PCB have developed from single layer to double-sided, multi-layer and flexible, and still maintain their respective development trends. Due to the continuous development in the direction of high precision, high density and high reliability, continuous reduction in size, cost reduction and performance improvement, the printed board still maintains a strong vitality in the development of electronic equipment in the future. So how is the schematic diagram of the PCB circuit board designed? Read the following seven steps to understand.
1. Preliminary preparation
Including preparation of component library and schematic diagram. Before designing the PCB, first prepare the schematic SCH component library and the PCB component package library.
The PCB component package library is best established by the engineer based on the standard size data of the selected device. In principle, the component package library of PC is established first, and then the schematic SCH component library is established.
The PCB component package library has high requirements, which directly affects the PCB installation; the schematic diagram SCH component library requirements are relatively loose, but pay attention to defining the pin attributes and the corresponding relationship with the PCB component package library.
2. 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).
3. PCB layout design
The layout design is to place the devices in the PCB frame according to the design requirements. Generate a netlist in the schematic tool (Design→Create Netlist), and then import the netlist in the PCB software (Design→Import Netlist). After the network table is imported successfully, it will exist in the software background. All devices can be called out through the Placement operation, and there are flying wires between the pins to indicate the connection. At this time, the layout design of the device 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 can directly affect the difficulty of the later wiring.
The layout design depends on the circuit board designer's basic circuit skills and rich design experience, which is a higher level requirement for the circuit board designer. Junior circuit board designers have little experience and are suitable for small module layout design or PCB layout design tasks that are less difficult for the entire board.
4. PCB wiring design
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, wiring generally has three realms:
The first is the layout, which is the most basic entry requirement for PCB design;
The second is the satisfaction of electrical performance, which is the standard to measure whether a PCB board is qualified. After the circuit is laid out, carefully adjust the wiring to make it achieve the best electrical performance;
Again, neat and beautiful, messy wiring, even if the electrical performance is passed, it will bring great inconvenience to the later board modification optimization and testing and maintenance.
5. Wiring optimization and silk screen placement
PCB design is not the best, only better, "PCB design is a flawed art", this is mainly because PCB design needs to meet the design requirements of all aspects of hardware, and individual requirements may conflict, fish and bear's paw Can't have both.
For example: a PCB design project needs to be designed as a 6-layer board after evaluation by the circuit board designer, but the product hardware must be designed as a 4-layer board due to cost considerations, so the signal shielding ground layer can only be sacrificed, resulting in adjacent wiring. Signal crosstalk between layers increases and signal quality degrades.
The general design experience is: the time to optimize the routing is twice the time of the initial routing. After the PCB wiring optimization is completed, post-processing is required. The first thing to deal with is the silk screen logo on the PCB surface. When designing, the bottom silk screen characters need to be mirrored to avoid confusion with the top silk screen.
6. Network DRC inspection and structure inspection
Quality control is an important part of the PCB design process. General quality control methods include: design self-inspection, design mutual inspection, expert review meetings, special inspections, etc. Schematic diagram and structural element diagram are the most basic design requirements. Network DRC inspection and structural inspection are to confirm that the PCB design satisfies the two input conditions of schematic netlist and structural element diagram respectively.
Generally, circuit board designers will have their own accumulated design quality inspection Checklists, in which some of the entries come from the company or department's specifications, and the other part comes from their own experience summary. The special inspection includes the Valor inspection and DFM inspection of the design. These two parts focus on the back-end processing of the PCB design and the processing of light-painting files.
7. PCB board
Before the PCB is formally processed and made, the circuit board designer needs to communicate with the PE of the PCB A board supply factory to answer the manufacturer's confirmation questions about the PCB board processing.
This includes but is not limited to: selection of PCB sheet types, adjustment of line width and spacing of circuit layers, adjustment of impedance control, adjustment of PCB stack thickness, surface treatment process, aperture tolerance control and delivery standards, etc.
The above is the whole process of PCB design.
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