Advanced AOI techniques provide manufacturers and OEM
customers with exceptional inspection reliability.
Automated optical inspection (AOI) technology is making significant
progress as PCB designs continue to pack in more circuitry per square
inch and create new inspection challenges. Nowhere is this felt more
than in assembly. Both designers and EMS providers need to stay abreast
of the latest advances in AOI tooling to ensure that designs can be
inspected to yield highly reliable assembled products. Precise optics,
advanced AOI algorithms, powerful computing, and easy-to-read graphical
user interfaces (GUIs) are critical to the success of PCB inspection
whether it is at the bare board level or in pre- post-assembly.
The common goal among AOI system manufacturers is to improve lighting,
computing power, vision software, and signal-to-noise ratio (SNR).
These improvements make AOI products more intelligent, flexible, and
with far more repeatable results that are superior to human visual
inspection. In AOI terms, SNR refers to the signal or image represented
by a featured component and its markings of interest. Noise comes in a
variety of forms that need to be suppressed to achieve a cleaner image.
Lighting is a critical factor for managing SNR. In the case of
post-assembly inspection, the AOI tool must have the capability to view
all the features and markings of a component to efficiently perform an
inspection. Appropriate light conditions amplify the features of
interest and verify them, while suppressing any features considered
noise. LED-based lighting, now standard for newer AOI gear, is stable
and easily controlled. Older style incandescent and fluorescent
lighting are still used in some newer systems.
Further, the increasing complexity of PCB manufacturing is severely
challenging AOI’s computing power. Hence, increasingly
powerful
data crunching capability is critical to acquiring an image, comparing
it to a golden board, processing the inspection, and communicating the
information to the database. One of the biggest challenges is that all
this computing needs to happen continuously and in real time.
High-end AOI systems operate on 3-dimensional
CAD data. They render an
image in 3D to perform component verification for cracks or flaws, lead
contamination or oxidization, scratches, and other defects. These
advanced AOI systems use 4 to 6 cameras to capture an image the cameras
translate the light pattern from the inspected component into an
electronic image.
FIGURE 1
shows an AOI image
while the software is being “trained” to view a
component
and verify markings. In this process, the system uses progressive
scanning to acquire a series of images and then compares them against
each other. In this training sequence, the computer analyzes the image
of the board’s features to exact specifications. Here, the
“brains” of the AOI system is the vision computer
and its
software, where complex algorithms store details and verify board
images while performing the initial inspection. The results of the
initial inspection are an acceptance or rejection of the stored
parameters of the acquired data.
An image from a golden board is then used as a reference to compare it
with images of inspected boards. Cameras scan the device under test
(DUT) for catastrophic failures or missing components and quality
defects, also known as manufacturing processes defects. These defects
include fillet size, the shape of solder joints, and component skew,
among others. The design of AOI cameras plays an important role in
boosting their effectiveness. The objective here is to place the camera
at an angle so that it can achieve maximum light collection for
component inspection. This allows the camera to capture as much area as
possible to apply a wider field of coverage to an inspected device.
AOI can be applied at various stages of the manufacturing process,
including bare board inspection, and can be used to verify the inner
layer stack up, lamination tolerances, and registration of layers at
the bare board manufacturing level. At the PCB assembly level, AOI can
perform solder joint inspection at pre- and post-reflow stages. At
pre-reflow, AOI inspects for the placement accuracy of the components
that the pick-and-place machine has dispensed. At post-reflow, it can
check for accurate component placement after the board is processed
through a reflow oven.
In the bare board application, AOI can be used to verify trace and
space width and violations, as well as inspect for excessive or
insufficient copper removal during or after the etching process. AOI
also can inspect for trace shorts, cuts, jumps, and other defects in
the manufacturing process.
Therefore, it is important for the PCB fabricator and EMS provider to
use AOI inspection at various stages of the process. AOI can also serve
as a process validation indicator and reduce the amount of rework and
touch up at the end of the assembly process, increasing the process
yield in a PCB fabrication operation.
Specifically, at board assembly, AOI checks for such problems as area
defects, component offsets, component polarity, component presence or
absence, component skew from surface mount pads, excessive solder
joints, flipped components, insufficient solder joints and paste around
leads, tombstoning, solder bridges, and solder paste registration. In
FIGURE 2, AOI debugs
and fine-tunes component verification by comparing it to a similar
component in the database parts library.
