Understanding 3D AOI

What it is, and What it isn’t

7Three-D inspection has become a buzzword in the industry, but neither the meaning nor the definition of three-dimensional inspection has been standardized. What is behind the buzzword, and what are the different approaches currently offered to electronics manufactures?

A number of systems and vendors have successfully used 3D measurement for years, but predominantly in the form of solder paste inspection (SPI) or, more correctly, solder paste measurement. It is necessary to differentiate between measurement and inspection, since 3D systems actually measure the solder paste volume deposited on the individual pads.

Deploying different measurement technologies such as laser triangulation or phase shift profilometry achieves reasonably good results in performing 3D volumetric measurement, but the task is relatively simple: simply measuring bricks of dull, grey solder paste.

The success of 3D solder paste measurement during the past decade is proven through the process improvements that these systems have contributed to electronics manufacturing, particularly to the paste printing process. The adoption rate of 3D SPI systems in electronics manufacturing has grown from approximately 15% for all SMT lines a decade ago to market coverage of nearly 60% today.

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Figure 2: The tall capacitor casts a shadow on adjacent parts, requiring AOI setting modifications.

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Figure 3: A chip resistor with (a) 15% solder volume and (b) 96% solder volume

Whether the same measurement technologies used for solder paste can be applied to solder joints depends on the technology. One proven technology in conducting real 3D measurements of solder joints is phase shift profilometry. The task of measuring curved and highly reflective solder joints is complex. The shiny surface and the multi-angled curvature of the solder joint complicate the ability for a laser to map all areas. Similarly, when applying light as the part of the measurement technology regardless of LED of flash, white or multi-colored, specula reflection has to be overcome as an obstacle to obtaining information from certain regions. Multiple directions of light projections must be used in order to compensate for this specula reflection. Too few light directions will not permit such compensation.

In the current debate, 2D AOI technology, which has been deployed for the past 25 years, can’t be overlooked. AOI was the only technology available to replace manual human inspection of assembled PCBs. That there are more than 25 AOI companies worldwide illustrates the need and importance of end-of-line or solder joint inspection in electronics manufacturing. These systems are now active on more than 90% of all SMT lines, and thus much more than that of SPI systems.

Since 2D systems deploy light from multiple angles onto the PCB, components and solder joints, all results for inspection depend on either contrast (e.g., location and polarity measurement) or location of reflection points (solder joint inspection). Figure 1 shows examples of chip resistors with different solder volumes under different light angles.

visible solder joint of every component, why would you just satisfy yourself with counting the bright pixels that the solder curvature is reflecting from an angled light source? Figure 3 shows a comparison of solder joints, measured in terms of their respective solder volumes.

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Figure 4: A resistor location shown in 2D

Contrary to common opinion, the polarity of components is typically a geometric feature rather than just a print. Those chamfers, dimples or corners can easily be measured in the third dimension, rather than relying on varying color and intensity of the print on the surface.

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Figure 5: The same part in 3D

Should you measure certain aspects of component in 3D, such as component location, and other attributes in traditional 2D? The solder joint is a three-dimensional structure as well. From a measurement perspective, we think applying the lower level 2D technology to the most critical feature on the PCB, the solder joint, doesn’t make sense. Instead, applying 3D measurement to as many aspects of the components and solder joints on a PCB is the best approach to improving the quality assurance of assembled PCBs, by utilizing true measurement technology with quantifiable measurement thresholds. For example, it is easy to understand that a lead is lifted for 170um and hence is bad (out of range) if the applied threshold is 150um. No more interpretation of this result is necessary, no image processing knowledge required and certainly no room for false calls or false accepts. Quality requirements can be translated into AOI thresholds. It makes these values transparent and understandable for everyone involved, e.g., the AOI programmer, the quality department and the operator at the AOI review station. 3D measurement is not just a buzz-word, but the future technology for AOI. As 3D has proven its capability in volumetric solder paste measurement, we would expect the capability to view components and measure solder joints in 3D will prevail in AOI as well.

The advantages of 3D measurement lie in the word measurement. If you can measure the volume of each and every

Do the limits of 2D cameras cast a shadow on their future? BY AXEL LINDLOFF

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