenience of production and installation, 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, such as the same components. They should be arranged 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.

Four, 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 routed, there is nothing to affect the performance of the electrical appliances, but at first glance it looks messy, plus 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.

Five, the main principles when wiring

Under normal circumstances, the power line and ground line 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)

First, wire the lines with strict requirements (such as high-frequency lines). The edges 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.

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;

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

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;

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

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".

Test points should be reserved for key signals to facilitate production and maintenance testing. 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 unwiring area is filled with ground wire, a large area of copper layer is used as the ground wire, and the unused places on the printed circuit board are connected to the ground as Ground wire. Or it can be made into a multi-layer circuit board, and the power supply and ground wires occupy one layer each.