SMT assembly involves processes that require control and standardization to ensure the production of high-quality circuit boards on the assembly line. The following are the main steps of SMT FPC assembly, which must be optimized to achieve a robust process.
Solder printing
Use templates and scrapers to distribute solder paste onto the printed circuit board at a specified pressure and speed. The template contains openings called "holes" used as patterns for solder paste. The scraper carries the solder paste through the wire mesh to wet the FPC pads. It is very important to control the amount of solder paste to achieve good solder joints.
Pick and place components
A highly automated pick and place machine with multiple gantry frames, robotic arms, component feeders, and cameras. A gantry is a device for horizontally and vertically moving robotic arms. Each arm has a nozzle for picking up and placing components onto the circuit board. The workbench is used for fixing and securing FPC. Placement accuracy is a key quality attribute for achieving a good picking and placing process. If there is misalignment when placing components, welding bridges may occur.
reflow process
Printed circuit boards with wet solder paste components need to undergo reflow soldering process. SMT printed circuit board assembly lines typically use conveyor belts, where each process is arranged according to the manufacturing process. Engineering evaluation must be conducted to verify the uniformity and consistency of the temperature curve inside the reflow soldering furnace. The supplier's recommendations for the thermal curve can serve as a benchmark, including preheating, soaking, reflow, and cooling. This step is to activate the flux, completely melt the solder paste, and achieve a fully cured joint. There is inert nitrogen gas inside the reflow soldering furnace to maintain a pollution-free process.
Inspection techniques
In SMT process, inspection is very important because it can provide early feedback on quality issues caused by assembly process. Automatic Optical Inspection (AOI) is a commonly used method for detecting FPC at extremely high speeds using high-resolution cameras and image processing techniques. AOI machines can be programmed according to the design of FPC to achieve flexible conversion. Calibration and maintenance are necessary to avoid false alarms during AOI inspections.
Another non-destructive testing technique is through X-ray inspection. In this type of inspection, X-rays can pass through materials to generate grayscale images that display internal structures, especially solder joint interfaces. X-ray detection can use 2D or 3D systems, where 2D can provide planar images and 3D can provide volume information of FPC.
Electrical Test
Electrical testing in SMT assembly can be conducted through online testing (ICT) or flying pin testing, each of which has specific advantages. ICT settings include fixtures or nail beds for obtaining electrical measurement values from FPC. It is most suitable for large-scale testing. Flying needle testing involves using a testing probe with a robotic arm that can flexibly move around the FPC to test products. Functional testing can also be performed to test the FPC, while simulating the working conditions that the FPC will be subjected to.
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