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December, 2012
Combining 2D and 3D in a Single AOI Platform
By Brian D’Amico, Mirtec Corp., Oxford, CT
manufacturing process becomes pro- gressively more complicated, there is the probability for defects on many finished PCB assemblies. For years the Automated Optical Inspection (AOI) industry has relied solely upon two-dimensional (2D) inspection principles to test the quality of work- manship on electronic assemblies. Advances in conventional 2D
A
optical inspection have made this technology suitable for detecting such defects as missing components, wrong components, proper compo- nent orientation, insufficient solder, and solder bridges. However, there is an inherent limitation to the ability to inspect for co-planarity of ultra- miniature chips, leaded devices, BGA and LED packages. True co-planari- ty inspection of these challenging devices is an absolute necessity and literally requires the addition of a third dimension in inspection capa- bility — 3D inspection technology.
Higher PCB densities and miniaturization of electronic packaging technologies have resulted in an unprecedented level of complexity.
It is important to understand
that there are advantages and disad- vantages associated with both 2D and 3D inspection technology. In order to achieve the highest level of quality assurance, the AOI system must, therefore, employ a combina- tion of both 2D and 3D inspection technology in a single platform.
2D Inspection Today’s 2D inspection technolo-
gy is the most widely adapted AOI solution available to electronics man- ufacturers. The most advanced sys- tems are configured with multiple ultra high resolution cameras, 10 to 15 MegaPixel resolution, precision telecentric lenses and multi-tiered lighting technology to properly illu- minate regions of interest across the assembly. The most technologically advanced hardware, however, is not
dvanced miniature packaging has led to ever-increasing PCB density and complexity. As the
enough. The AOI system must also provide a sophisticated arsenal of 2D
ty indications is sufficient to make 2D technology an absolute require-
There are basically two differ-
ent methodologies by which 3D inspection is performed in AOI machines. The first is Laser Measurement, the second is Multi- Frequency Moiré Phase Shift Image Processing. Laser Measurement methodology uses a laser subsystem to scan across gull-wing lead tips as well as provide 3D measurement of BGA and other height-sensitive devices. This proven methodology works very well for detecting co-pla- narity defects that might otherwise be missed with conventional 2D inspection. Unfortunately, this tech- nology does not provide volumetric measurement of a given region of interest and is primarily reserved for testing gull-wing leaded devices and BGAs. Another limitation of Laser Measurement is the inability to inspect solder fillets post reflow.
Advanced Methodology Multi-Frequency Moiré is by far
MV9 AOI system provides both 2D and 3D imaging.
inspection algorithms to characterize and inspect for manufacturing defects on finished PCB assemblies.
The obvious advantages of 2D: l
l Cost effective inspection solution. l High speed inspection capability.
Mature Technology. l l l
Ability to inspect SMT and thru-hole technology.
l Color inspection capability.
Less susceptible to shadowing issues.
Ability to inspect J-lead solder fillets.
l Ability to inspect tall devices >5mm. l
Extremely flexible inspection capability.
The disadvantages are fewer: l
Incapable of true co-planarity inspection.
l
l Increased probability for escapes. l
Does not provide volumetric measurement.
Increased false call rate. As we can see, although there
are some inherent limitations with 2D inspection technology, there are some significant advantages which make 2D inspection an extremely viable technology. Even the simple ability to test for two-dimensional component nomenclature and polari-
Multi-frequency Moiré imaging (left to right) original image; pattern optical image; Moiré pattern image; 3D shape image.
directly after the screen printing process. Over the past few years, however, 3D inspection has emerged as a viable technology for examining gull-wing and BGA devices as well as a host of other co-planarity sensitive circuitry on finished PCB assemblies. But the main reason for the industry trend toward 3D inspection technolo- gy is to compensate for the limita- tions of conventional 2D inspection.
phase unwrapping techniques, the 3D profile of test surface can be recon- structed for precise measurement. Multi-Frequency Moiré takes
this one step further in that two or more line patterns of differing fre- quencies are projected onto a given test surface in order to characterize objects of varying heights. The most advanced systems
Continued on page 57
ment for automated inspection. 3D Inspection Technology Although 3D inspection technol-
ogy has existed for many years in the electronic inspection industry, this technology has primarily been reserved for inspecting solder paste depositions on printed circuit boards
the most advanced methodology for testing true co-planarity of virtually any given region of interest on the PCB. Moiré 3D Phase Shift Image Processing is a methodology by which a single or multiple projectors are used to project a shifting pattern of lines on a given region of interest. A digital camera then captures the image of deformed lines as they are shifted across the test surface. By applying phase shift analysis and
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