For designers, they must follow the PCB design process. A specification design flow can help avoid errors that lead to later design revisions when dealing with complex circuits. For example, A designer A, under the premise of structural engineers providing structural DXF, did not follow the specification process to first import the structure and hurriedly layout and wiring. When the product is close to production, he remembers that the structure is not processed, and finds that the entire PCB layout almost has to be readjusted, which seriously delay the development progress.
Due to the differences in software and the complexity of circuits, some circuits may have problems such as single-ended networks, open and short circuits, etc.. Blindly design and produce without inspection by relevant testing tools, and when the board is finished, the error will not be recovered, so process design is necessary!
1. Preparation of files
Before PCB designing, it is necessary to ensure that the following files are prepared, which can ensure the accuracy of subsequent design PCB and avoid extending design cycle caused by repeated design.
1) Schematic: the final version of the project design schematic. Note: Paginated schematics should be included in the project.
2) Packaging library: PCB packaging library or component Datasheet file for creating a new packaging library;
3) Structure diagram: DXF structure file containing the placement position and height limit of the main connector;
4) Design description: including the processing of key signals, special design requirements of chips and board making process descriptions;
5) Important device data sheet: recommended layout and wiring for reference when designing.
2. Import of netlists
Import the schematic network connection and device packaging into the PCB to ensure that there is no deviation in the import, and if the schematic design is updated, remember to re-import the latest schematic into the PCB.
3. Import of structure
Import the board outline, device positioning information, etc. into the PCB, if the structure is updated, remember to re-import the latest structure diagram DXF into the PCB for verification.
4. Project analysis
1) Block diagram: analyze the main functions, modules and interface information used in the entire project design, retrieve such module information, and understand its corresponding design requirements.
2) Interpretation of design instructions: including the processing of key signals, the special design requirements of the chip and the description of the board making process, which is convenient to integrate into the corresponding PCB design.
3) Power supply binary tree: analyze the power supply situation of the entire project, and clarify the power flow direction, current size, etc., which are included in the general schematic design.
5. Functional module grabbing
Due to the increasing degree of product integration, most of the product functions are clearly separated, PCB design can follow the schematic function module page to grab the devices on the corresponding module, convenient for the designer to sort out the signal flow between the functional modules later, and can draw module auxiliary lines in the PCB board to evaluate the approximate position and occupation range of the module in the PCB.
6. Structural device layout
According to the DXF structure file, import the board outline, place the device with location requirements in the correct position, and remember to update the structure as soon as possible if the structure is updated, so as not to lead to the duplication of subsequent work.
7. Key device layout
The main IC is placed in the appropriate position on the PCB according to the signal flow direction, which is actually the pre-layout of the PCB. The pre-layout can effectively plan the signal flow direction of the overall PCB and the balance of PCB layout density, and improve the rationality of the layout.
The captured modules are laid out separately and then placed in the appropriate position on the PCB, which is the detailed PCB layout. Based on the pre-layout part and layout requirements to standardize the layout, please refer to the specification requirements of the module in the following article for layout specifications, and pay attention to the location of the holes and traces.
9. Layout optimization
Fine-tune the already laid out devices and components to make the entire PCB layout more reasonable and beautiful.
10. PCB stacking settings
According to the flying line after the PCB layout, it is necessary to estimate the number of PCB layers required for the design, and calculate the line width and line spacing according to the number of layers and board thickness using the lamination software (or according to the existing stack template).
11. Rule settings
Set line width and line spacing rules on the PCB to facilitate wire routing, and the rule setting needs to check the design requirements and the process requirements of the PCB manufacturer, otherwise it may lead to the entire rerouting.
12. Class settings
According to the IC signal requirements, the signals with design requirements are grouped, set to different Classes, and set the corresponding Class rules.
To connect the signals of the same network, after the wiring is completed, the entire PCB wiring requirements need to be checked. The power wiring is checked to see whether it meets the current carrying requirements and whether it meets various certification requirements.
14. Equal length winding
The signal lines required by the time sequence are processed at equal length, and attention is paid to the error requirements of the signal when the length is equal.
15. Wiring optimization
Optimize the already connected traces to make the traces more beautiful and reasonable. Critical signal priorities are checked and processed optimally.
16. Power treatment
This step is mainly to optimize the power signal in the single board and divide the power plane, confirm that the current carrying requirements are met. When splitting in a plane, important signal lines should not be split across the plane.
17. DRC inspection
1) Connectivity check: Check whether all the signals in the project are connected and whether there is an open circuit.
2)Other inspections: Check whether the signal in the project has a short circuit condition, whether there is a keepout layout zone, whether the device height limit is avoided, etc.
18. Screen printing treatment
Reasonably place the device tag silk screen on the PCB, add the corresponding version number, production logo (anti-static, SN number, enterprise LOGO, etc.) and other content. Pay attention to the PCB silkscreen printing when placing it to ensure readability and facilitate production.
19. Project inspection
Finally, check all the design requirements of the customer and check the production process.
20. Gerber file output
Set and output Gerber files in EDA software. If it is the same type of product, you can set the template for Gerber output, so that you can output it with one click when the next update.
21. Gerber file inspection
Although the EDA tool has effectively checked the designed PCB, the output Gerber should also check the production manufacturability problem with tools so that the production problem can be solved in the design stage.
22. Document Archiving
A good design file archiving can help us effectively improve the communication between departments and parts and the sending of data, according to experience, it is recommended that designers file documents as follows:
1) ASM folder: contains assembly files, assembly drawings of top and bottom layers, the components for customers to assemble → send to the SMT factory.
2) CAM folder: contains Gerber files, IPC netlists, PCB production instructions→ send to PCB manufacturer.
3) DXF folder: contains the structure drawing files used by the project→ send to the structural engineer
4) PCB folder: contains the final PCB file of the project→ engineering designer’s revision and design
5) SCH folder: contains the final schematic file of the project→ engineering designer’s revision and design
6) SMT folder: contains SMT coordinate files and stencil files→ send to SMT manufacturers