Automated Optical Inspection (AOI)

Quality Control

At UnityPCB, Automated Optical Inspection (AOI) is a critical step in our quality control process. We use the state of the art AOI technology, which comes with cutting-edge imaging systems and algorithms to detect faulty circuit boards that a human eye might not detect.

What Is Automated Optical Inspection (AOI)?

Automated Optical Inspection is a visual inspection method of identifying defects that are likely to happen in the manufacturing and assembly of the PCBs. In this technology, the cameras focused their lenses on the circuit board to inspect the surface features for stains, scratches, opened circuit, and short circuit. Also it can identify lack of components and wrong positions of components .
PCBs will be passed through the AOI machine after all components have been placed on the PCB and again in the final after the soldering process is over. In the process, the several capturing images are produced and used as a golden standard. In further production runs, AOI acquires new image and compares it with the past golden standard image. If it comes across area not in compliance with the standard, it points at probable defects in the area. And then technicians identify these errors and come up with solutions to correct them.

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How Does Automated Optical Inspection Work?

Automated Optical Inspection (AOI) systems rely on 3 key components: lighting, machine vision cameras, and software to processing the acquired data.
1.Lighting is of extreme importance in AOI. LED modules containing red, white, green, and blue lights are utilized in today’s AOI systems for proper and efficient lighting as opposed to incandescent and fluorescent lamps. In addition, its lighting angle addresses issues of different component height to get a proper view of all the parts.
2.High-resolution machine vision cameras are the eyes of AOI systems. With the ability to record as many as millions of pixels in one snap, these cameras can go as high as 100 frames per second. The optical system’s resolution refers to the ability to define the details in an image, and the FOV(field of view) specifies the speed at which AOI scans and the precision of the scanning procedure.
3.The processing software used is actually the commanding control of the AOI system. It also tries to match what is captured against a “golden standard– an idea of what a perfect PCB should be. This standard can be created either by scanning a known good board or by programming according to the specifications of the product. The software can recognize size, shape, position, and quality of each component, checking if there is any issue such as misalignments, solder problems, or missing parts.
All these components work together to accomplish the inspection of printed circuit boards and the identification of defects, ensuring the high quality standards throughout the production process.

Advantages and Disadvantages of AOI

AOI inspection has many advantages as listed below:

Positional Accuracy: AOI inspection is features with sub-pixel accuracy, which enables it to detect even very small deviations. We can use it to check tiny components like 01005.
Cost-Effective: Auto Optical Inspection increases the yield of PCB passes which in turn decreases the general cost.
Versatility: It can be used to examines both bare PCB and assembled boards, checking for different defects such as shorting, opening, poor soldering, wrong polarity and values.
Programmable Lighting: The dynamic and customizable LED lighting in AOI systems can detect different issues regardless of the components or PCB types.
Flexibility: PCB automated optical inspection can be applied at any fabrication process. It is used particularly after the process of reflow soldering as it enables correction of parameters of subsequent products on the production line.

Though it brings a lot of benefits, we cannot ignore the limitations of AOI inspection:

Inability to Detect Internal Defects: AOI only looks at the surface, it cannot detect some problems inside the layer structure such as improper alignments of layers or presence of voids that require to use other inspection method such as X-ray.
Difficulty with complex geometries: Complex or 3-D geometries on the PCB can be difficult for AOI systems to detect efficiently.
Color and contrast limitations: AOI may not perform well on checking low contrast defects or certain color combinations.
High Initial Setup Costs: One of disadvantages of AOI inspection is its high initial setup costs due to the expensive equipment and maintenance, which can be a barrier for small manufacturing companies.
Limited Electrical Performance Assessment: AOI does not assess the electrical functionality, therefore additional tests such as ICT are required for the optimal assurance of product quality.

What Defects Can Be Detected through AOI Testing?

AOI inspection can identify a wide range of defects, they are:

Component Flaws

Misaligned Components

Missing Components

Incorrect Components

Tombstoning

Bent or Damaged Leads

Solder Joint Issues

Insufficient solder

Solder Bridges

Excessive solder

Cold Solder Joints

Solder Balling

Lifted Solder Pads

PCB Defects

Short Circuits

Open Circuits

Copper Trace Defects

Scratches and Surface Defects

Etching or plating issues

Marking Flaws

Misaligned Markings

Blurry or Incomplete Markings

Barcode or QR Code Errors

Incorrect or Missing Markings

FAQs about Automated Optical Inspection (AOI)

1.How does AOI differ from manual inspection?

Automated Optical Inspection delivers a consistent and objective evaluation of the status of printed circuits by using high-resolution cameras and advanced algorithms. While for manual inspection, there is a possibility of human error and the efficiency is relative low.

2.Can AOI be used for all types of PCBs?

Yes, AOI can be used on the single and double side PCB as well as multilayer PCBs of different sizes and level of complexity.

3.Can AOI be used for BGA inspection?

Yes, AOI can also be used for BGA inspection but there are limitations such as hard to inspect hidden joints and false negatives and positives.

4.How Is AOI Different from X-Ray Inspection?

AOI targets external aspects of PCBs; on the other hand, defects such as solder voids and hidden joints can only be seen through X-ray. X-ray is normally used to check the complicated multilayer board that has internal problem, while AOI is ideal for surface-level inspections.

5.When Should AOI Be Implemented in the PCB Manufacturing Process?

AOI is usually carried out after the stages of depositing the solder paste on PCB pads, placement of the components and reflow soldering process.

6.Can AOI Be Used for Both SMT and Through-Hole Assemblies?

Absolutely, AOI can be used in SMT and through hole assemblies too. It checks for SMT parts for alignment, placement, and solder quality. For PTHs, it checks for soldering and their component orientation.

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