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What are the requirements for PCB boards in PCBA soldering?
Regarding PCBA welding and processing, there are usually many requirements for PCBs; only boards that meet these requirements can be accepted for welding and processing. So, why does the welding process have so many requirements for circuit boards?
Regarding PCBA soldering processing, PCBs usually have many requirements; only boards meeting these requirements can be accepted for soldering. So, why does the soldering process have so many requirements for circuit boards?
Practice has shown that many special processes are involved in PCBA soldering, and the application of special processes immediately brings requirements for the PCB. If there is a problem with the PCB,
it will increase the difficulty of the PCBA soldering process, and may ultimately lead to soldering defects and unqualified boards. Therefore, to ensure the smooth completion of this special process and facilitate PCBA soldering,
the PCB must meet the manufacturability requirements in terms of size, pad distance, etc.
The following are the requirements for PCBs in PCBA soldering processing:
1. PCB Size
The width of the PCB (including the board edge) should be greater than 50mm and less than 460mm, and the length of the PCB (including the board edge) should be greater than 50mm. If the size is too small, assembly is required.
2. PCB Edge Width
Edge width > 5mm, board spacing < 8mm, distance between board and edge > 5mm.
3. PCB Bending Degree
Upward bending: <1.2mm, Downward bending: <0.5mm, PCB deformation: Maximum deformation height ÷ diagonal length < 0.25.
4. PCB Mark Points
Mark shape: Standard circle, square, triangle
Mark size: 0.8~1.5mm
Mark material: Gold, tin, copper, and platinum
Mark surface requirements: Smooth surface, no oxidation, no dirt
Requirements around the mark: Within 1mm around the mark, there should be no obstacles with significantly different colors from the mark, such as green oil.
Mark position: More than 3mm from the board edge, no holes, test points, etc., within 5mm of the mark.
5. PCB Solder Pads
There are no through-holes on the solder pads of surface mount components. If there are through-holes, the solder paste will flow into the holes, resulting in less solder inside the components, or solder flowing to the other side, causing an uneven surface and making it impossible to print solder paste.
When designing and manufacturing PCBs, it is necessary to understand some knowledge of PCB soldering processes to make the product suitable for production. Understanding the factory's requirements in advance can make the subsequent manufacturing process smoother and avoid unnecessary trouble.
These are the requirements for PCBs in PCBA soldering processing. Production of PCBs cannot be neglected; only high-quality PCBs that meet the requirements can allow the circuit board to better accept other special processes, giving the PCB life and injecting the soul of function.
SMT-PCB Design Principles
I. Layout of Components on SMT-PCB
1. When the circuit board is placed on the reflow soldering furnace conveyor belt, the major axis of the component should be perpendicular to the equipment's conveying direction. This can prevent the component from drifting or "standing up" on the board during the soldering process.
2. The components on the PCB should be evenly distributed, especially high-power components should be dispersed to avoid local overheating stress on the PCB during circuit operation, affecting the reliability of the solder joints.
3. For double-sided mounted components, larger components on both sides should be staggered in installation position; otherwise, the soldering process will be affected by the increase in local heat capacity, affecting the soldering effect.
4. Components with pins on four sides, such as PLCC/QFP, cannot be placed on the wave soldering surface.
5. The major axis of large SMT components installed on the wave soldering surface should be parallel to the direction of the solder wave flow to reduce solder bridging between electrodes.
6. Large and small SMT components on the wave soldering surface should not be arranged in a straight line; they should be staggered to prevent the "shadow" effect of the solder wave from causing cold solder joints or missing solder during welding.
II. SMT-PCB Solder Pads
1. For surface mount components on the wave soldering surface, the solder pads of larger components should be appropriately enlarged. For example, the solder pads of SOT23 can be extended by 0.8-1mm to avoid empty soldering causing the "shadow effect" of the components.
2. The size of the solder pad should be determined according to the size of the component. The width of the solder pad is equal to or slightly larger than the electrode width of the component, resulting in better soldering results.
3. Avoid using a single large solder pad between two connected components, because the solder on the large solder pad will connect the two components in the middle. The correct approach is to separate the solder pads of the two components and connect them with a thinner wire in the middle of the two solder pads. If the current passing through the wire is large, several wires can be connected in parallel, and the wires can be coated with green oil.
4. There should be no through-holes on or near the solder pads of SMT components; otherwise, during the reflow process, the solder on the solder pads will melt and flow along the through-holes, causing cold solder joints, insufficient solder, or even flowing to the other side of the board, causing a short circuit.
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