The general PCB basic design process is as follows:
1: Early preparation
This includes preparing the packaging library and schematICs. Before PCB design, the logic package of schematic SCH and PCB package library should be prepared first. Packaging library can be a PADS library, but generally it is difficult to find suitable, it is best to do their own packaging library according to the standard size of the selected devices. In principle, first do PCB packaging library, and then do SCH logic packaging. The packaging library of PCB has high requirements, which directly affects the installation of the board. SCH logic packaging requirements are relatively loose, as long as pay attention to the definition of pin attributes and the corresponding relationship with PCB packaging. (Note the hidden pins in the standard library) After the schematIC design, ready to start the PCB design.
In this step, according to the size of the circuit board and various mechanical positioning, PCB board surface is drawn in the PCB design environment, and the required connectors, keys/switches, screw holes, assembly holes and so on are placed according to the positioning requirements. And fully consider and determine the wiring area and non-wiring area (such as how much of the area around the screw hole belongs to the non-wiring area).
3: mesh guide table
It is recommended to enter the plate frame before the mesh guide. Import a DXF or emn format board and frame
4: Rule setting
Reasonable rules can be set based on the specific PCB design. These rules are the PADS constraint Manager (PADS), which is used to determine the width and safe spacing of lines at any point in the design process. DRC MARKers will be used to identify areas that do not conform to the limits during subsequent DRC tests.
The general rules are set before the layout, because sometimes some fanout work will be completed during the layout, so the rules should be set well before fanout, when the design project is large, it can be more efficient to complete the design. Note: Rules are set for better and faster completion of the design, in other words, for the convenience of the designer. Common Settings are: 1. Default line width/line distance for common signals. 3. Set line widths and colors for important signals and power supplies. 4. Set the board layer.
5: PCB layout
The general layout is carried out according to the following principles:
(1) According to the electrical performance reasonable partition, generally divided into: digital circuit area (fear of interference, and interference), analog circuit area (fear of interference), power driving area (interference source);
(2) To complete the same function of the circuit, should be placed as close as possible, and adjust the components to ensure that the connection is the most concise; At the same time, adjust the relative position between the functional blocks to make the connection between the functional blocks the most concise;
(3) Installation position and installation strength should be considered for components with high quality; The heating element should be placed separately from the temperature sensitive element, and the heat convection measures should be considered if necessary;
(4) I/O driving device as close as possible to the edge of the printing plate, close to the outlet connector;
(5) Clock generator (such as crystal oscillator or clock oscillator) should be as close to the device that uses the clock as possible;
(6) In each integrated circuit between the power input pin and ground, need to add a decoupling capacitor (generally using high frequency performance good monolithic capacitor); When the board space is dense, a tantalum capacitor can also be added around several integrated circuits.
(7) the discharge diode (1N4148) should be added to the relay coil;
(8) The layout requirements should be balanced, dense and orderly, not top-heavy or heavy
Special attention should be paid to the actual size (area and height) and relative position between components when placing components, so as to ensure the electrical performance of the circuit board and the feasibility and convenience of production and installation. At the same time, under the prEMIse of ensuring that the above principles can be reflected, the placement of devices should be appropriately modified to make them neat and beautiful. Such as the same device to be placed neatly, in the same direction, can not be placed "patchwork". This step relates to the overall image of the board and the next level of wiring difficulty, so a little to take great effort to consider. Layout, not sure of the place can be made preliminary wiring, full consideration.