PCB Tech - Important factors for PCB manufacturability

PCB plays a key role in realizing the functions of electronic products, which has led to the increasing importance of PCB design, because the performance of PCB design directly determines the function and cost of electronic products. Excellent PCB design can keep electronic products away from many problems, so as to ensure that products can be manufactured smoothly and can meet all the needs of practical applications.

Among all the elements that contribute to PCB design, Design for Manufacturing (DFM) is absolutely an essential element, because it links PCB design with PCB manufacturing so that problems can be discovered and resolved in time during the entire life cycle of electronic products. As the manufacturability of electronic products is considered in the PCB design stage, the complexity of PCB design will increase. In the life cycle of electronic product design, DFM can not only make electronic products smoothly participate in automated production, and save labor costs in the manufacturing process, but also can effectively shorten the manufacturing and production time to ensure the timely completion of the final electronic product.

PCB manufacturability

Due to the combination of manufacturability and PCB design, manufacturing design is a key element leading to efficient manufacturing, high quality and low cost. The scope of PCB manufacturability research is wide, and it can usually be divided into PCB manufacturing and PCB assembly.

PCB manufacturing

As far as PCB manufacturing is concerned, the following aspects should be considered: PCB size, PCB shape, process edges and Mark points. Once these aspects are not fully considered in the PCB design stage, unless additional processing measures are taken, the automatic chip placement machine may not be able to accept the prefabricated PCB. To make matters worse, some boards cannot be automatically manufactured using manual welding. As a result, the manufacturing cycle will be extended and labor costs will also increase.

Each chip mounter has its own required PCB size, which varies according to the parameters of each mounter. For example, the maximum PCB size accepted by the chip mounter is 500mm * 450mm, and the minimum PCB size is 30mm * 30mm. This does not mean that we cannot handle PCB components smaller than 30mm * 30mm, and when smaller sizes are needed, we can rely on jigsaw puzzles. When only manual installation is required and labor costs rise and the production cycle is out of control, chip placement machines will never accept PCBs that are too large or too small.

Therefore, in the PCB design stage, the PCB size requirements set by the automatic installation and manufacturing must be fully considered, and they must be controlled within the effective range.

PCB shape

In addition to the PCB size, all chip placement machines have requirements for the shape of the PCB. The general PCB shape should be rectangular, and the ratio of length to width should be 4:3 or 5:4 (best). If the shape of the PCB is irregular, additional measures must be taken before SMT assembly, resulting in increased costs. In order to prevent this from happening, the PCB must be designed into a common shape during the PCB design stage to meet SMT requirements. However, it is difficult to do this in actual situations. When the shape of some electronic products must be irregular, a stamp hole must be used to make the shape of the final PCB have a normal shape. After assembly, the redundant auxiliary baffle can be omitted from the PCB to meet the requirements of automatic installation and space

In order to meet the needs of automated manufacturing, process edges must be placed on the PCB to fix the PCB.

In the PCB design stage, a 5mm wide process edge should be reserved in advance, without any components and traces. 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 assembling, the craft side that is used as an auxiliary production role can be dismantled.

Reference point (Mark point)

For PCBs with components installed, Mark points should be added as a common reference point to ensure that each assembly equipment can accurately determine the location of the components. Therefore, the Mark point is the SMT manufacturing benchmark required for automatic manufacturing.

Components need 2 Mark points, while PCB needs 3 Mark points. These marks should be placed on the edge of the PCB and cover all SMT components. The center distance between the Mark point and the edge of the board should be at least 5mm. For PCBs with double-sided SMT components, there should be Mark points on both sides. If the components are placed too densely to place the Mark points on the board, you can place them on the edge of the process.

PCB assembly

PCB assembly, PCBA for short, is actually the process of soldering components on a bare board. In order to meet the requirements of automated manufacturing, PCB assembly puts forward some requirements for component packaging and component layout.

Component packaging

In the PCB design process, if the package of components does not meet the appropriate standards and the distance between components is too close, automatic installation will not be performed.

In order to obtain the best component packaging, professional EDA design software should be used to be compatible with international component packaging standards. In the PCB design process, the bird's-eye view area must not overlap with other areas, and the automatic IC placement machine will be able to accurately identify and surface mount.

Component layout

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

During the component layout process, the assembly surface of SMD components and THD components should be determined. Here, we set the front side of the PCB as the component A side and the back side as the component B side. The component layout should consider the assembly form, including single-layer single-package assembly, double-layer single-package assembly, single-layer mixed packaging assembly, A-side mixed packaging and B-side single-package assembly, and A-side THD and B-side SMD components. Different assembly requires different manufacturing processes and technologies. Therefore, in terms of component layout, the best component layout should be selected to make manufacturing simple and easy, thereby improving the manufacturing efficiency of the entire process.

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

Generally speaking, the component orientation should be consistent. The component layout conforms to the principle of the shortest tracking distance. Based on this principle, the polarity direction of the components with polarity marks should be consistent, and the components without polarity marks should be neatly arranged on the X or Y axis. The height of the component should be at most 4mm, and the transmission direction of the component and PCB should be kept at 90°.

In order to increase the welding speed of the components and facilitate later inspection, the spacing between the components should be kept 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. The silk screen and the pad must not overlap, otherwise the component will not be installed.

Due to the actual operating temperature of the PCB and the thermal characteristics of the electrical components, heat dissipation issues should be considered. Component layout should focus on heat dissipation, and fans or heat sinks should be used when necessary. Suitable heat sinks should be selected for power components, and heat-sensitive components should be placed away from heat generation. High components should be placed behind low components.

There are more details that should be focused on PCB DFM, and experience should be accumulated in practice. For example, high-speed signal PCB design requires special requirements for impedance. Discuss with the circuit board manufacturer before actual manufacturing to determine impedance and layering information. In order to prepare for production on some small-sized and densely routed PCB boards, the minimum trace width and through-hole diameter manufacturing capabilities should be discussed with the PCB manufacturer to ensure the smooth production of these PCBs.