All You Need to Know About Automated Optical Inspection (AOI)

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Manufacturing the highest quality products is of utmost importance for every manufacturer. Visual inspection of PCBs is an important aspect of quality assurance. Traditionally, the visual inspection process has been carried out by humans. However, technological advancement has also introduced automated AI solutions in PCB inspection. The availability of the Automated Optical Inspection method has significantly revolutionized the quality control process.

Let’s get to know about all the important aspects of Automated Optical Inspection (AOI) below.

What is Automated Optical Inspection (AOI)?

AOI is a quality control method in PCB assembly that has been used since earlier assembly days. Automated Optical Inspection helps in finding PCB errors earlier. It also provides valuable data that you can further use in root cause analysis.

In this process, cameras help to scan the PCBs in fine detail to check for any component failures and defects. The machine scanner will use light imaging to evaluate workmanship quality in PCBs.

How does Automated Optical Inspection work?

An Automated Optical Inspection (AOI) system more often contains multiple lighting sources and cameras (also video cameras). The lighting sources help in illuminating the PCBs under inspection from different angles. At the same time, cameras will record video and capture multiple images to perform comprehensive image processing.

The measurement of AOI generally utilizes machine scanners to perform PCBs scanning autonomously without any human visual inspection need. It is because the scanner can check possible defaults that can lead to product failure. It will check critical components' absence and other flaws that could diminish the performance and quality of PCBs. Smaller defects often include soldering defects, components’ misalignment, etc.

Automated Optical Inspection (AOI) is highly useful for different stages of production. It can perform well for:

  • Post-reflow and pre-reflow inspection
  • Solder paste inspection
  • Bare board inspection

Each of these inspection forms can unveil defects in the design of PCB boards. Fortunately, AOI testing can help you reach these errors before even the production of PCBs proceeds on a flawed or defective board.

PCB manufacturing

Defects to review during an AOI

Automated Optical Inspection (AOI) can review different components and soldering errors. If you don't detect these errors in earlier production or assembly stages, these can lead to malfunctioning or even product failure.

Let’s get to know the type of defects reviewed during an Automated Optical Inspection below.

So, here we go:

Soldering defects

A low-temperature melting allows used to solder PCBs. Soldering is based on a lead-free alloy or a tin or leads alloy to create a connection between metal components of PCBs. PCB soldering is similar to welding but doesn't melt the board’s metal components.

Soldering errors can occur when components fail to develop connections as they should. AOI can check for different soldering defects to ensure the accuracy and high-performance of your PCBs.

The common type of soldering defects that are reviewed during Soldering inspection include:

  • Solder shorts or solder bridges
  • Open circuits
  • Excess solders
  • insufficient solders

Lifted Lead

A lifted lead problem occurs when a small metal pad fails to obey on PCB. Excessive heat or mishandling of PCBs during production can also cause lifted lead issues because of flexion in the Printed Circuit Board.

Regardless of the reason that led to the lifted lead problem, this error can cause instability of PCBs. These pads on boards let the pins connect to the underneath copper plates. With lifted leads, pins will be unable to create proper connections. As a result, this problem will cause PCB failure and negatively affect your board's performance.

Misplaces or misaligned components

Components misalignment and misplacement can occur when PCB components are placed incorrectly or shift after placement. During reflow, component shifting problem isn't uncommon. It is because the parts can move easily over molten solder. The PCBs may still work with misaligned or misplaced components, but the performance will be sluggish. At times misalignments are two minor to inspect with the human eyes. However, AOI can help you to detect these easily.

Missing components

A component can be missing on PCBs for different reasons. These include skipped production processes or Inadequate soldering. In any case, missing components on PCBs are major flaws that can lead to catastrophic failure. Having a reliable and consistent inspection with AOI can let you configure and fix missing component errors more effectively.

Why are AOI tests popular?

PCBs are an important part of AOI tests. It should be delivered without any faults when manufacturing or assembling a board. Otherwise, the final implications of PCBs can be disheartening.

