PCB board design needs to provide information:

(1) Schematic diagram: a complete electronic document format that can generate the correct netlist (netlist);

(2) Mechanical size: to provide the identification of the specific position and direction of the positioning device, as well as the identification of the specific height limit position area;

(3) BOM list: it mainly determines and checks the specified package information of the equipment on the schematic diagram;

(4) Wiring guide: description of specific requirements for specific signals, as well as impedance, lamination and other design requirements.

The basic design process of PCB board is as follows:

Prepare – & gt; PCB structure design – & GT; PCB layout – & GT; Wiring – & gt; Routing optimization and screen – > Network and DRC inspections and structural inspections – > PCB board.

1: Preliminary preparation

1) This includes preparing component libraries and schematics. “If you want to do something good, you have to hone your tools first.” In order to build a good board, in addition to designing principles, you must draw well. Before proceeding with the PCB design, you must first prepare the schematic SCH component library and the PCB component library (this is the first step – very important). Component libraries can use libraries that come with Protel, but it is often difficult to find the right one. It is best to build your own component library based on standard size data for your chosen device.

In principle, execute the PCB’s component library first, and then SCH’s. PCB component library has high requirement, which directly affects PCB installation. The SCH component library is relatively relaxed, as long as you are careful to define pin attributes and their correspondence to PCB components.

PS: Note the hidden pins in the standard library. Then comes the schematic design, and when it’s ready, the PCB design can begin.

2) When making the schematic library, note whether the pins are connected to the output/output PCB board and check the library.

2. PCB structure design

This step draws the PCB surface in the PCB design environment according to the determined board dimensions and various mechanical positions, and places the required connectors, buttons/switches, nixie tubes, indicators, inputs, and outputs according to the positioning requirements. , screw hole, installation hole, etc., fully consider and determine wiring area and non-wiring area (such as the scope of screw hole is non-wiring area).

Special attention should be paid to the actual size (occupied area and height) of the payment components, the relative position between components – the size of the space, and the surface on which the equipment is placed to ensure the electrical performance of the circuit board. While ensuring the feasibility and convenience of production and installation, appropriate modifications should be made to the equipment to keep it clean while ensuring that the above principles are reflected. If the same device is placed neatly and in the same direction, it cannot be placed. It’s a patchwork. “

3. The PCB layout

1) Make sure the schematic diagram is correct before layout – this is very important! —- – is very important!

Schematic diagram has been completed. Check items are: power grid, ground grid, etc.

2) The layout should pay attention to the placement of surface equipment (especially plug-ins, etc.) and the placement of equipment (vertically inserted horizontal or vertical placement), to ensure the feasibility and convenience of installation.

3) Place the device on the circuit board with white layout. At this point, if all of the above preparations are complete, you can generate a network table (design-gt; CreateNetlist), and then import the network table (Design- > LoadNets) on the PCB. I see the complete device stack, with flying wire prompt connections between pins, and then device layout.

The overall layout is based on the following principles:

In the layout when I’m lying down, you should determine the surface on which to place the device: in general, patches should be placed on the same side, and plug-ins should look for specifics.

1) According to the reasonable division of electrical performance, generally divided into: digital circuit area (interference, interference), analog circuit area (fear of interference), power drive area (interference source);

2) Circuits with the same function should be placed as close as possible, and components should be adjusted to ensure the simplest connection; At the same time, adjust the relative position between the function blocks, so that the connection between the function blocks is the most concise;

3) For high-quality parts, the installation position and installation intensity should be considered;Heating elements should be placed separately from temperature sensitive elements and, if necessary, thermal convection measures should be considered;

5) The clock generator (e.g. crystal or clock) should be as close as possible to the device using the clock;

6) Layout requirements should be balanced, sparse and orderly, not top-heavy or sunken.

4. The wiring

Wiring is the most important process in PCB design. This will directly affect the performance of PCB. In PCB design, wiring generally has three levels of division: the first is the connection, and then the most basic requirements of PCB design. If no wiring is laid and the wiring is flying, then it will be a substandard board. It’s safe to say it hasn’t started yet. The second is electrical performance satisfaction. This is a measure of printed circuit board conformity index. This is connected after careful adjustment of the wiring to achieve optimal electrical performance, followed by aesthetics. If your wiring is connected, then there is no place to affect the electrical performance, but in the past glance, there are a lot of bright, colorful, then how good your electrical performance, in the eyes of others is still a piece of garbage. This brings great inconvenience to testing and maintenance. Wiring should be neat and uniform, without rules and regulations. These must be achieved while ensuring electrical performance and other personalized requirements.

Wiring is carried out in accordance with the following principles:

1) Under normal circumstances, the power cord and ground wire should be wired first to ensure the electrical performance of the circuit board. Within these conditions, try to widen the power supply and ground wire widths. Ground cables are better than power cables. Their relation is: ground wire > The power cord & gt; Signal lines. Generally, the signal line width is 0.2~0.3mm. The thinnest width can reach 0.05~0.07mm, and the power cord is generally 1.2~2.5mm. For digital PCBS, a wide ground wire can be used to form loops for the grounding network (analog grounding cannot be used like this);

2) Pre-processing of higher requirements (such as high frequency line), input and output edges should avoid adjacent parallel, in order to avoid reflection interference. If necessary, coupled with grounding, two adjacent layers of wiring should be perpendicular to each other, parallel prone to parasitic coupling;

3) The oscillator housing is grounded, and the clock line should be as short as possible and cannot be quoted anywhere. Below the clock oscillation circuit, the special high-speed logic circuit part should increase the grounding area, should not use other signal lines, in order to make the surrounding electric field close to zero;

4) Use 45° polyline as far as possible, do not use 90° polyline to reduce the radiation of high frequency signal; (high line is required to use double arc);

5) Do not loop on any signal lines. If unavoidable, loop should be as small as possible; The number of through-holes for signal cables should be as small as possible.

6) The key line should be as short and thick as possible, and protection should be added on both sides;

7) When transmitting sensitive signals and noise field signals through flat cables, they should be extracted through “ground signal – Ground wire”;

8) Key signals should be reserved for test points to facilitate debugging, production and maintenance testing;

9) After schematic wiring is completed, wiring should be optimized. At the same time, after the initial network check and DRC check are correct, the grounding of the wireless area is performed, and a large copper layer is used as the ground, and a printed circuit board is used. Unused areas are connected to the ground as ground. Or make a multi-layer board, power supply, grounding each accounted for a layer.

5. Add tears

A tear is a dripping connection between a pad and a line or between a line and a guide hole. The purpose of the teardrop is to avoid contact between the wire and the pad or between the wire and the guide hole when the board is subjected to a large force. In addition, disconnected, teardrop Settings can make the PCB board look prettier.

In the circuit board design, in order to make the pad stronger and prevent the mechanical plate, welding pad and welding wire between the fracture, welding pad and wire is usually set up between the transition strip copper film, shape like tears, so it is usually called tears.

6. In turn, the first check is to look at Keepout layers, top layer, bottom topoverlay and bottom overlay.

7. Electrical rule check: through hole (0 through hole – very incredible; 0.8 boundary), whether there is a broken grid, minimum spacing (10mil), short circuit (each parameter analyzed one by one)

8. Check power cables and ground cables – interference. (Filter capacitance should be close to the chip)

9. After completing the PCB, reload the network marker to check if the netlist has been modified – it works fine.

10. After PCB completion, check circuit of core equipment to ensure accuracy.