Electronic Design - What is PCB design?

The circuit board refers to the use of Electronic design automation software (EDA) to design the circuit board and design the layout and circuit connection of the printed circuit board (PCB). The PCB design mainly refers to layout design, which needs to consider the layout of external connections. Various factors such as optimized layout of internal electronic components, optimized layout of metal connections and through-holes, electromagnetic protection, thermal dissipation, etc. Circuit boards are an important component of electronic products, which can connect electronic components and transmit signals through wires.

How to design a PCB?

Design requires different point settings at different stages, and large grid points can be used for device layout during the layout stage; For large devices such as ICs and non-positioning connectors, a grid accuracy of 50-100 mils can be selected for layout, while for passive small devices such as resistors, capacitors, and inductors, a grid accuracy of 25 mils can be used for layout. The accuracy of large grid points is beneficial for device alignment and layout aesthetics.

1.PCB layout rules

1) In general, all components should be arranged on the same side of the circuit board. Only when the top layer components are too dense can some highly limited and low heat-generating devices, such as chip resistors, chip capacitors, and chip ICs, be placed on the bottom layer.

2) On the premise of ensuring electrical performance, components should be placed on a grid and arranged parallel or perpendicular to each other, to be neat and beautiful. In general, components are not allowed to overlap; The arrangement of components should be compact, and the components should be evenly distributed and densely distributed throughout the entire layout.

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

4) The distance from the edge of the circuit board is generally not less than 2 mm. The best shape of the circuit board is rectangular, and the Aspect ratio is 3:2 or 4:3. When the size of the circuit board is greater than 200 mm by 150 mm, the mechanical strength that the circuit board can withstand should be considered.

2.PCB layout skills

In the layout of pcb design, it is necessary to analyze the units of the circuit board and design the layout based on their functions. When laying out all components of the circuit, the following principles should be followed:

1) Arrange the positions of each functional circuit unit according to the circuit flow,making the layout convenient for signal flow and keeping the signal in the same direction as much as possible.

2) Layout around the core components of each functional unit as the center. The components should be evenly, integrally, and compactly arranged on the PCB, minimizing and shortening the leads and connections between each component as much as possible.

3) For circuits operating at high frequencies, consideration should be given to the distribution parameters between components. Generally, circuits should be arranged in parallel with components as much as possible, which is not only aesthetically pleasing but also easy to assemble and solder, making it easy for mass production.

3.PCB design steps

Layout design

In PCB, special components refer to key components in the high-frequency part, core components in the circuit, easily disturbed components, components with high voltage, components with high heat generation, and some heterosexual components. The positions of these special components need to be carefully analyzed, and the layout of the ribbon should meet the requirements of circuit functions and production needs. Improper placement of them may cause circuit compatibility problems and Signal integrity problems, thus leading to the failure of PCB design.

When designing how to place special components, the first consideration is the size of the PCB. Kuaishou pointed out that when the PCB size is too large, the printing line is long, the impedance increases, the anti-drying ability decreases, and the cost also increases; When it is too small, the heat dissipation is poor, and the adjacent lines are easily disturbed. After determining the size of the PCB, determine the square position of the special components. Finally, lay out all components of the circuit according to the functional units.

The position of special components should generally follow the following principles when laying out:

1) Try to shorten the connection between high-frequency components as much as possible, and try to reduce their distribution parameters and electromagnetic interference with each other. Components that are susceptible to interference should not be too close to each other, and input and output should be kept as far away as possible.

2) Some components or wires may have high potential differences, and their distance should be increased to avoid accidental short circuits caused by the discharge. High-voltage components should be kept out of reach as much as possible.

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

4) For the layout of adjustable components such as a Potentiometer, adjustable inductance coil, variable capacitor, microswitch, etc., the structural requirements of the whole board should be considered. Some switches often used should be placed in places easily accessible to hands if the structure allows. The layout of components should be balanced, with appropriate density, and should not be top-heavy.

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.

Layout check

1) Does the circuit board size match the machining dimensions required by the drawing?

2) Is the layout of the components balanced, neatly arranged, and fully laid out?

3) Whether there are conflicts at all levels. Are the components, frames, and layers that require screen printing reasonable?

4) Are the commonly used components convenient to use? Such as switches, plug-in boards inserted into equipment, components that need to be replaced frequently, etc.

5) Is the distance between thermal sensitive components and heating components reasonable?

6) Is the heat dissipation good?

7) Does the interference issue of the line needs to be considered?

Common problems in PCB design

1.Size mismatch

A common problem is size mismatch,which is particularly important in PCB design because it relates to the connection of the PCB to other components. If the size is inaccurate, it may lead to soldering difficulties, connection failure and other situations,which will seriously affect the performance and reliability of the PCB.

2.Design specification issues

Design does not meet the electrical or mechanical specifications is also a common problem.Designers need to ensure that all design elements (such as pads,apertures, alignment width and spacing) in line with IPC standards to avoid subsequent connection and electrical performance problems.

3.Signal and Power Integrity Issues

Signal and power integrity issues often occur intermittently and are difficult to recognize. The best approach is to find the root cause of the problem and address it during the design process, rather than trying to fix it at a later stage, which can lead to production delays.

4.Unwanted connections and layer design errors

Unnecessary connections on some graphic layers can cause misunderstandings, especially when designing multilayer boards. Sometimes a board originally designed for four layers is mistakenly designed for five or more layers, resulting in design errors.

5.Hole and Pad Problems

Improper selection of the attributes of the holes in the design, such as choosing blind buried holes instead of through holes, which will lead to the inability to generate the drill file, and in the worst case, will miss the drilling of holes, which will affect the PCB manufacturing process. In addition, poorly designed SMD pads on the holes may also cause poor connection problems.

6.Material Selection Problems

Selection of inappropriate materials is also a common problem in design, which may lead to substandard mechanical and electrical performance. Ensure that the material selection is appropriate to meet the specific design requirements is an important step to ensure the quality of the PCB.

7.Design and Cabling Check

Comparative differences between design and wiring are one of the factors that cause major errors in the final stage of PCB design. Therefore, repeated checks of device dimensions, over-hole quality, pad sizes, etc. are required. This can greatly reduce potential errors and improve the reliability of the final product.

PCB design is the foundation for connecting electronic components, and it is the key to achieving electronic system functions. If the PCB design is not reasonable, the functionality of the entire electronic system will be affected.It can also help us better control the parameters of electronic components, such as power, signal, and power supply.It also can help us better control the quality of electronic products, thereby improving product reliability and safety.