As a platform for the transmission of many components and circuit signals, the printed circuit board (PCB) has always been regarded as a key part of electronic information products, and its quality determines the quality and reliability of the final product. Due to the development trend of high density, lead-free and halogen-free environmental requirements, if professional and timely inspections are not completed, various failure problems may occur, such as poor wettability, cracks, delamination, etc.
Generally, PCBA assembly inspection technology is divided into two types: visual inspection and automatic process inspection.
A. Visual inspection
After a large number of steps in the PCB assembly process, visual inspection can be used, and the visual inspection equipment is selected according to the position of the inspection target. The effectiveness of the visual inspection depends on the competence of the inspectors, the consistency and applicability of the inspection standards.
Inspectors must be fully aware of the technical requirements for each type of solder joint, as each type of solder joint may contain up to 8 defect criteria, and there may be more than 6 solder joints on different assembly equipment. Therefore, visual inspection is not suitable for quantitative measurement of effective structural process control.
B. Structural Process Test System (SPTS)
Digitization and analysis systems for real-time and automatic video capture can significantly improve the tolerance and repeatability of visual inspection. Therefore, structural process test systems rely on certain forms of emitted light, such as visible light, laser beams, and X-rays. All of these systems process images to obtain information to identify and measure defects related to solder joint quality.
C. Automatic / Automatic Optical Inspection (AOI)
The AOI system relies on multiple light sources, a programmable LED library, and some cameras to illuminate the solder joints and shoot. Under reflected light, the leads and solder joints reflect, reflecting most of the light, while PCB and SMD reflect very little light. The light reflected from the solder joint does not provide actual height data, while the pattern and intensity of the reflected light provides information about the curvature of the solder joint. A professional analysis is then performed to determine if the solder joints are complete, whether the solder is sufficient, and if wetting has not occurred.
D. Automatic laser test (ALT) measurement
ALT is a more direct technique for testing height and shape solder joints or paste deposits. When the image of the laser beam is focused on one or more position-sensitive detectors at an angle to the laser beam, the system is used to measure the height and reflectance of some surface parts.
During the ALT measurement, the surface height is determined by the position of the light reflected by the position-sensitive detector, and the surface reflectance is calculated from the power of the reflected beam. Due to the secondary reflection, the beam may illuminate a position-sensitive detector at multiple locations, which requires a solution to distinguish the correct measurement.
In addition, when traveling along the light of the position sensitive detector, the reflected light beam may be shielded or interfered by interfering materials. To eliminate multiple reflections and prevent shielding, the system should test the reflected laser beam along an adjusted independent optical path.
How to determine the PCB assembly inspection method?
Despite the variety of detection methods, there is a big difference between AOI inspection and X-ray inspection. The three factors to consider when determining the inspection method are defect type, cost, and inspection speed. When it comes to defect types AOI and X-ray coverage, AOI is usually used for inner layer testing before lamination. Defect items include solder paste amount, component location, missing and polarity, and solder joint defects.
The above is a detailed introduction to the common methods of testing printed circuit boards. If you have any questions about our answer, you can contact us.