What is an AOI Inspection Machine?-The Top Flaws Found by AOI Inspection

AOI Inspection Machine Introduction

AOI Inspection Machine is an inspection method used in PCB manufacturing to automatically detect defects on assembled printed circuit boards using image processing software and computer vision algorithms.

Automated Optical Inspection came into use in the 1980s as a way to improve quality control and reduce manufacturing costs compared to manual visual inspection. The first AOI systems used simple 2D image cameras mounted above a moving PCB to scan for defects.

Modern AOI systems use advanced imaging techniques like 3D scanning, multiple angled cameras, and coherent light to detect microscopic defects and verify component presence and placement. This allows inspection of solder joints, height, coplanarity, and other attributes.

There are three main types of AOI systems:

  • 2D AOI – Uses a single top-down camera for quick inspection. Limited in defect detection ability.
  • 2.5D AOI – Uses multiple angled cameras to create a 2.5D model of the PCB, providing more inspection data than a 2D system.
  • 3D AOI – Uses lasers or structured light to scan a true 3D model of the PCB with accurate height and volume information. Provides most comprehensive inspection.

Key benefits of AOI inspection:

  • Increased quality – AOI detects defects early that could escape visual inspection like shorts, opens, missing components, misalignments. This reduces rework costs.
  • Faster process – AOI performs inspection in seconds, much faster than manual inspection.
  • Consistency – AOI provides reliable and repeatable inspection not subject to human error or fatigue.
  • Insightful data – AOI systems collect detailed data like defect type, location, frequency that provides production insights.

By automating optical inspection, AOI improves quality control efficiency, reducing manufacturing costs. Advanced 3D systems provide the most complete inspection data.

Common Defects Detected

Automated optical inspection (AOI) machines are designed to detect a wide variety of defects that can occur during the PCB assembly process. Here are some of the most common types of defects that AOI inspection is used to catch:

Open Circuits

Open circuits occur when there is a break in the conductive path of a circuit board trace. This prevents electrical current from flowing properly. Open circuits are often caused by too little solder, solder splashes, or damage to the copper trace. AOI inspection uses image comparison to identify missing solder and compare joint shapes against a reference image.


Short circuits result when two or more conductive paths touch that are not supposed to connect. This provides an erroneous current path. Shorts commonly happen due to excess solder creating a bridge between adjacent pads or traces. AOI systems use algorithms to analyze pad shapes and determine if a short is present.

Missing Components

AOI machines verify that all components are present and positioned correctly. Missing components are detected by comparing the PCB to a database reference file and identifying any components that are absent. This may occur due to pick and place errors, incorrect component reels, or human assembly mistakes.

Wrong Components

Placing the wrong component is a serious but avoidable defect. AOI inspection compares the shapes, sizes, and features of components to identify any mismatches from the reference design. For example, a capacitor may be placed where a resistor should be.

Solder Defects

Flaws in solder joints represent a common issue AOI systems detect. This includes things like excessive solder, insufficient solder, solder balls or satellites, skewed parts, tombstoning, etc. By analyzing the volume, shape, and position of solder, defects can be automatically flagged.

Assembly Errors

Human errors during manual assembly and rework represent another source of defects. AOI can check for reversed components, misaligned parts, uneven glue application, scratched PCBs, and more. The system screens the final product and catches any defects arising from manual process flaws.

In summary, AOI inspection is highly effective at catching these and many other assembly defects through optical image analysis. It serves as a final quality control check before products ship to customers. AOI saves manufacturers time and money by automatically detecting errors in real-time that could lead to field failures down the road.

Optical Inspection

Optical inspection uses cameras and lighting to visually inspect PCBs, looking for defects in the artwork, solder mask, silkscreen, and other visual aspects of the board. This is one of the first stages of AOI inspection and focuses purely on the appearance of the PCB rather than its function.

Some key aspects optical inspection examines:

  • Artwork – Checks for issues with the copper etching, making sure there are no shorts, open traces, or other defects. Looks at line widths, spacing, etc.
  • Solder mask – Inspects the solder mask layer, ensuring full coverage with no voids, scratches, or misalignment. Checks that solder mask openings are the correct size.
  • Silkscreen – Verifies the silkscreen layer meets specifications for alignment, text size and readability, and coverage.
  • Legends – Checks any on-board legends, labels, or markings are correct, readable, and properly aligned.
  • Via – Examines plated through-hole vias to make sure the plating is continuous from one side to the other, with no holes or gaps.
  • Colors – Confirm colors match required specifications. Common colors checked are solder mask, silkscreen ink, and legend/label colors.

