What are the SMT processing processes?

The basic components of the SMT patch processing process are: solder paste printing, SPI, patch, first-piece inspection, reflow soldering, AOI inspection, X-ray, rework, and cleaning:

1. Printing solder paste: Its function is to print the solder-free paste on the pads of the PCB to prepare for the welding of components. The equipment used is a screen printer, which is located at the forefront of the SMT production line.
Both solder paste and patch are thixotropic and have viscosity. When the solder paste printer moves forward at a certain speed and angle, it exerts a certain pressure on the solder paste, pushing the solder paste to roll in front of the scraper, generating the pressure required to inject the solder paste into the mesh or leak hole.
The sticky friction of the solder paste causes the solder paste to shear at the junction of the scraper and the stencil of the solder paste printer. The shear force reduces the viscosity of the solder paste, which is conducive to the smooth injection of the solder paste into the steel mesh opening leak hole.

2. SPI: SPI plays a considerable role in the entire SMT patch processing. It is a fully automatic non-contact measurement used after the solder paste printer and before the patch machine.
Relying on technical means such as structured light measurement (mainstream) or laser measurement (non-mainstream), the solder after PCB printing is measured in 2D or 3D (micron-level accuracy).
Principle of structured light measurement: A high-speed CCD camera is set in the vertical direction of the object (PCB and solder paste), and a projector is used to illuminate the object with periodically changing striped light or images from the upper side. When there is a high component on the PCB, an image of the stripes shifted relative to the base surface will be captured. Using the principle of triangulation, the offset value is converted into a height value.
Thus, the defect of solder paste can be discovered in time before reflow oven soldering, thereby avoiding the occurrence of unqualified finished PCBs as much as possible, which is a quality process control method.

3. Chip mounter: The chip mounter is configured after SPI. It is a device that accurately places surface mount components on the PCB pads by moving the mounting head.

It is a device used to achieve high-speed and high-precision placement of components. It is the most critical and complex equipment in the entire SMT patch processing and production.

4. First-article detector: It is a machine used for SMT first-article inspection. The principle of this equipment is to automatically generate the inspection program by integrating the BOM table, coordinates and high-definition scanned first-article images of the PCBA to be the first-article, quickly and accurately inspect the patch processing components, and automatically determine the results, generate the first-article report, so as to improve production efficiency and capacity, and enhance quality control.

5. Reflow soldering: Reflow soldering is to re-melt the solder paste solder pre-allocated to the PCB pad to achieve the mechanical and electrical connection between the solder end or pin of the surface assembly patch processing component and the PCB pad.
The reflow soldering machine used in the reflow soldering process is at the end of the SMT production line.

6. AOI: The scanning image is formed by using the reflection principle of light and the characteristics of copper and substrate with different reflection capabilities for light. The standard image is compared with the actual board layer image, analyzed, and judged whether the object to be inspected is OK.

7. X-ray: The X-ray inspection equipment penetrates the PCBA to be inspected through x-rays, and then maps an x-ray image on the image detector.
The quality of the image is mainly determined by the resolution and contrast.
This equipment is usually placed in a separate room in the SMT workshop.

8. Rework: It is to repair the PCB board with bad solder joints detected by AOI.
The tools used include soldering iron, hot air gun, etc.
The rework position is configured at any position of the production line.

9. Cleaning: Cleaning is mainly to remove the solder slag of the flux on the PCBA board processed by the patch.
The equipment used is a cleaning machine, which is fixed at the offline back end or packaging.