Automated Optical Inspection and X-Ray Inspection
a free tutorial or summary about auomatic or automated optical inspection
as well as automatic X-ray inspection.
Automatic or automated optical inspection (AOI) and X-ray inspection are two
key techniques that are used in the manufacture and test of electronics printed
circuit boards. They are two test technologies that enable fast and accurate
inspection of electronics assemblies to ensure that the quality of product
leaving the production line is high and the items are built correctly and
without manufacturing faults.
Despite the major improvements that have been made, modern circuits are far
more complicated than boars were even a few years ago. The introduction of
surface mount technology, and the subsequent further reductions in size mean
that boards are particularly compact. Even relatively average boards have
thousands of soldered joints, and these are where the majority of problems are
found.
This increase in the complexity of boards also means that manual inspection
is not a viable option these days. Even when it was an accepted approach, it was
realised that it was not particularly effective as inspectors soon tired and
poor and incorrect construction was easily missed. With the marketplace now
requiring high volume, high quality products to be brought to market very
quickly very reliable and fast methods are needed to ensure that product quality
remains high. AOI and X-ray inspection are valuable tools in an integrated
electronics test strategy that ensure costs are kept as low as possible by
detecting faults early in the production line.
One of the solutions to this is to use automatic or automated optical
inspection systems. These can be placed into the production line just after the
solder process. In this way they can be used to catch problems early in the
production process. This has a number of advantages. With faults costing more to
fix the further along the production process they are found, this is obviously
the optimum place to find faults. Additionally process problems in the solder
and assembly area can be seen early in the production process and information
used to feedback quickly to earlier stages. In this way a rapid response can
ensure that problems are recognised quickly and rectified before too many boards
are built with the same problem.
How they work
AOI systems use visual methods to monitor printed circuit boards for defects.
They are able to detect a variety of surface feature defects such as nodules,
scratches and stains as well as the more familiar dimensional defects such as
open circuits, shorts and thinning of the solder. They can also detect incorrect
components, missing components and incorrectly placed components. As such they
are able to perform all the visual checks performed previously by manual
operators, and far more swiftly and accurately.
They achieve this by visually scanning the surface of the board. The board is
light by several light sources and there are likely to be a number of high
definition cameras. This enables them to monitor all areas of the board, even
those hidden in one direction by other components.
The AOI system takes time to �learn� the board. The system needs to be able
to add the board to its memory, looking at what components are there, their
positions and also look at the quality of the soldered joints. It must be able
to recognise and adapt to differences in the appearances of the board that
result from the normal production process variations. However it must be able to
recognise any that affect performance. To achieve this it is normal to run a
number of good boards through the system before full production starts so that
the system can �learn� the board.Today the best AOI machines can be set up
relatively quickly, but older or less capable machines may require a
considerable period of �learning� the board as production levels increase. Only
after this period are they reliably able to inspect boards.
X-Ray Inspection
The growing trend to use ICs with packages such as ball grid arrays where the
connections are underneath the chip and not visible means that ordinary optical
inspection is not viable. Additionally as the connections are underneath the
chip package there is a greater need to ensure that the manufacturing process is
able to accommodate these chips correctly. Additionally the chips that use BGA
style connections are generally the larger ones with many connections and it is
essential that all the connections are made correctly.
As they are not visible the only alternative is to use a low level X-Ray
inspection. These inspection systems are more costly than ordinary optical
systems, but they are able to check all the connections, even those underneath
the chip package.
Summary
It is essential that any printed circuit board manufacturing area is able to
check the quality of the boards coming off the end of the line. Only in this way
are they able to monitor quality and when problems are detected to rectify the
process so that further boards are not affected by the same problems. In this
way optical and where necessary X-ray inspection are two essential tools for the
manufacturing industry.
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