Impact of various illumination concepts on fault detection in optical inspection systems
In the past, automated optical inspection systems (AOI) focused on increasing inspection speed along with the number of cameras and megapixels. For a successful AOI process, however, false call rate and safe fault detection is as important as the overall inspection speed. Highly sophisticated optical inspection systems are equipped with various illumination modules. Fault detection and false calls reduction depends on the right selection of light-frequency, -angle and -intensity, not just cameras and algorithms used.
- Infra-Red for enhanced contrast on IC bodies
- Ultra-Violet for enhanced contrast on tin-plated surfaces
- Coaxial to avoid unwanted reflections
- Colored imaging for polarity tests
- Multi-Directional light for better OCR/OCV results
- Optical filter for nano-coating defect detection
Although white orthogonal illumination is able to cover approximately 85% of all the inspection tasks, which include presence check and solder quality, there are applications where this simple top-white light will fail. Special lighting, infra-red light for low contrast component detection or ultra-violet illumination for enhanced fiducial mark detection, for example, can help in achieving an almost 100% test coverage.LEDs mounted at certain angles (0°, 30°, 45°, or 50°) can improve certain attributes of the image of components and help reading labels or finding solder bridges. Conventional ring illumination modules, mounted around the light source, will always emit the light with an offset angle to the camera, which, in some cases, can cause interfering reflections. A true top illumination, where the light source (LEDs) and image capturing device (camera) are aligned coaxial, will be able to suppress such interference and enhance contrast and recognition quality.
To meet today's challenges, an AOI system must be able to apply such varied illumination for each test step independently without sacrificing inspection speed. As a result the inspection system needs to be capable of flexibly and rapidly triggering light pulses with various directionality, intensities and wavelengths, while at the same time, capturing resulting images and applying test algorithms in real-time.