PCB Design

PCB design 

Software

1. The most commonly used software in China are Protel, Protel 99se, Protel DXP, Altium, they are from the same company and constantly upgraded; the current version is Altium Designer 15 which is relatively simple, the design is more casual, but not very good for complex PCBs.

2. Cadence SPB. The current version is Cadence SPB 16.5; the ORCAD schematic design is an international standard; PCB design and simulation are very complete. It is more complicated to use than Protel. The main requirements are in complicated settings. ; But there are rules for the design, so the design is more efficient, and it is significantly stronger than Protel.

3. Mentor’s BORDSTATIONG and EE, BOARDSTATION is only applicable to a UNIX system, not designed for PC, so fewer people use it; the current Mentor EE version is Mentor EE 7.9, it is in the same level with Cadence SPB, its strengths are pulling wire and flying wire. It is called a flying wire king.

4. EAGLE. This is the most widely used PCB design software in Europe. The PCB design software mentioned above is used a lot. Cadence SPB and Mentor EE are well-deserved kings. If it is a beginner design PCB, I think Cadence SPB is better, it can develop a good design habit for the designer, and can ensure good design quality.

Related skills

Setting tips

The design needs to be set at various points in different stages. In the layout stage, large grid points can be used for device layout;

For large devices such as ICs and non-positioning connectors, you can choose a grid accuracy of 50 to 100 mils for layout. For passive small devices such as resistors, capacitors, and inductors, you can use 25 mils for layout. The accuracy of the large grid points is conducive to the alignment of the device and the aesthetics of the layout.

PCB layout rules:

1. Under normal circumstances, all components should be placed on the same surface of the circuit board. Only when the top layer components are too dense, can some high-limit and low-heat devices, such as chip resistors, chip capacitors, paste Chip ICs are placed on the bottom layer.

2. On the premise of ensuring electrical performance, the components should be placed on the grid and arranged parallel or perpendicular to each other to be neat and beautiful. Under normal circumstances, the components are not allowed to overlap; the components should be arranged compactly, and the components should be on the entire layout Uniform distribution and uniform density.

3. The minimum spacing between adjacent pad patterns of different components on the circuit board should be above 1MM.

4. It is generally not less than 2MM away from the edge of the circuit board. The best shape of the circuit board is rectangular, with a length to width ratio of 3: 2 or 4: 3. When the board size is greater than 200MM by 150MM, the affordability of the circuit board should be considered Mechanical strength.

Layout skills

In the layout design of the PCB, the unit of the circuit board should be analyzed, the layout design should be based on the function, and the layout of all the components of the circuit must meet the following principles:

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

2. With the core components of each functional unit as the center, layout around him. The components should be evenly, integrally and compactly arranged on the PCB to minimize and shorten the leads and connections between the components.

3. For circuits operating at high frequencies, the distribution parameters between components should be considered. The general circuit should arrange the components in parallel as much as possible, which is not only beautiful, but also easy to install and solder, and easy to mass-produce.

Design steps

Layout design

In the PCB, the special components refer to the key components in the high-frequency part, the core components in the circuit, the components that are easily interfered, the components with high voltage, the components with large heat generation, and some heterosexual components The location of these special components need to be carefully analyzed, and the layout needs to meet the circuit function requirements and production requirements. Improper placement of them may cause circuit compatibility problems and signal integrity problems, which can lead to failure of PCB design.

When placing special components in the design, first consider the PCB size. When the PCB size is too large, the printed lines are long, the impedance increases, the anti-drying ability decreases, and the cost also increases; if it is too small, the heat dissipation is not good, and the adjacent lines easily interfere. After determining the size of the PCB, determine the pendulum position of the special component. Finally, according to the functional unit, all the components of the circuit are laid out. The location of special components should generally observe the following principles during layout:

1. Shorten the connection between high-frequency components as much as possible, try to reduce their distribution parameters and mutual electromagnetic interference. The susceptible components cannot be too close to each other, and the input and output should be as far away as possible.

2 Some components or wires may have a higher potential difference, and their distance should be increased to avoid accidental short circuits caused by the discharge. High-voltage components should be kept out of reach.

3. Components weighing more than 15G can be fixed with brackets and then welded. Those heavy and hot components should not be placed on the circuit board but should be placed on the bottom plate of the main chassis, and heat dissipation should be considered. Thermal components should be kept away from heating components.

4. The layout of the adjustable components such as a potentiometer, adjustable inductance coils, variable capacitors, micro switches, etc. should consider the structural requirements of the whole board. Some frequently used switches should be Place it where you can easily reach it with your hands. The layout of the components is balanced, dense and dense, not top-heavy.

One of the successes of a product is to pay attention to inner quality. But it is necessary to take into account the overall beauty, both are relatively perfect boards, in order to become a successful product.

Sequence

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.

Layout check

1. Whether the size of the circuit board and the drawings meet the processing dimensions.

2. Whether the layout of the components is balanced, neatly arranged, and whether they have all been laid out.

3. Are there conflicts at all levels? Such as whether the components, the outer frame, and the level requiring private printing are reasonable.

3. Whether the commonly used components are convenient to use. Such as switches, plug-in boards inserted into equipment, components that must be replaced frequently, etc.

4. Is the distance between the thermal components and the heating components reasonable?

5. Whether the heat dissipation is good.

6. Whether the problem of line interference needs to be considered.

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