The increasing modern PCBs complexity has made manual inspection processes obsolete. Therefore, checking PCBs with AOI has become vital to find its defects, from soldering problems to bare board inspection.

Moreover, contemporary markets now demand top-notch, high-volume products that can reach their potential customers faster without compromising product quality inspection. In this situation, AOI is the fundamental solution to consider for an effective PCB test strategy. This inspection method ensures that defects are caught as quickly as possible from the production phase to products reaching the market.

Importance of AOI in PCB manufacturing

Automated Optical Inspection (AOI) is a highly important aspect of PCB production. It is important to ensure that PCBs are manufactured rightly to perform high-performance tasks, even in complicated electronics applications. However, if you still don’t know why AOI is highly important in PCB manufacturing, then look at the key reasons below. These reasons will certainly let you understand things better.

So, here we go:

  • Earlier error detection

AOI is a helpful inspection method that indicates production defects in solder paste applications and bare boards. AOI testing can also help determine PCBs' assembly problems in earlier stages. This inspection method can even offer you quick results to perform quick responses for addressing PCB problems.

Manufacturers can catch defects in their PCBs earlier in the manufacturing process. This practice will reduce the cost of correcting these errors significantly. Ultimately, you can avoid issues re-occurring while running a large-scale production. Otherwise, this could be highly time-consuming and expensive.

  • Real-time inspection

Most importantly, you can get real-time Automated Optical Inspection data and results. Real-time results can help improve the PCBs' production process and offer better quality control by rectifying potential problems.

The algorithms of AOI can track and document defective and non-defective assemblies of PCBs. So, you can experience enhanced efficiency in aspects of process management, documentation, and operational integrity.

  • Improved product quality

Automated Optical Inspection (AOI) helps in ensuring that your PCBs will function as intended. Ensured product quality can help to improve customer satisfaction. Consequently, it will enhance the reputation of your business and provide you peace of mind about the service quality you offer.

  • The customizability of the AOI process

Although Automated Optical Inspection is an automated process, you can customize the process per your testing requirements. You can set different testing parameters in AOI to tell your scanner what you want it to test in your PCBs.

You can adjust your AOI system’s settings accordingly with every changed design specification. This practice will make it easier to test and detect the errors you want to find effortlessly.

  • Consistency in evaluating complex board

Currently, the modern PCBs contain above 100 components on our boards. Inspecting these components physically can become harder. However, all the components will undergo a thorough inspection process in AOI. This testing method can inspect even the intricate PCB designs with the same accuracy level. This accuracy level can’t be matched with a human eye.

Principles of AOI testing

Different technologies are available to test and check the errors in PCBs. Some of these technologies are robust in-circuit testing methods. These are suitable for full-coverage PCB design testing that guarantees more human-error-free coverage than other testing methods. AOI is one of such technologies.

However, AOI inspection is based on three principles: illumination, cameras, and processing tools. Let’s get into the details of these below.

Lighting used for AOI testing

Highly illuminated lighting sources are used to project different PCB defects more easily. PCBs are placed under a highly illuminated surface to perform the inspection process. Earlier different lighting styles were used in AOI scanning. These include:

  • Fluorescent Lighting
  • UV or infra-red lighting
  • LED Lights

At present, LEDs are the most widely used lighting system in AOIs. This lighting type offers more unified or stable illumination applications than others.

Camera configurations for AOI

Two types of cameras are used in AOI. These include streaming video cameras and still image camera systems. 3D cameras can provide better imaging. However, any camera used in AOI is moveable under the application software control to capture and detect faults. Speed, accuracy, and image quality are the key factors influencing camera choice use in AOI.

Processing software

Two types of processing software are used to Program AOI machines for PCBs inspection. These include:

  • Algorithm-based programming
  • Use of Golden Board

You can choose adequate processing software based on your inspection preferences.


When it’s about using PCBs in your products, ensuring they’ll perform as intended is important. Automated Optical Inspection (AOI) can help test PCBs for possible errors and defects. Consequently, getting perfectly working PCBs will become easier.


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