As an essential part of electronic products, printed circuit board (PCB) plays a key role in realizing the function of electronic products, which leads to the increasingly prominent importance of PCB design, because the performance of PCB design directly determines the function and cost of electronic products. Good PCB design can keep electronic products from many problems, thus ensuring that products can be smoothly manufactured and meet all the needs of practical applications.

Of all the elements that contribute to PCB design, manufacturing Design (DFM) is absolutely essential because it links PCB design with PCB manufacturing in order to find problems early and solve them in time throughout the life cycle of electronic products. One myth is that the complexity of PCB design will increase as manufacturability of electronics is considered at the PCB design stage. In the design life cycle of electronic products, DFM can not only make electronic products participate in automatic production smoothly, and save labor costs in the manufacturing process, but also effectively shorten the manufacturing time to ensure the timely completion of the final electronic products.

PCB manufacturability

By combining manufacturability with PCB design, manufacturing design is a key factor leading to efficient manufacturing, high quality and low cost. The research of PCB manufacturability covers a wide range, usually divided into PCB manufacturing and PCB assembly.

LPCB production

For PCB manufacturing, the following aspects should be considered: PCB size, PCB shape, process edge and Mark point. If these aspects are not fully taken into account at the PCB design stage, automatic chip laminating machines may not be able to accept prefabricated PCB boards unless additional processing measures are taken. To make matters worse, some plates cannot be made automatically using manual welding. As a result, the manufacturing cycle will be longer and labor costs will increase.

1. PCB size

Each chip installer has its own desired PCB size, which varies according to the parameters of each installer. For example, the chip installer accepts a maximum PCB size of 500mm * 450mm and a minimum PCB size of 30mm * 30mm. This does not mean that we cannot handle PCB board components smaller than 30mm by 30mm, and can rely on jigsaw boards when smaller sizes are required. When you can only rely on manual installation and labor costs are rising and production cycles are out of control, chip SMT machines will never accept PCB boards that are too large or too small. Therefore, in the PCB design stage, the PCB size requirements set by automatic installation and manufacturing must be fully considered, and it must be controlled within the effective range.

The following figure illustrates the PCB board design document completed by Huaqiu DFM software. As a 5×2 board, each square unit is a single piece, measuring 50mm by 20mm. The connection between each unit is achieved by V-cut/V-scoring technology. In this image, the entire square is shown with a final size of 100mm by 100mm. According to the above requirements, it can be concluded that the size of the board is within the acceptable range.

2. PCB shape

In addition to PCB size, all chip SMT machines have requirements for PCB shape. A normal PCB should be rectangular in shape with a length to width ratio of 4:3 or 5:4 (best). If the PCB is irregularly shaped, additional measures must be taken prior to SMT assembly, resulting in increased costs. To prevent this from happening, the PCB must be designed in a common shape during the PCB design phase to meet SMT requirements. However, this is difficult to do in practice. When the shape of some electronic products must be irregular, stamp holes must be used to give the final PCB a normal shape. When assembled, redundant auxiliary baffles can be removed from the PCB to meet automatic installation and space requirements.

The picture below shows the PCB with irregular shape, and the processing edge is added by Huaqiu DFM software. The whole circuit board size is 80mm * 52mm, and the square area is the size of the actual PCB. The size of the upper right corner area is 40mm by 20mm, which is the auxiliary craft edge produced by the bridge of the stamp hole.

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3. Process side

To meet the requirements of automatic manufacturing, process edges must be placed on the PCB to secure the PCB.

In PCB design stage, 5mm wide process edge should be set aside in advance, without leaving any components and wiring. The technical guide is usually placed on the short side of the PCB, but the short side can be selected when the aspect ratio exceeds 80%. After assembly, the process edge can be removed as an auxiliary production role.

4. Mark point

For PCBS with components installed, Mark points should be added as a common reference point to ensure that component locations are accurately determined for each assembly device. Therefore, Mark points are SMT manufacturing benchmarks required for automated manufacturing.

Components require 2 Mark points and PCBS require 3 Mark points. These marks should be placed on the edges of the PCB board and cover all SMT components. The center distance between Mark point and plate edge should be at least 5mm. For PCBS with double-sided SMT components, Mark points should be placed on both sides. If the components are too close together to place Mark points on the board, they can be placed on the edge of the process.

LPCB assembly

PCB assembly, or PCBA for short, is actually the process of welding components onto bare boards. In order to meet the requirements of automatic manufacturing, PCB assembly has some requirements for assembly package and assembly layout.

1. Packaging of components

During PCBA design, if component packages do not meet appropriate standards and components are too close together, automatic installation will not occur.

In order to obtain the best component packaging, professional EDA design software should be used to comply with international component packaging standards. During PCB design, the aerial view area must not overlap with other areas, and the automatic IC SMT machine will be able to accurately identify and mount the surface.

2. Component layout

Component layout is an important task in PCB design because its performance is directly related to the complexity of PCB appearance and manufacturing process.

During component layout, the assembly surfaces of SMD and THD components should be determined. Here, we set the front of the PCB as the component A side and the back as the component B side. The assembly layout should consider the assembly form, including single layer single package assembly, double layer single package assembly, single layer mixed package assembly, Side A mixed package and side B single package assembly and side A THD and side B SMD assembly. Different assembly requires different manufacturing processes and techniques. Therefore, in terms of component layout, the best component layout should be selected to make manufacturing simple and easy, so as to improve the manufacturing efficiency of the whole process.

In addition, consideration must be given to the orientation of component layout, spacing between components, heat dissipation, and component height.

In general, component orientation should be consistent. Components are laid out in accordance with the principle of minimum tracking distance, based on which components with polarity markers should have uniform polarity directions, and components without polarity markers should be neatly aligned along the X or Y axis. The height of the component should be up to 4mm, and the transmission direction between the component and the PCB should be 90°.

To improve the welding speed of components and facilitate subsequent inspection, the spacing between components should be consistent. Components in the same network should be close to each other and a safe distance should be left between different networks according to the voltage drop. Silkscreen and pad must not overlap, otherwise components will not be installed.

Due to the actual operating temperature of the PCB and the thermal characteristics of the electrical components, heat dissipation should be considered. The layout of components should focus on heat dissipation. If necessary, use a fan or heat sink. Appropriate radiators should be selected for the power components and heat sensitive components should be placed away from heat. The high component should be placed after the low component.

More details should be focused on PCB DFM, and experience should be accumulated in practice. For example, high-speed signal PCB design requirements have special impedance requirements and should be discussed with the board manufacturer prior to actual manufacture to determine impedance and layering information. In order to prepare for production on small sized PCB boards with dense wiring, the minimum wiring width and through-hole diameter manufacturing capacity should be discussed with the PCB manufacturer to ensure smooth production of these PCBS.