A.O.I.

How AOI Works 

Image Capture 

• Color or monochrome camera mounted above the PCB
• Angled lighting to detect height variations
• PCBs are passed over the camera on a conveyor system, achieving 100% inspection coverage

Image Analysis 

• Software compares the captured image to a gold plate
• Image processing algorithms detect anomalies
• Defect detection and classification algorithms can be trained and optimized

Inspection Results

• Defect location coordinates and dimensional data
• Image or video of detected defects
• Report with failure statistics
• Pass/Fail determination

AOI Detectable Defect Types

Component Defects

• Missing Components
• Wrong or Misplaced Components
• Component Position Offset
• Component Orientation Error
• Tombstoning

Solder Joint Defects

• Insufficient Solder
• Excessive Solder
• Solder Balls/Spatter
• Solder Bridges (Shorts)
• Solder Voids

Marking Defects

• Missing Silk Screen
• Misplaced or Unclear Silk Screen
• Unreadable Barcodes

PCB Defects

• Track Damage
• Clogged Vias
• Residual Foreign Material
• Etching or Plating Issues
• Lifted Pads

• Track Damage

• Clogged Vias
• Residual Foreign Material
• Etching or Plating Issues
• Lifted Pads

AOI Inspection Process

1. Setup

 Program the inspection process
 Optimize lighting, camera, and focus
 Load a golden reference plate

2. Verify setup

 Test samples to adjust inspection performance
 Optimize algorithms and thresholds
 Confirm accurate fault determination and no missed defects

3. Production Inspection

 Automatic plate transport and scanning
 Continuous operation with pass/fail notifications
 Process monitoring and trend analysis

4. Fault Review

 View fault images and coordinates
 Reject boards with critical defects
 Categorize by defect type and severity

5. Reporting and Analysis

 Generate defect rate reports
 Identify defect patterns and trends
 Conduct root cause analysis
 Implement corrective actions to reduce defect rates

AOI Equipment Types

• Inline AOI

Integrated into the production line, it performs inspection immediately after the SMT process, quickly identifying defect sources.

• Standalone AOI

Flexible, independent inspection method, allowing for random sampling and verification of process quality.

• Dual Lane AOI

Featuring two independent inspection lanes, it doubles production capacity and provides redundant inspection capabilities.

• Mini AOI

Benchtop system with lower cost but limited inspection area.

AOI Equipment Types

• Inline AOI

Integrated into the production line, it performs inspection immediately after the SMT process, quickly identifying defect sources.

• Standalone AOI

Flexible, independent inspection method, allowing for random sampling and verification of process quality.

• Dual Lane AOI

Featuring two independent inspection lanes, it doubles production capacity and provides redundant inspection capabilities.

• Mini AOI

Benchtop system with lower cost but limited inspection area.

Limitations of AOI Technology

• Low-contrast defects may be missed
• Component and marking confusion
• Shadows beneath or behind components
• Misidentification caused by PCB structure
• Limited underfill inspection capabilities
• Difficulty detecting defects within the board/under the surface

Comparison of AOI and Other SMT Inspection Methods 

Comparison with ICT (In-Circuit Test)
• AOI detects assembly defects, while ICT performs electrical testing
• AOI provides more detailed data on defect location and type
• AOI can be used before electrical testing
Comparison with X-Ray
• AOI is lower cost and faster
• X-Ray can detect internal defects that AOI cannot detect
• AOI has higher inspection speeds on the production line
Comparison with SPI (Solder Paste Inspection)
• AOI inspects assembly quality after reflow soldering
• SPI inspects solder paste print quality before assembly 

Key Points for Implementing AOI Inspection 

• Select the appropriate AOI technology based on your needs
• Carefully develop inspection procedures
• Understand limitations and avoid overreliance
• Leverage AOI data for targeted repairs and root cause analysis
• Correlate AOI results with other testing methods
• Continuously improve inspection procedures based on feedback
• Deploy AOI in-line for fastest defect detection
• Implement AOI as part of your quality management system

Conclusion

Automated optical inspection (AOI) is a critical quality control technology in the SMT assembly process. This article provides an overview of AOI's operating principles and its role in detecting surface defects in PCB assemblies. A thorough understanding of AOI's capabilities, false positives, and limitations helps manufacturing engineers optimize its application within a comprehensive quality strategy. When implemented properly, AOI provides valuable inspection data that helps improve yield, reduce missed detections, and achieve consistent product quality.