PCB Technical - Basic knowledge of PCB design

ipcb is a PCB design company specializing in electronic product circuit board design (layout wiring design), mainly undertaking multi-layer, high-density PCB design circuit board design proofing business. Next, I will introduce the basic knowledge of PCB design.

Basic knowledge of PCB design
The design of the printed circuit board is based on the schematic diagram of the circuit to realize the functions required by the circuit designer. The design of the printed circuit board mainly refers to the layout design, which needs to consider the layout of factors such as external interfaces, internal electromagnetic protection, and heat dissipation. Our commonly used design software are Altium Designer, Cadence Allegro, PADS, etc. design software. If you are a beginner, it is recommended to learn Altium Designer software. As a PCB design engineer, we need to be proficient in two softwares.

PCB Design Company

In high-speed design, the continuity of the controllable impedance board and the line impedance is a very important issue. The common impedance is 50 ohms single-ended and 100 ohms differential. How to ensure signal integrity? In our usual way, the adjacent layers of the signal line have a complete GND plane, or a power plane. We use single-chip microcomputers as products. Under normal circumstances, we don't need to make impedance, and its working frequency is generally very low.

1. Setting Techniques
Different settings are required at different stages of the design. In the layout stage, large grid points can be used for device layout; for large devices such as ICs and non-positioned connectors, a grid point accuracy of 50 to 100 mils can be used for layout. Small passive components such as inductors can be laid out with 25mil grid points. The accuracy of the large grid points is conducive to the alignment of the device and the aesthetics of the layout. In high-speed wiring, we generally use millimeters as the unit, and most of us use mil as the unit.

2. PCB layout rules
1. Under normal circumstances, all components should be placed on the same side of the circuit board as much as possible. Only when the top components are too dense, can some devices with limited height and low heat generation, such as chip resistors, chip capacitors, and paste Chip chips etc. are placed on the bottom layer.
2. Under the premise of ensuring electrical performance, the components should be placed on the grid and arranged in parallel or perpendicular to each other for neat and beautiful appearance; the arrangement of the components should be compact, and the components should be evenly distributed and densely distributed on the entire layout.
3. The device should be no less than 2mm away from the edge of the circuit board as far as possible. The process is 5mm away from the device. When it is less than 5mm, we should add a process edge. Of course, the mechanical strength that the circuit board can withstand must also be considered.

3. Layout skills
In the layout design of the PCB, the units of the circuit board should be analyzed, and the layout design should be based on the starting function. When laying out all the components of the circuit, the following principles should be met:

1. Arrange the position of each functional circuit unit according to the circuit flow, so that the layout is convenient for signal circulation, and the signal is kept in the same direction as possible.

2. Take the core components of each functional unit as the center and lay out around it. The components should be uniformly, integrally and compactly arranged on the PCB to minimize and shorten the leads and connections between the components. When the interface is fixed, we should lay out the interface and then the core components. The shortest high-speed signal is the principle.

3. For circuits operating at high frequencies, the distribution parameters between components must be considered. Low-frequency and high-frequency line circuits should be separated, and digital and aalog circuits should be designed separately.

4. Layout design
In the PCB, special components refer to the key components of the high-frequency part, the core components in the circuit, the components that are susceptible to interference, the components with high voltage, the components with high heat generation, and some components of the opposite sex., The location of these special components needs to be carefully analyzed, and the belt layout meets the requirements of circuit functions and production requirements. Improper placement of them may cause circuit compatibility problems, signal integrity problems, and lead to failure of PCB design.

5. Placement order
1. Place components that closely match the structure, such as power sockets, indicator lights, switches, connectors, etc.
2. Place special components, such as large components, heavy components, heating components, transformers, ICs, etc.
3. Place small components.

6. Layout check
1. Whether the circuit board size and the processing size required by the CAD drawing are consistent.
2. Whether the layout of the components is balanced, neatly arranged, and whether they have all been laid out.
3. Whether there are conflicts at all levels. Such as whether the components, frame, and connectors are reasonable.
4. Whether the commonly used components are convenient for maintenance and installation. Such as switches, plug-in board insertion equipment, components that must be replaced frequently, etc.
5. Whether the distance between thermal components and heating components is reasonable.
6. Whether the heat dissipation is good.
7. Whether the line interference problem needs to be considered.

7. Wiring principles
1. Avoid arranging important signal lines on the edge of the PCB, such as clock and reset signals.
2. The distance between the chassis ground wire and the signal wire is at least 2 mm
3. High-speed signals should be punched as small as possible to ensure signal integrity.
4. Whether the digital and analog signals are separated. Whether the high frequency and low frequency signals are separated.
5. When designing large-area copper coating, there should be opening windows on the copper coating, adding heat dissipation holes, and designing the opening windows into a net shape.
6. Vibrate the crystal, and check whether the wires are routed under the transformer. These basic knowledge of introductory knowledge can only be improved by our diligent practice.

8. Silk screen placement
1. The screen printing position number is not available for solder mask, and it is missing after the screen printing is placed in production.
2. The screen printing position number is clear, and the recommended font width/height dimensions are 4/25mil, 5/30mil, and 6/45mil.
3. Maintain the uniformity of direction. Generally, one PCB should not be placed in more than two directions. It is recommended that the letters be on the left or on the bottom.

9. 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 meets the two input conditions of the schematic netlist and structural element diagram.

10. Regular assembly
In the design, from the perspective of PCB board assembly, the following parameters should be considered:
1. The diameter of the hole should be determined according to the maximum material condition (MMC) and the minimum material condition (LMC). The diameter of a hole of an unsupported component should be selected such that the MMC of the pin is subtracted from the MMC of the hole, and the difference is between 0.15-0.5 mm. And for ribbon pins, the difference between the nominal diagonal of the pin and the inner diameter of the unsupported hole will not exceed 0.5mm and not less than 0.15mm.
2. Place smaller components reasonably so that they will not be covered by larger components.
3. The thickness of the solder mask should not be greater than 0.05mm.
4. The screen printed logo cannot intersect with any pads.
5. The upper half of the circuit board should be the same as the lower half to achieve structural symmetry. Because an asymmetrical circuit board may bend