How Does AOI PCB Inspection work During Circuit Manufacturing

PCB inspection is a crucial stage in PCB fabrication. Proper inspection verifies the functionality and performance of a circuit board. A failed or ineffective circuit board can only be confirmed by proper inspection. With the constant development of electronic devices, PCB inspection is vital.

Printed circuit boards are the backbone of any electrical or power circuit. Hence, it is crucial to ensure these boards are functional and effective. In the PCB industry, there are different types of inspection. However, we look further into automatic optical inspection in this article.

Automatic Optical Inspection – What is it?

The automatic optical inspection (AOI) is a type of inspection that checks and tests circuit boards for any possible defect. AOI PCB inspection uses automated machines to test a PCB. This PCB inspection technique checks for errors like component placement defects.

As PCB becomes more complex and vast, it is impossible to check these boards manually. Although manual inspection is still in place, AOI PCB inspection is crucial. AOI PCB inspection ensures accurate and quick inspection of the overall PCB. This inspection technique verifies the quality and functionality of a circuit board. It is indeed a reliable inspection technique in PCB manufacturing.

The AOI machine features high-resolution cameras that help to capture several images. Also, this system features LED illumination sources for making a comparison. PCB manufacturers first of all carry out a manual inspection before the AOI test. Once the manufacturer confirms the board as effective, it then runs through an AOI machine.

AOI PCB inspection checks and tests the different phases of PCB manufacturing. It inspects the assembly and fabrication process of PCBs.  The AOI system uses a camera to scan the circuit board and then detects any defects in it. This technique enables PCB manufacturers to evaluate the quality of PCB fabrication. It also helps to correct any defects or errors at any point in the PCB fabrication.

The machine scans the PCB and makes comparisons between the data it collects and the qualified specifications.  The AOI system replaced manual inspection of PCB due to its effectiveness.

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How Does AOI System Work?

aoi test

The fundamental principle of the AOI system is image processing and analysis. The optics and integrated analysis are what this system works with. The AOI machine generates an array of images for a circuit board. It then applies some techniques to determine if the PCB passes the inspection.

It compares images of the PCB with images of good boards. In this system, a series of good and defective circuit boards start accumulating. The image processor compares the test circuit board in order to match a pattern. The inspection will complete faster if the system finds a matched pattern quicker. If the AOI system can’t find a matched pattern, it will reject the PCB and issue a report. If the system has many images to check it can slow down inspection speed.

The AOI system uses another method to find components. This method is based on algorithms. It looks for patterns in an image. For example, the skeletal outline of a PCB component. This processing technique is more powerful. This is because it doesn’t have to compare the captured images with other library images. Also, it doesn’t consider minor discrepancies in color between identical PCB components.

AOI PCB inspection uses both 2D and 3D systems to facilitate inspection. There is an additional camera for the capture of 3D images. LED lamps in the AOI machine provide lighting. They ensure a consistent lighting output throughout. The 2D imaging system detects defects such as;

  • Offset ad skewed placements
  • Excess or sufficient solder
  • Missing and wrong components
  • Bridging
  • Polarity reversals

While 3D imaging system detects defects like;

  • Lifted leads
  • Billboarding
  • Package coplanarity
  • Foreign material presence
  • Presence and absence of components

Common Defects the AOI Test Detects

The AOI test detects several detects in printed circuit boards. However, we will discuss some of these defects here.

Lifted lead

This happens when one of the metal pads on a circuit board doesn’t adhere well. Sometimes, this defect occurs due to mishandling during manufacturing or excessive heat. Also, it could be a result of flexion in the PCB. Sometimes, a lifted lead can occur when leads rise when they mix with solder.

A lifted lead defect is unstable, therefore, it is problematic. The pads on a circuit board enable pins to link to the copper plate under. However, if this lead lifts, the pins can’t make the connections. Improper connections can lead to component failure. This can jeopardize the performance of the PCB.

Insufficient or excessive solder

Solder helps to connect various components to the board. If this solder isn’t enough, there is insufficient solder joint problem. The solder may not connect the components well. Cases such as this are technically referred to as solder skips. An insufficiently soldered PCB component connection will be unstable. This instability leads to an unstable electrical connection. Therefore, the board will function poorly.

Excess solder can also be a common defect in PCB manufacturing. This occurs when there is excess solder on the PCB. This sometimes happens when the soldering tips are extremely wide. Excess solder can also be a result of overapplication of solder. This defect can lead to several difficulties. It can cause solder bridges and solder balls. An AOI System will solve this problem and send the PCB back for reworking.

Missing components

This is another defect the AOI test can detect. PCBs may lack an electronic component due to some reasons. The manufacturing process may skip some necessary parts. A missing component is a serious defect on a circuit board. It can result in catastrophic failure. An AOI machine is crucial for scanning this defect.

Open circuits

AOI PCB inspection also detects open circuits. This is a common defect in circuit board manufacturing. Open circuits are problematic since they prevent the electrical current from flowing. Furthermore, open circuits jeopardize the functionality of a circuit board.

Open circuit defects can happen due to vias and traces that are improperly formed. Also, it can happen due to soldering not getting to its targets. Sometimes an X-ray imaging or visual inspection can help to detect an open circuit. However, AOI test PCB can detect even the slightest open circuits.

