A person must have an overall view, and the same is true for PCB design. Before starting a PCB project, designers must understand the overall process of the entire project from start to finish, as well as some key points that require special attention during the operation of the project. Next, we will share the process of PCB project.
The overall process and key points of the PCB project
1. At the beginning of the project, it is necessary to check whether all the materials required for the project are complete: including schematic diagram, structural diagram, packaging library, signal flow diagram of complex products, power supply tree diagram, key signal description, power supply current size, design requirements, etc.
2. Design information input: including the import of netlists and structural diagrams. After the structure diagram is imported, special attention should be paid to the size of screw holes and some positioning holes, the prohibited areas for devices and wiring, the height limit area, and the location of connectors.
3. Layout: On the basis of comprehensive consideration of signal quality, EMC, thermal design, DFM, DFT, structure, safety regulations and other requirements, the devices should be placed on the board reasonably. The basic idea of layout is generally to combine the signal flow direction and the power flow direction in addition to structural restrictions.
4. Wiring constraints: Wiring constraints are mainly divided into line width, spacing size, equal length, etc. Some rules require pre-simulation guidance, such as line length, impedance size, topology, stacked structure, etc.
example:
Through hole: The definition of the minimum aperture of the finished board depends on the thickness of the board. The board thickness-to-aperture ratio should be less than 10. The commonly used thickness-to-diameter ratio is 8. Vias with larger apertures are preferred.
Spacing: The recommended line width/space for PCB processing is: 6mil/6mil; the minimum line width/space that can be used is: 4mil/5mil; the ultimate minimum line width/space: 4mil/4mil.
Electrical rules:
1) The BUS line requires a defined topology and a certain stub length;
2) For signal lines with timing requirements, timing constraint rules must be defined;
3) Signal lines with time delay or relative delay requirements require setting delay rules;
4) Signal lines with crosstalk control requirements need to be set up with crosstalk constraint rules;
5) For signals with differential requirements, differential constraint rules need to be set
6) For high-speed signal lines with impedance requirements, set impedance control rules;
7) All rule requirements have been verified by simulation or meet clear requirements (such as theoretical impedance values).
5. Wiring: Wiring is the most workload-intensive aspect of PCB design, and there are many things that need attention. Such as line impedance, reference plane continuity, EMC, SI/PI, DFM, etc.
Example: Basic requirements for wiring processing
1) When rule-driven wiring, ensure the rationality of the rules, use and provide process Do files;
2) Avoid using extreme values for vias, line widths, and safe spacing.
3) Consider ICT test point design as much as possible during the rule-driven wiring process;
4) The pin leads should be led out from the center of the PIN as much as possible;
5) Keep the distance between the signal line and the PIN as much as possible;
6) No through holes or mechanical blind holes on the pad;
7) The distance from the trace to the edge of the board usually needs to be ≥ 2mm. If the conditions cannot be met, it must be at least no less than 20mil;
8) Except for the short interconnection lines and Fanout short lines on the surface, the signal lines should be placed on the inner layer as much as possible;
9) No via holes or surface wiring are allowed under the metal shell device;
10) Try to provide dedicated wiring layers for key signals such as clock signals, high-frequency signals, sensitive signals, etc., and ensure the minimum loop area. Use methods such as shielding and increasing safety distances to ensure signal quality;
11) The EMC environment between the power layer and the ground layer is poor, so signal lines that are sensitive to interference should be avoided;
12) Networks with impedance control requirements should be arranged on the impedance control layer;
13) Route wiring as close to a plane as possible and avoid spanning splits. These situations are only allowed to exist in low-speed signal lines if they must span splits or cannot be close to the power ground plane;
14) The corresponding power plane or ground plane in the high-speed signal line area should be kept as intact as possible;
15) The plane layer and wiring layer are symmetrically distributed, the media thickness is symmetrically distributed, and the vias are symmetrical across layers;
16) Avoid right or sharp angles when dividing plane layers;
17) When laying copper over a large area, the ground network is used as the reference network;
18) Avoid right or sharp angles when applying copper, and the upper and lower copper sheets must be connected by vias, especially at the edges of the copper sheets. The distance between adjacent vias at the edges is about 200~400mils;
19) The wiring should be kept uniform. Large areas without wiring need to be covered with copper, but it is required not to affect the impedance control;
20) There are no DRC errors in wiring and no network errors with the same name;
21) All signal lines must be chamfered, and the chamfering angle is 45 degrees, except for special circumstances;
22) After the PCB design is completed, there are no unfinished nets, and the PCB netlist is consistent with the schematic netlist.
6. Review + post-simulation verification: After the wiring is completed, review and inspection by senior department personnel and simulation of key signals and power supplies are required.
7. Processing: After the PCB design is OK, the light drawing file can be output for production.