By thoroughly inspecting the PCB artwork and physical attributes, optical inspection provides an early means of detecting defects before further production and assembly. Issues identified can be addressed prior to populating components, avoiding more costly rework down the line. Optical inspection establishes a quality foundation for the rest of the PCB production and test process.

Solder Paste Inspection

Solder paste inspection is a critical process in PCB assembly to ensure the solder paste has been applied correctly before components are placed. Some of the key defects detected during solder paste inspection include:

Insufficient or Excess Paste – The solder paste print must deposit the right amount of solder. Too much can lead to shorts, while too little can result in poor solder joints. Optical inspection and 2D or 3D SPI analyze the solder paste height, area, and volume to catch defects.

Smearing – The solder paste should print a neat brickwall pattern. Smearing occurs when the stencil separates from the PCB unevenly, resulting in blurred or smeared paste deposits. Solder paste inspection detects these uneven patterns.

Slumping – After print, solder paste needs to retain its shape before component placement. Slumping happens when the height of the paste reduces and begins to spread out. This again is caught in inspection by monitoring deposits over time.

Proper solder paste laydown is critical for defect-free soldering. Automated SPI provides fast and consistent inspection of paste height, area, volume, slumping, smearing, and other factors to ensure a smooth assembly process. Catching errors early with SPI saves on rework costs down the line.

Component Placement Inspection

Automated optical inspection (AOI) is commonly used to detect defects in component placement after the pick-and-place stage of PCB assembly. Some key defects detected include:

Missing Components

AOI inspection can accurately detect missing components by comparing the assembled board to the bill of materials (BOM) and CAD data. This allows any components entirely missing from the board to be identified.

Wrong Components

Optical character recognition and pattern matching algorithms allow AOI systems to detect if the wrong component has been placed. This includes incorrect part numbers, orientations or values.


Component leads that are misaligned with pad centroids on the PCB can lead to poor solder joints. AOI inspection analyzes the position and alignment of component placements and flags any potential issues.


If one pad lifts during soldering, it can cause the component to stand upright like a tombstone. AOI inspects for partially lifted components and pad alignment to predict tombstoning defects.

By thoroughly inspecting component placement, AOI provides valuable feedback to optimize the pick-and-place process and avoid defects in subsequent manufacturing stages. Analyzing AOI data helps improve component packaging, feeder setups, and pick-and-place programming. This ultimately leads to higher production yields and quality.

Solder Joint Inspection

Solder joints are one of the most critical areas inspected by AOI systems. Defects in solder joints can lead to electrical connection issues and overall product failure. Some common solder joint defects detected by AOI include:

Insufficient Solder

  • Insufficient solder occurs when there is not enough solder paste applied or the solder fails to adequately reflow during the soldering process. This can result in poor electrical connections, intermittent contacts, or opens.
  • AOI inspection can identify insufficient solder by looking for solder joints that are smaller than expected or have an abnormal shape. Lack of solder fillet around component leads is also a telltale sign.
  • Causes of insufficient solder include misaligned paste printing, low paste volume, non-wetting surfaces, or reflow issues like low peak temperature.

Excess Solder

  • Excess solder is the opposite problem, with too much solder paste applied or overflow of solder during reflow.
  • AOI can detect excess solder by flagging solder joints that are larger than the allowable tolerances. Blobs, icicles, or solder bridging between joints are clear signs.
  • Applying too much paste, insufficient paste tack, or excessive reflow temperatures can result in excess solder defects.

Solder Shorts

  • Solder shorts refer to solder that bridges between adjacent component leads or lands, creating an unintended electrical connection.
  • AOI easily identifies shorts by optical recognition of any solder joining points that should remain electrically isolated.
  • Common causes are solder paste bridging, solder spatter, or component misalignment.

Solder Opens

  • Solder opens describe intended solder joints that did not form properly, leaving an open circuit.
  • AOI detects opens by inspecting for missing solder between component leads and PCB pads.
  • Potential causes include insufficient solder paste, non-wetting surfaces, shifted components, or clogged vias.

By reviewing solder joints for these common defects, AOI provides critical quality control to ensure proper board assembly. This automated optical inspection protects against latent defects and avoids functional issues down the line.

In-Circuit Testing

In-circuit testing (ICT) is an inspection technique that checks the electrical connections between components on a printed circuit board (PCB). ICT utilizes test probes that make contact with test points on the PCB to verify that the right components are installed correctly and are connected properly.

