What is AOI? What defects in PCB assembly can 2D AOI detect?

AOI stands for Automatic Optical Inspection, and it is now widely used in the electronics industry for visual inspection on PCB assembly production lines, replacing traditional manual visual inspection.

The basic principle of AOI technology is to use image processing to compare the object under inspection with a standard reference image to determine whether there are significant differences. Based on this comparison, it judges whether the object meets the required standards. Therefore, the performance of an AOI system largely depends on its image resolution, imaging capability, and image analysis technology.

In its early days, AOI was mainly used to check for surface printing defects on integrated circuit (IC) packages. However, as technology progressed, it began to be widely used in SMT (Surface Mount Technology) production lines to inspect the quality of soldering and assembly on PCBs (Printed Circuit Boards) after component placement.

The biggest advantage of AOI is that it can replace manual inspection before and after the reflow oven in SMT processes, and it can often detect assembly defects more accurately than the human eye. However, just like human visual inspection, AOI is primarily limited to inspecting surface-level defects. It struggles to detect solder joints hidden under components or at their edges.

One major drawback of AOI is its tendency to produce false rejects in areas with ambiguous grayscale or shadow contrast. While using different lighting colors may help resolve some of these issues, components that are covered by others or solder joints located beneath components remain problematic. This is because traditional AOI can only inspect areas that direct light can reach. For example, components under shielding frames or near their edges often escape detection due to these limitations.

As a result, it's rare for a PCB assembly production line to rely solely on AOI to ensure product quality. Typically, additional tests like ICT (In-Circuit Test) and FVT (Functional Verification Test) are performed. Some lines even incorporate AXI (Automatic X-ray Inspection) to check the quality of solder joints under components (e.g., BGA packages) using X-ray, ensuring 100% test coverage for the PCB.

What defects can AOI detect on assembled PCBs?

Modern AOI systems can fully detect the following placement defects, which can also generally be identified by a careful manual inspection:

Missing components、Skewed components、Tombstoning

Additionally, due to limitations involving lighting, angle, and resolution, the following defects can only be detected under certain conditions:

Wrong component、Incorrect polarity、Lifted leads、Deformed leads、Solder bridges、Insufficient solder、Cold solder joints、False solder joints

In summary, although AOI is a powerful tool, it does have some inherent limitations. However, it can provide real-time quality analysis during SMT processes and offer immediate feedback to help improve SMT operations, ultimately boosting production yield.

Furthermore, with the advancement of 3D imaging technology and improved MCU computing power, many equipment manufacturers are now developing 3D AOI systems. These not only offer technological differentiation but also provide more realistic imaging than traditional 2D AOI, making it easier to identify defects accurately.