What preparations are needed for PCB design?

1. Preparation for PCB design

Including preparation of component libraries and schematics. Before doing PCB design, you must first prepare the schematic SCH component library and PCB component packaging library. The PCB component packaging library is best established by engineers based on the standard size data of the selected devices. In principle, the PC component package library should be established first, and then the schematic SCH component library should be established. The requirements for the PCB component package library are relatively high, which directly affects the installation of the PCB. The requirements for the schematic SCH component library are relatively loose, but attention should be paid to defining the pin attributes and the corresponding relationship with the PCB component package library.

(1) PCB structure design

According to the determined circuit board size and various mechanical positioning, draw the PCB board frame in the PCB design environment, and place the required connectors, buttons/switches, screw holes, assembly holes, etc. according to the positioning requirements. ? Fully consider and determine the wiring area and non-wiring area (such as how much area around the screw hole belongs to the non-wiring area).

(2) PCB layout design

Layout design is to place devices in the PCB board frame according to the design requirements. Generate the network list in the schematic tool (Design→Create?Netlist), and then import the network list in the PCB software (Design→Import?Netlist). After the network table is successfully imported, it will exist in the software background. All devices can be called out through the Placement operation. There are flying wires between each pin to prompt connection. At this time, the device layout design can be carried out. PCB layout design is the first important process in the entire PCB design process. The more complex the PCB board, the more the quality of the layout will directly affect the difficulty of later wiring. Layout design relies on the circuit board designer’s basic circuit skills and design experience, and has higher-level requirements for circuit board designers. Junior circuit board designers have little experience and are suitable for small module layout design or less difficult PCB layout design tasks for the entire board.

PCB wiring design is the process with the largest workload in the entire PCB design, which directly affects the performance of the PCB board. In the PCB design process, there are generally three realms of wiring: first, layout, which is the most basic entry requirement for PCB design; second, satisfaction of electrical performance, which is the standard to measure whether a PCB board is qualified. After passing through, carefully adjust the wiring to achieve the best electrical performance; again, neat and beautiful wiring, messy wiring, even if the electrical performance is passed, will bring great inconvenience to later board modification optimization, testing and maintenance, and neat wiring is required Uniform, not criss-crossed and unorganized.
Wiring optimization and silk screen placement “There is no best in PCB design, only better” and “PCB design is an art of defects”. This is mainly because PCB design must meet the design requirements of all aspects of hardware, and individual requirements may vary. It’s conflicting, and you can’t have your cake and eat it too. For example, a PCB design project needs to be designed as a 6-layer board after evaluation by the circuit board designer. However, due to cost considerations, the product hardware must be designed as a 4-layer board. In this case, the signal shielding ground layer can only be sacrificed, resulting in adjacent wiring. Signal crosstalk between layers increases and signal quality decreases. A general design rule of thumb is that optimizing wiring takes twice as long as initial wiring. After PCB wiring optimization is completed, post-processing is required. The first thing to deal with is the silk screen logo on the PCB board. During design, the bottom silk screen characters need to be mirrored to avoid confusion with the top silk screen.

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2.PCB post-processing work

(1) DRC inspection: that is, design rule inspection. Through inspection methods such as Checklist and Report, the focus is on avoiding major design defects such as circuits and short circuits. During the inspection, the PCB design quality control process and methods are followed;

(2) DFM inspection: After the PCB design is completed, whether it is the processing of the bare PCB board or the patch assembly processing of the PCBA support board, it is necessary to use relevant inspection tool software or Checklist to check the processing-related design;

(3) ICT design: Some PCB boards will undergo ICT testing during mass processing and production, so such PCB boards need to add ICT test points during the design stage;

(4) Silk screen adjustment: Clear and accurate silk screen design can improve the convenience and accuracy of subsequent testing and assembly processing of circuit boards;

(5) Drill layer labeling: The information marked on the Drill layer is the processing requirement drawing provided to the PCB processing factory PE. It needs to follow industry standards and ensure the accuracy and completeness of the Drill processing information;

(6) PCB design file output: The final file of PCB design needs to be output into different types of packaged files according to specifications for subsequent testing, processing, and assembly;

(7) CAM350 inspection: The CAM files (also called Gerber files) that the PCB design outputs to the PCB processing factory need to use CAM350 software for design review before board release.

Other matters to be dealt with before listing

In-group QA review

After receiving the board submission process, the QA within the group first confirms that the designer has conducted sufficient self-inspection. If the process is not completed, it will return to the process and require the designer to submit the process again after completion.

Review and record review results, handling opinions, etc. If the review fails, the process will be returned to the designer.

The QA review opinions within the group must also be filled in the single board design review record database.

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Check for short circuit and open circuit problems

1. There is a single point grounding; the plane layer is hollowed out; when modifying the processing of the pad flower pad and the hot pad, be careful and write the processing results into the ground source and ground division of the design file, which is conducive to self-inspection and QA review And the next board change.

2. When performing negative division on the electrical ground layer of P software, it must be carefully checked. The method for checking electrical ground layer negatives using P software is as follows: Set PLANENETs assigned to the same layer to different colors.

Shows only all features of the layer to be inspected.

Check whether there is a network across partitions (different colors are in the same partition). If there is no network, it means the partition is successful.

If there is a cross-segmentation situation, you need to edit the PLANETHERMAL properties of the spanned pads and vias so that they do not produce flowerpots in this area, and then ensure their connectivity through wiring.

3. Where there are windows or slots, a forbidden area for wiring must be added to prevent the wiring from being dug out and causing an open circuit. structural elements.The prohibited areas defined in the figure must also meet the requirements.

4. The correct DRC must be set and all DRC checks must be turned on.

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