Some key aspects of in-circuit testing:

  • Checks for short circuits – Probes check for any unwanted connections between nodes.
  • Checks for opens – Probes check that there is continuity in required connections.
  • Checks component values – The electrical properties of components like resistors and capacitors are verified to match specifications.
  • Detects wrong components – Checks if the right component is placed compared to the reference design.
  • Verifies polarity – Confirms that polarized components like diodes and electrolytic capacitors are inserted correctly.
  • Tests integrated circuits – Applies signals to ICs to ensure they are functioning per design.
  • Detects soldering defects – Identifies issues like cold solder joints and solder bridges.
  • Tests multiple boards – ICT testers allow testing of multiple PCBs for automated inspection.
  • Generates reports – Results are output to pinpoint fail location and support repair activities.

ICT provides a comprehensive inspection of PCB assemblies by actively testing connections and components. It serves as an important automated quality check before functional testing of boards. ICT enhances defect detection and helps avoid problems during system integration or end-use of the electronics.

Box Build Inspection

Box build inspection occurs after the assembly process is complete and inspects the finished assembly. This end-of-line inspection ensures the product is properly assembled, meets quality standards, and functions as intended before it leaves manufacturing.

Box build AOI checks for a variety of defects including:

  • Missing or incorrect components – The AOI scans each assembly and verifies all required components are present and in the correct positions. It checks that the correct parts are used based on the bill of materials.
  • Loose parts or wires – High resolution cameras on the AOI inspect for any loose wires, connectors or components that may have come detached during assembly. This prevents both functionality and safety issues.
  • Board damage – The system scans for any cracks, scratches or other damage that could have occurred during production. Damaged boards can cause operational failures.
  • Panelization defects – For PCB panels, it confirms proper spacing between boards and no damage occurred during depaneling.
  • Wire routing – Optical inspection validates all wires and cables follow the correct paths and are securely connected. Improper wire routing can cause shorts.
  • Missing labels, markings or text – Text, symbols, markings and labels are checked to ensure critical information is present for identification, warnings and instructions.
  • Overall workmanship – The AOI gauges overall build quality and assembly practices. A neat, orderly assembly indicates attention to detail.

By thoroughly inspecting completed assemblies, box build AOI provides final assurance of quality and reliability before products ship. This prevents flawed units from reaching customers.

3D AOI Inspection

3D AOI inspection utilizes laser profiling and scanning to generate a height map of the PCB, allowing for analysis of component height and solder joint height. This enables detection of defects that are difficult to identify with traditional 2D inspection such as head-in-pillow, non-wet opens, and solder voiding.

Key aspects of 3D AOI inspection:

  • Laser profiling – A line laser is projected onto the PCB at an angle while a camera captures the laser line profile from above. This creates a 3D model of the PCB surface.
  • Scanning for height analysis – The PCB moves under the laser allowing the entire board to be scanned. The peak heights are analyzed to identify component and solder joint defects.
  • Generates a height map – The scanning process creates a detailed 3D map of the PCB with x, y, and z data for every point. This enables precise measurement of heights.
  • Detects hard-to-find defects – By measuring height, 3D AOI can identify issues like head-in-pillow, non-wet opens, voids under components, and more.
  • Complementary to 2D AOI – 3D inspection can operate alongside traditional 2D optical inspection for a comprehensive analysis. 2D detects missing parts while 3D detects height anomalies.

3D AOI provides immense value in inspection coverage and defect detection. It overcomes the limitations of 2D optical inspection alone. The 3D mapping capabilities open up analysis of PCB quality and assembly integrity beyond what was previously possible.


Automated optical inspection (AOI) is crucial for detecting defects and ensuring quality in printed circuit board (PCB) manufacturing. This article reviewed the key capabilities of AOI machines in identifying flaws across multiple stages of production.

The inspection process begins even before components are placed, with AOI systems able to detect issues with solder paste printing like excess paste, insufficient paste, and misalignment. Once components are mounted, AOI checks for missing, misaligned, or incorrectly oriented parts.

Solder joint inspection is also a vital AOI function, validating that connections between components and the PCB are properly formed. Other checks enabled by AOI include verifying part markings, board fiducials, board finish, and more. 3D AOI provides additional defect detection abilities by creating 3D models of the assembled board for comparison to the original CAD data.

By performing rapid and automated inspection at multiple points along the assembly line, AOI systems catch defects early when they are easiest to correct. This prevents wasting further value-added steps on defective boards. The end result is PCBs that reliably meet quality standards, ensuring customer satisfaction and avoiding costly product failures in the field.

In summary, utilizing AOI is a best practice for any PCB assembly operation seeking to deliver flawless boards while minimizing escapes and maximizing yields. The multiple inspection capabilities of AOI machines allow comprehensive testing for the complete range of assembly flaws. The inspection data further provides helpful feedback to improve production processes and first-time quality yields over the long-term.

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