Skip to content

What is an AOI test in SMT ?

Automated optical inspection is a key technique used in the manufacture and test of electronics board. AOI enables fast and accurate inspection of PCB PCBA and in particular PCB to ensure that the quality of product leaving the production line is high and the items are built correctly and without manufacturing faults

Bare PCB AOI Test

For Bare printed circuit boards, The AOI test verifies the finished conductor trace image for deviations from the Gerber data and finds errors that the E-Test may not discover, such as (for example) narrowed, but still unbroken conductor traces.

The AOI test is especially important for the following application areas:

1. High frequency
2. High power loads
3. High data transmission rates
4. Op-amps with high amplification factors and input resistances

The inner layers of multilayer PCB use AOI to scan before they are pressed together. This guarantees the high reliability of multilayer boards.



Automated optical inspection (AOI) is a key test method used in surface mount technology (SMT) printed circuit board (PCB) assembly to detect defects and ensure quality. This article provides an overview of AOI covering:

  • How AOI works
  • Defect types detected
  • 2D vs 3D AOI
  • Inspection process flow
  • AOI machine types
  • False call reduction
  • Limitations of AOI
  • AOI vs other test methods
  • Implementing AOI inspection

Understanding AOI technology and capabilities is important for quality management and process control in electronics manufacturing.

How AOI Works

Automated Optical Inspection (AOI)

AOI machines capture high resolution images of assemblies and use image processing software to compare against a “golden board” reference to identify defects.

Image Capture

  • Color or black/white cameras mounted above board
  • Angled lighting to detect height variations
  • Board conveyed under cameras for 100% coverage

Image Analysis

  • Software compares images against golden board
  • Image processing algorithms detect anomalies
  • Trainable defect detection and classification


  • Defect location coordinates and size data
  • Images/video of detected flaws
  • Reports with failure statistics
  • Pass/fail outcome

Defect Types Detected by AOI

AOI inspection can identify a wide range of assembly-related defects:

Component Flaws

  • Missing parts
  • Wrong or misloaded parts
  • Shifted component position
  • Incorrect orientation
  • Tombstoned parts

Solder Joint Issues

PCB Defects

  • Trace damage
  • Hole plugging
  • Foreign object debris
  • Etching or plating issues
  • Pad lifting

Marking Flaws

  • Missing silkscreen print
  • Misaligned or unclear legends
  • Barcode readability

2D vs 3D AOI


  • Lower cost inspection
  • Images from single angle
  • Limited height inspection
  • Can miss some defects


  • Constructs 3D model of board
  • Multi-angle image capture
  • Accurate height measurement
  • Detects more defect types
Types of Faults AOI can Inspect

AOI Inspection Process Flow

A typical AOI inspection sequence is:

1. Setup

  • Program inspection routine
  • Optimize lighting, cameras, focus
  • Load golden board reference

2. Verify Settings

  • Test sample boards to tune detection
  • Refine algorithms and thresholds
  • Confirm proper failure calls and none missed

3. Production Testing

  • Automatic board handling and scanning
  • Continuous operation with pass/fail indication
  • Process monitoring and trend analysis

4. Review Failures

  • Review failure images and coordinates
  • Reject boards with critical defects
  • Categorize faults by type and severity

5. Reporting and Analysis

  • Generate reports with failure rates
  • Identify failure patterns and trends
  • Perform root cause analysis
  • Implement corrective actions to reduce defects

Types of AOI Machines

There are different configurations of AOI equipment:

Inline AOI

  • Integrated into production line
  • Inspects boards right after SMT process
  • Fastest defect detection at source

Standalone AOI

  • Flexible off-line operation
  • Samples boards from line
  • Verifies process quality

Dual Lane AOI

  • Two independent inspection lanes
  • Double throughput capacity
  • Redundant capability

Mini AOI

  • Benchtop systems
  • Lower cost solution
  • Limited inspection area

Reducing False Calls


False calls where AOI reports a defect incorrectly reduce efficiency. Strategies to minimize false calls:

  • Optimize lighting, thresholds, algorithms
  • Enhance golden board with all acceptable variations
  • Mark acceptable design features as allowed
  • Train AOI system with representative samples
  • Perform regular maintenance and calibration
  • Tune detection based on inspection history
  • Review calls before rejecting boards

Limitations of AOI Technology

Despite advanced imaging technology, AOI has limitations including:

  • Low contrast defects can be missed
  • Confusion between components and markings
  • Shadowing underneath or behind components
  • False calls from irregular board features
  • Limited underfill inspection capability
  • Difficulty detecting subsurface defects

AOI vs Other SMT Inspection Methods

Compared to ICT

  • AOI finds assembly flaws vs electrical testing
  • More detailed defect and location data
  • Can be applied before electrical test

Compared to X-Ray

  • AOI is lower cost and faster
  • X-ray reveals subsurface defects missed by AOI
  • AOI has higher throughput for production

Compared to SPI

  • AOI checks assembly after reflow
  • SPI checks solder paste print before assembly

Implementing AOI Inspection

Key steps for utilizing AOI effectively:

  • Select right AOI technology based on needs
  • Program inspection routines carefully
  • Understand limitations to avoid over-reliance
  • Use AOI data for focused repair and root cause
  • Correlate AOI findings with other test methods
  • Continuously improve programs through feedback
  • Apply AOI inline for fastest defect detection
  • Implement as part of quality management system


Automated optical inspection is a critical quality control technique used throughout the SMT assembly process. This article provided an overview of how AOI works to detect surface defects on PCB assemblies. Understanding AOI capabilities as well as factors like false calls and limitations helps manufacturing engineers apply AOI optimally as part of a comprehensive quality strategy. Implemented effectively, AOI provides valuable inspection data to improve yields, reduce escapes and achieve consistent product quality.

Frequently Asked Questions

Here are some common questions about AOI testing for SMT:

Q: What types of defects can AOI detect?

AOI detects missing, shifted or wrong components, solder defects, PCB flaws, barcode issues and more. It finds assembly-related rather than electrical defects.

Q: Does AOI replace functional PCB testing?

No, AOI complements electrical and functional testing. AOI verifies component placement while circuit tests confirm electrical operation.

Q: Can AOI be used for BGA inspection?

Yes, AOI can reliably inspect BGA solder joints. 3D AOI provides accurate measurement of solder ball heights.

Q: What is the difference between inline and standalone AOI?

Inline AOI inspects boards directly on the assembly line while standalone AOI allows off-line sampling.

Q: Does AOI completely eliminate escapes and field failures?

AOI dramatically improves defect detection but cannot find all failure mechanisms like latent defects. Multiple test methods are still required.