Preliminary preparations for PCB layout design

PCB Layout     |      2021-10-16 17:15

Table of Content:

1. Preliminary preparation

2. PCB structure design

3. PCB layout design

4. Network DRC inspection and structure inspection

 PCB layout design

 

 

1. Preliminary preparation

Including the preparation of component libraries and schematic diagrams. Before proceeding with PCB layout design, we must first prepare the schematic SCH component library and PCB component packaging 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 PCB component package library is established first, and then the schematic SCH component library is established. PCB component packaging library requirements are high, which directly affects the installation of PCB; schematic diagram SCH component library requirements are relatively loose, but pay attention to define the pin attributes and the corresponding relationship with the PCB component packaging library.

A.Check the schematic diagram carefully: the design of any printed board is inseparable from the schematic diagram. The accuracy of the schematic diagram is the prerequisite basis for the correctness of the printed board. Therefore, before the design of the printed board, the signal integrity of the schematic must be carefully and repeatedly checked to ensure the correct connection between the devices.

B.Device selection: The selection of components is a very important link for the design of printed boards. Devices with the same functions and parameters may have different packaging methods. The package is different, and the solder holes (disks) of the components on the printed board are different. Therefore, before embarking on the printed board layout design, we must determine the packaging form of each component.

Multilayer boards must be positioned in the selection of surface mount components (SMD) in terms of device selection. SMD is widely used in various electronic products due to its advantages of miniaturization, high integration, high reliability, and installation automation.

At the same time, in the selection of devices, not only should the characteristic parameters of the device meet the needs of the circuit, but also the supply of the device to avoid the problem of device shutdown. At the same time, it should be aware that many domestic devices such as chip resistors, capacitors, and connections The quality of potentiometers and potentiometers has gradually reached the level of imported components, and it has the advantages of sufficient supply, short delivery time, and low price. Therefore, under the condition of circuit permission, domestic devices should be considered as far as possible.

 

2. PCB structure design

According to the determined circuit board size and various mechanical positioning, draw the PCB frame in the PCB layout 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 printed board layout design is to place the components in the PCB frame according to the design requirements. Generate the netlist (Design→Create Netlist) in the schematic tool, and then import the netlist (Design→Import Netlist) in the PCB software. After the netlist is successfully imported, it will exist in the background of the software. Through the Placement operation, all devices can be called out, and there is a flying line prompt connection between the pins. At this time, the layout design of the device can be carried out.PCB layout design is the first important process in the entire printed board layout design process. The more complex the PCB board, the better the layout can directly affect the difficulty of later wiring. Layout design depends on the circuit board designer’s basic circuit knowledge and rich design experience, which is a higher level requirement for the circuit board designer. Elementary circuit board designers have little experience and are suitable for small module layout design or PCB layout design tasks with low overall board difficulty.

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: first, routing, which is the most basic entry requirement for PCB layout design; second, the satisfaction of electrical performance, which is a standard for measuring whether a PCB board is qualified. After passing, carefully adjust the wiring to achieve the best electrical performance; again, the neat and beautiful, messy wiring, even if the electrical performance is passed, it will bring great inconvenience to the later modification of the board and the test and maintenance. The wiring is required to be neat. Uniformity, not criss-cross and disorderly.

Wiring optimization and silk screen placement “printed board layout design is not the best, only better”, “PCB layout design is an art of defects”, this is mainly because PCB design needs to achieve the design requirements of all aspects of hardware, and individual requirements may vary. It is conflicting, and you cannot have both fish and bear’s paws. 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 The signal crosstalk between layers increases and the signal quality decreases. The general design experience is: the time to optimize the wiring is twice the time of the first wiring. After the PCB layout optimization is completed, post-processing is required. The first thing to do is the silk screen logo on the PCB surface. The bottom silk screen characters need to be mirrored during the design to avoid confusion with the top silk screen.

 

4. Network DRC inspection and structure inspection

Quality control is an important part of the PCB layout 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 printed board layout design meets the two input conditions of the schematic netlist and structural element diagram. Generally, circuit board designers will have their own accumulated design quality check Checklist, where the entries partly come from the company or department’s specifications, and the other part comes from their own experience summaries. Special inspections include Valor inspection and DFM inspection of the design. These two parts focus on the PCB design and output back-end processing gerber files.

PCB manufacturing, before the PCB is officially processed and manufactured, the circuit board designer needs to communicate with the PE of the PCB supplier to answer the manufacturer’s confirmation questions about PCB processing. This includes but is not limited to: selection of PCB board model, adjustment of circuit layer line width and line spacing, adjustment of impedance control, adjustment of PCB stacking thickness, surface treatment processing technology, aperture tolerance control and delivery standards, etc.