PCB was born in 1936, and the United States used this technology in military radios in 1943; since the mid-1950s, PCB technology has been widely used. At present, PCB has become the “mother of electronic products”, and its application has penetrated into almost every terminal field of the electronics industry, including computers, communications, consumer electronics, industrial control, medical equipment, national defense, aerospace and many other fields.
1. Preliminary preparation
This includes preparing component libraries and schematics. Before PCB design, the schematic SCH component library and PCB component package library must be prepared first.
The PCB component package library is best established by engineers based on the standard size data of the selected device. In principle, the PC component package library should be established first, and then the schematic SCH component library should be established.
The PCB component package library has high requirements, which directly affects the PCB installation; the schematic SCH component library has relatively loose requirements, but attention should be paid to defining the pin properties 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
Layout design is to place devices in the PCB frame according to the design requirements. Generate the 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 successfully imported, it will exist in the software background. Through the Placement operation, all devices can be called out, and there are flying wires between each pin to prompt 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 better the layout will directly affect the difficulty of later wiring.
The layout design depends on the basic circuit skills and design experience of the circuit board designer, which is a relatively high 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 with low difficulty for the whole board.
4. PCB wiring design
PCB layout 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:
The first is Bouton, 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 wiring is completed, carefully adjust the wiring to achieve the best electrical performance;
Once again, it is neat and beautiful, and the messy wiring, even if the electrical performance passes the test, will bring great inconvenience to the optimization, testing and maintenance of the later board modification.
5. Wiring optimization and silk screen placement
“PCB design is not the best, only better”, “PCB design is an art of defects”, this is mainly because PCB design needs to realize the design requirements of all aspects of hardware, and there may be conflicts between individual requirements. You can’t have both.
For example: a PCB design project needs to be designed as a 6-layer board after the evaluation of 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 wiring is twice the time of the initial wiring. After the PCB layout optimization is completed, post-processing is required. The first thing to be processed is the silkscreen logo on the PCB board surface. During design, the bottom silkscreen characters need to be mirrored to avoid confusion with the top layer silkscreen.
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 meeting, special inspection, etc.
The schematic diagram and structural element diagram are the most basic design requirements. The network DRC inspection and structural inspection are to confirm that the PCB design meets the two input conditions of the schematic netlist and structural element diagram respectively.
Generally, circuit board designers will have their own accumulated design quality inspection checklist, some of which are derived from the company or department’s specifications, and the other part is derived from their own experience summary. The special inspection includes the Valor inspection of the design and the DFM inspection. These two parts focus on the output of the PCB design output and the back-end processing light drawing file.
7. PCB board
Before the PCB is formally processed and made, the circuit board designer needs to communicate with the PE of the PCB supplier factory to answer the manufacturer’s confirmation questions about the PCB board processing.
This includes but is not limited to: selection of PCB board type, adjustment of circuit layer line width and line spacing, adjustment of impedance control, adjustment of PCB stack thickness, surface treatment process, aperture tolerance control and delivery standards, etc.