Sensors, Vision

Detecting defects with 2-D and 3-D automated optical inspection

2-D and 3-D automated optical inspection (AOI) scan printed circuit boards (PCBs) for both catastrophic failure and quality defects, but each inspection system has particular strengths and weaknesses.
By AIA June 23, 2019
Courtesy: CFE Media

The miniaturization trend in packaging has led to increased printed circuit board (PCB) density and complexity. Because boards are more complicated, there is a higher probability for defects to occur on finished PCBs. Automated optical inspection (AOI) scan PCBs for both catastrophic failure and quality defects.

AOI is often used in the manufacturing process because it’s a non-contact test method. It has primarily been used after solder reflow or in post-production. Post-reflow AOI systems inspect for many kinds of defects, including component placement issues, solder shorts, or missing solder. Faulty boards can be reworked and acceptable boards are sent to the next stage.

Detecting defects with AOI systems

Some AOI systems check a specific task, such as paste, pre-reflow, or post-reflow. AOI cost is justifiable if manufacturers spend a lot of time inspecting. AOI machines are fast and reliable. They can notify of problems that occur repetitively. And they can track main problems and help improve manufacturing processes.

AOIs inspect the following, among others:

  • Area defects
  • Component offset
  • Component presence or absence
  • Component skew
  • Excessive solder joints
  • Flipped component
  • Insufficient solder joints
  • Presence of foreign material
  • Severely damaged components
  • Wrong part.

The AOI industry has long relied on 2-D inspection principles to test workmanship quality. This technology is suitable for detecting many defects. But ultra-miniature chips, leaded devices, and LED packages make a 3-D inspection technology necessary. However, there are advantages and limitations to both technologies. Therefore, AOI systems are most efficient if they employ both 2-D and 3-D inspection technology.

2-D AOI systems

2-D inspection technology is the most common solution in use. The most advanced systems have multiple high-resolution cameras, 10 to 15 MP resolution, and precise lenses. They also use sophisticated inspection algorithms to inspect for defects.

Advantages of 2-D systems:

  • Mature technology
  • Cost-effective
  • High speed
  • Less susceptible to shadowing issues
  • Able to inspect tall devices > 5mm
  • Flexible inspection capabilities.

Limitations of 2-D systems:

  • Not capable of true-co-planarity inspection
  • Can’t provide volumetric measurement data
  • Increased false call rate.

3-D AOI systems

3-D inspection tech has existed for many years, but it’s typically only been used to inspect solder paste deposition on PCDs right after the screen-printing process. But 3-D inspection has recently been added to other areas. Laser measurement is used to provide a 3-D measurement of height sensitive devices. This method helps detect co-planarity defects that may be missed with 2-D inspection methods.

Advantages of 3-D systems:

  • True co-planarity inspection capability
  • Volumetric inspection data
  • Reduced false call rate.

Limitations of 3-D systems:

  • Emerging technology
  • Incapable of inspecting 2-D elements
  • Significant cost increase
  • Significant reduction in speed
  • Height limitation, about 5mm max
  • Shadowing issues
  • No color inspection.

2-D AOI has long been relied on for testing and inspection of defects. However, with the unique inspection needs of ultra-miniature chips, leaded devices, and LED packages, 3-D inspection technology is also necessary. Manufacturers are now able to leverage 2-D and 3-D AOI to offset the limitations of each to meet more of their testing and inspection requirements.

This article originally appeared in Vision Online. AIA is a part of the Association for Advancing Automation (A3), a CFE Media content partner. Edited by Chris Vavra, production editor, CFE Media, cvavra@cfemedia.com.


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