Misplaced or misaligned component

Component misplacement or misalignment can happen due to incorrect placement of components. Also, it could happen when PCB components shift after placing them. Component shifting happens during reflow since components easily move atop solder. Components can slide due to the solder’s surface tension.

A circuit board may still function with a misplaced or misaligned component. However, it may function slowly. Sometimes, it could even fail to function at all. Detecting misalignment with the naked eye is difficult. Therefore AOI test helps to detect this kind of defect.

Solder shorts or solder bridges

This defect happens when solder links up two components on a PCB that shouldn’t connect electrically. These shorts can happen when a PCB manufacturer uses excess solder. Also, it can occur when solder gets into an area of the PCB meant to be solder-free.

Solder bridges will happen when solder sticks with its leads before solidifying. The leads are responsible for connecting two locations on a circuit board. The solder may stick to the leads if the leads are very large. This defect is very problematic since it affects the flow of current via the PCB. Solder bridges can burn up vital traces or components on the circuit. Detecting them with the human eye is difficult. Hence, the need for AOI test PCB.

Comparing AOI with Other Methods of PCB Inspection

There are several inspection methods for PCBs. However, the AOI test seems to be superior to these inspection techniques. This is because of its flexibility, efficiency, and speed. AOI test PCB is capable of detecting complex defects on PCBs. Here we will compare other PCB inspection methods with the AOI test.

In-circuit testing

This testing method incorporates an electrical probe to assess how a PCB will function. In-circuit testing utilizes electrical current to ensure components are in the right place. This method uses electrical current instead of imaging. Therefore, ICT provides limited soldering information, unlike AOI. Although ICT can evaluate components’ quality, it can’t provide much information about excessive soldering. Therefore, AOI test PCB is unarguably more reliable.

Automated X-ray Inspection

Here, X-rays inspect circuit boards, unlike the AOI test which uses light imaging. PCB manufacturers use automated X-ray to inspect densely constructed PCBs. This is because X-rays move through the board to image it. X-ray inspection is more expensive. Therefore, test AOI is a more budget-friendly option for inspection.

Manual visual inspection

AOI is more preferred among PCB manufacturers. This is because AOI doesn’t need to touch the PCB, unlike manual inspection. For manual visual inspection, a human inspector checks the boards. Improper handling can cause defects to a perfect board. AOI scans circuit boards without disrupting delicate components. The AOI test PCB preserves the structural integrity of circuit boards. Also, AOI is more reliable than manual visual inspection.

With the improvements in AOI test, this system can detect errors without failures. When comparing the efficiency of other tests, the benefits of AOI test PCB is clear. This system features a GPU card which enhances its speed in the inspection process.

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Key Elements in the AOI System

aoi test pcb

The AOI system has some basic elements it works with. These elements are what it uses to inspect and detect errors in circuit boards.

AOI light source

Lighting is an important element in the AOI test PCB. Selecting the right lighting source can help to figure out various defects easily. The advancement in lighting technology enabled AOI to enhance images. Therefore, this enables the system to highlight defects much easier.

Test AOI systems feature a defined light setting. However, the setting depends on the type of product it is testing. Fluorescent lighting and LED lighting are widely used for AOI test. These lightings help to view defects on PCB clearly. However, fluorescent lighting degrades over time. LEDs are better than fluorescent lightings.

AOI image capture and analysis

This is a key element of the automated optical inspection system. The image capture and analysis of test AOI capture an image of the PCB. The processing software in the AOI then analyses the image. There are several image capture systems in the AOI.

The imaging systems may feature more than one camera. These cameras can move under the control of software. Still image camera system is available in the AOI System. Another type of camera is streaming video. When the AOI system is analyzing a board’s image, it checks for various features. It considers the board fiducials, component placement, and background color among others.

AOI machine programming

The AOI system must store information for an acceptable PCB to test a PCB assembly. This system must carry out the programming effectively so as to detect any fault. There are two methods for programming an AOI machine. The algorithm-based programming and the Golden board are these methods.

Advantages of AOI PCB Inspection

The AOI system offers a lot of benefits to PCB manufacturers. This system is unarguably a flexible and efficient inspection method. Some of its advantages include;

Evaluation of complex boards

A circuit board that features over 100 components should undergo test AOI. This inspection method is ideal for densely populated and complex boards.


The AOI PCB inspection method is ideal for any stage of PCB manufacturing. It is advisable to carry out an AOI test PCB after reflow soldering. This is because most defects result from issues in the soldering process. The flexibility of the AOI test PCB results in cost reduction. This is because it is an in-process inspection.

Budget-friendly option

AOI test is a budget-friendly option when compared to other forms of PCB inspection. This test is less expensive.

Network capable software

The ability of the AOI system to collect and retrieve data is one of its greatest advantages. The data it collects can be in the form of image collection or text output. Data collection is quite simple. However, data retrieval is more complex.

Several inspection objects

AOI PCB inspection works for both PCB assembly and manufacturing. For PCB manufacturing, it checks defects like open circuits, insufficient solder, and shorts. For PCB assembly, it checks component soldering and polarity among others.


In PCB production, inspection is an important step. Test AOI is a crucial and effective inspection system in the PCB industry. This inspection method ensures the accuracy and functionality of a circuit board. Therefore, its importance can never be underestimated.

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