1. Set the size of the board and frame according to the structural drawing, arrange the mounting holes, connectors and other devices that need to be positioned according to the structural elements, and give these devices non-movable attributes. Dimension the size according to the requirements of the process design specifications.
2. Set the prohibited wiring area and the prohibited layout area of the printed board according to the structural drawing and the clamping edge required for production and processing. According to the special requirements of some components, set the prohibited wiring area.
3. Select the processing flow based on comprehensive consideration of PCB performance and processing efficiency.
The preferred order of the processing technology is: single-sided mounting of component surfaces-component surface mounting, insertion and mixing (component surface mounting welding surface mounting once wave forming)-double-sided mounting-component surface mounting and mixing, Welding surface mounting.
4. Basic principles of layout operation
A. Follow the layout principle of "big first, then small, hard first, and easy", that is, important cell circuits and core components should be given priority.
B. Refer to the principle block diagram in the layout, and arrange the main components according to the rule of the main signal flow of the board.
C. The layout should meet the following requirements as far as possible: the total wiring is as short as possible, the key signal line is the shortest; high voltage, high current signal and small current, low voltage weak signal are completely separated; analog signal is separated from digital signal; high frequency signal Separated from low-frequency signals; the spacing of high-frequency components should be sufficient.
D. For the circuit parts of the same structure, use the “symmetrical” standard layout as much as possible;
E. Optimize the layout according to the standards of even distribution, center of gravity balance, and beautiful layout;
F. Device layout grid setting. For general IC device layout, the grid should be 50--100 mil. For small surface mount devices, such as surface mount component layout, the grid setting should not be less than 25 mil.
G. If there are special layout requirements, it should be determined after communication between the two parties.
5. The same type of plug-in components should be placed in one direction in the X or Y direction. The same type of discrete components with polarities should also strive to be consistent in the X or Y direction to facilitate production and inspection.
6. The heating elements should generally be evenly distributed to facilitate the heat dissipation of the single board and the whole machine. The temperature sensitive devices other than the temperature detection elements should be far away from the components with large heat generation.
7. The arrangement of the components should be convenient for debugging and maintenance, that is, large components cannot be placed around the small components, the components to be debugged, and there must be enough space around the device.
8. For the veneer produced by wave soldering process, the fastener mounting holes and positioning holes should be non-metallized holes. When the mounting hole needs to be grounded, it should be connected to the ground plane by means of distributed grounding holes.
9. When the wave soldering production technology is used for the mounting components on the welding surface, the axial direction of the resistance and the container should be perpendicular to the wave soldering transmission direction, and the axial direction of the resistance row and SOP (PIN pitch is greater than or equal to 1.27mm) and the transmission direction are parallel; IC, SOJ, PLCC, QFP and other active components with a PIN pitch of less than 1.27mm (50mil) should be avoided by wave soldering.
10. The distance between BGA and adjacent components is >5mm. The distance between other chip components is >0.7mm; the distance between the outside of the mounting component pad and the outside of the adjacent plug-in component is greater than 2mm; PCB with crimping parts, there can be no insertion within 5mm around the crimped connector Elements and devices must not be placed within 5mm of the welding surface.
11. The layout of the IC decoupling capacitor should be as close as possible to the power supply pin of the IC, and the loop formed between it and the power supply and ground should be the shortest.
12. In the component layout, due consideration should be given to the use of devices with the same power supply as far as possible to facilitate the separation of future power supplies.
13. The layout of resistance components used for impedance matching purposes should be reasonably arranged according to their properties.
The layout of the series matching resistor should be close to the driving end of the signal, and the distance should generally not exceed 500mil.
The layout of the matching resistors and capacitors must distinguish between the source end and the terminal of the signal, and the terminal matching of multiple loads must be matched at the farthest end of the signal.
14. After the layout is completed, print out the assembly drawing for the schematic designer to check the correctness of the device package, and confirm the signal correspondence between the single board, the backplane, and the connector. After confirming the correctness, the wiring can be started.
