PCB design, engineers how to avoid non streaming skills!

PCB design, engineers how to avoid non streaming skills!

Many people like design. When designing, people should first master the most basic design skills, and then continue to practice and research. For those who aspire to be engineers, the sketchpad is a hard martial art. If you do not practice, you will not succeed. Even if you can record all the characteristic curves of MOS tubes, you will not be popular after all. Today, I would like to share with you how engineers can avoid non streaming in PCB design!

General PCB basic design process is as follows:

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

1、 Pre preparation includes preparing catalogs and schematics

"If a worker wants to do his job well, he must first sharpen his tools." To make a good board, he must design the principle well and draw it well. Before PCB design, first prepare the component library of schematic SCH and PCB. You can use Peotel's own library for component library, but it is difficult to find a suitable one in general. It is better to make your own component library according to the standard size information of the selected components. In principle, the component library of PCB should be built first, and then the component library of SCH.

PCB component library has high requirements, which directly affects the installation of boards; The component library requirements of SCH are relatively loose. Just pay attention to defining the pin attributes and the corresponding relationship with PCB components. PS: Note the hidden pins in the standard library. After that, it is the schematic design. After that, it is ready to start PCB design.

2、 PCB structure design

In this step, the PCB surface is drawn in the PCB design environment according to the determined PCB size and various mechanical positioning, and the required connectors, keys/switches, screw holes, assembly holes, etc. are placed according to the positioning requirements. And fully consider and determine the wiring area and non wiring area (such as how large the area around the screw hole belongs to the non wiring area).

3、 PCB layout

The layout is to put devices on the board. At this time, if all the preparations mentioned above are completed, you can generate a network list (Design ->Create Netlist) on the schematic diagram, and then import a network list (Design ->Load Nets) on the PCB diagram. The device is piled up in a crash, and there are flying wires between pins to prompt connection. Then you can lay out the device. The general layout shall follow the following principles:

1 According to the reasonable division of electrical performance, it is generally divided into: digital circuit area (i.e., fear of interference and generate interference), analog circuit area (fear of interference), power drive area (interference source);

2 Circuits that complete the same function shall be placed as close as possible, and all components shall be adjusted to ensure the most concise connection; At the same time, adjust the relative position between the function blocks to make the connection between the function blocks the most concise;

3 For components with high quality, the installation position and strength shall be considered; The heating element shall be placed separately from the temperature sensing element, and thermal convection measures shall be considered when necessary;

4 The I/O driver shall be close to the edge of the printed board and the outlet connector as far as possible;

5 The clock generator (such as crystal oscillator or clock oscillator) shall be as close as possible to the device using the clock;

6. A decoupling capacitor shall be added between the power input pin of each integrated circuit and the ground (generally, a monolithic capacitor with good high-frequency performance is used); When the circuit board space is dense, a tantalum capacitor can also be added around several integrated circuits.

7. Discharge diode shall be added at relay coil (1N4148 is enough);

8 The layout shall be balanced, dense and orderly, and shall not be top heavy or head heavy. Special attention shall be paid to the actual size (area and height) of components and the relative position between components when placing the element, so as to ensure the electrical performance of the circuit board and the feasibility and convenience of production and installation. At the same time, the placement of components shall be properly modified on the premise that the above principles can be reflected, Make them neat and beautiful. The same devices should be placed in order and in the same direction. They should not be placed in an "orderly manner".

This step is related to the overall image of the board and the difficulty of wiring in the next step, so we should make great efforts to consider it. In the layout, preliminary wiring can be made for uncertain places, and full consideration can be given.

4、 Wiring

Wiring is the most important process in PCB design. This will directly affect the performance of PCB. In the process of PCB design, there are generally three levels of wiring: first, routing, which is the most basic requirement for PCB design. If the lines are not connected and there are flying lines everywhere, it will be an unqualified board. It can be said that we haven't 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 to carefully adjust the wiring after wiring to achieve the best electrical performance. Next is beauty. If your wiring is connected, there is no place that affects the performance of electrical appliances. But at a glance, it is chaotic, colorful and colorful. Even if your electrical performance is good, it is still a piece of garbage in the eyes of others. This brings great inconvenience to testing and maintenance. The wiring shall be neat and uniform, and shall not be crisscross and disorganized. All these should be achieved under the premise of ensuring the electrical performance and meeting other individual requirements, otherwise, it will be a waste of time.

V. Main Principles for Wiring

In general, the power line and ground wire shall be wired first to ensure the electrical performance of the circuit board. Within the allowable range, the width of power supply and ground wire shall be widened as much as possible. It is better that the ground wire is wider than the power wire. Their relationship is: ground wire>power wire>signal wire. Generally, the width of signal wire is 0.2~0.3mm, the thinnest width can reach 0.05~0.07mm, and the power wire is generally 1.2~2.5mm. The PCB of digital circuit can be used to form a loop with a wide ground wire, that is, to form a ground grid for use (the ground of analog circuit cannot be used in this way).

1. Wires with strict requirements (such as high-frequency lines) shall be wired in advance, and the side lines of input and output terminals shall not be adjacent and parallel to avoid reflection interference. If necessary, ground wires shall be added for isolation, and the wiring of two adjacent layers shall be perpendicular to each other, which may cause parasitic coupling.

2. The oscillator shell shall be grounded, and the clock wire shall be as short as possible, and shall not be led everywhere. The area of the ground under the clock oscillation circuit and special high-speed logic circuit should be added, 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 wiring to reduce the radiation of high-frequency signal; (Double arc lines are also used for lines with high requirements)

4. No signal line shall form a loop. If it is unavoidable, the loop shall be as small as possible; The vias of signal lines shall be as few as possible;

5. The key lines shall be as short and thick as possible, and protective areas shall be added on both sides.

6. When transmitting sensitive signals and noise field band signals through flat cables, they should be led out in the way of "ground wire signal ground wire".

7. Test points shall be reserved for key signals to facilitate production, maintenance and detection. After the schematic wiring is completed, the wiring shall be optimized;

8. At the same time, after the preliminary network inspection and DRC inspection are correct, fill the ground wire in the area without wiring, use a large area of copper layer as the ground wire, and connect the unused places with the ground on the printed board as the ground wire. Or it can be made into multilayer boards, with one layer for power supply and one layer for ground wire