Other Key Features
Foremost among AOI features is the offline programming option, which
allows a user to program the AOI system offline and avoids downtime,
helping to keep the system continuously inspecting boards. This feature
is both cost effective and saves time. Another important AOI feature is
solder bump (blob) analysis shown in
TABLE 1. This
analysis verifies the presence or absence of solder on component leads.
Optical character recognition (OCR) is another useful AOI feature. More
advanced AOI systems use optical character verification (OCV). Both
perform character matching, status checks, and data basing, where OCR
and OCV correlate text in the captured image with an image algorithm
saved in the database. Based on this comparison, OCR and OCV will flag
a particular image if there are flaws in the component.
AOI is especially valuable for visually defining component
manufacturers’ markings. Many manufacturers can make the same
component, but their markings will always be different. Therefore,
it’s vital for the AOI software to be sufficiently advanced
to
correctly associate specific markings with the correct component
manufacturer.
Staging and Lighting
Most AOI systems have built-in “intelligent”
staging and
lighting, which saves time in inspection and component verification.
Using these options sometimes requires the installation of macro
programs to utilize these AOI system features.
Intelligent AOI can utilize multiple forms of lighting. One source
reflects light and causes the highlighting of a particular feature,
while other light sources can be used to obscure undesirable component
features, which are considered noise. Hence, intelligent AOI determines
the required lighting parameters, either ambient or created light, and
can determine angles and the correct lighting intensity applied to a
given component.
Proper staging and lighting setup reduce variability in component
images. They act to properly present the image for the vision computer.
Without proper presentation, the vision inspection process can be
slowed, preventing AOI from properly inspecting the desired components
within given time constraints.
New AOI Requirements
Today’s AOI systems are being challenged by rapidly advancing
technologies that create more complex PCBs than ever before. It may
become evident to a manufacturer that the AOI system being used is
outdated when it can no longer detect newer and smaller packages or the
minute component markings on them. In these instances, extremely
high-powered camera magnification and specific angles may be necessary
to accurately inspect these small packages and their markings.
Accurate color detection should also be incorporated in newer AOI
models. Some through-hole components are color coded like resistors and
capacitors. AOI detects the colors on the bands of these devices, but
often, close colors in inspection images appear similar to an AOI
system, making inspection difficult. This is a problem that requires
attention in future AOI development. Recognizing odd-shaped PCBs and
non-symmetrical components is another improvement that can make AOI
systems operate more efficiently. Due to these unusual attributes, some
AOI models have difficulty inspecting these types of PCBs and
components.
As for its limitations, an AOI system can be more useful by giving it
the capability to import data from commonly available software
applications and platforms, and be able to manipulate that data to
obtain desired results. Some AOI systems have size and weight limits
for the PCB. For many tools, an 18-inch by 20-inch, three to five pound
board is the largest that can be inspected pre/post assembly.
The ability to produce statistical process control (SPC) data is one
particular feature that would make AOI more useful. SPC comes in handy
when there are recurring failures of specific components, and this
feature would allow tracking by lot and date code.
On the plus side, newer AOI systems have the capability to assign a
barcode to all boards passing their inspection. Plus, newer systems
incorporate lights that can automatically adjust intensity for the
board under inspection, increasing detection sensitivity and accuracy
of the system. But what’s missing and should be incorporated
into
future developments is a “grey test.” This feature
measures
the brightness level on the tip of an SMT pad to detect a defective
condition, such as an incorrect amount of solder. This problem area
could be improved by the use of different and perhaps better
illumination sources.
Finally, a good AOI machine has at least six cameras to inspect the
top, bottom, right, left, front and back areas of a board, as shown in
FIGURE 3.
The quality of cameras has come a long way from those in the past, and
are more powerful with considerably higher image capturing and
processing capabilities. These advances account for one of the more
important aspects in the accuracy of today’s AOI machines.
An advanced feature is the capability of AOI machine to be able to
check and detect shorts, especially those hidden under an IC.
Inspecting and catching a simple open circuit using an AOI inspection
is relatively an easy task, but an improvement in existing AOI machines
would be the ability to find the hard to find opens, which may require
more powerful algorithms and techniques.
The alternative to AOI is human visual inspection, which is not
acceptable in light of the growing complexity and reduced component
sizes used on PCBs. For effective human PCB inspection, technicians
typically must have 10 to 15 years experience visually inspecting PCBs,
but regardless of experience, while progressing through the workday, an
inspector will become tired and less efficient. As this occurs,
carelessness can lead to an increasing number of board defects.
That’s why advanced AOI is a “must” to
provide OEM
customers with the ultimate reliability in the inspection of their
highly complex PCBs.
PCD&F
Zulki Khan
is president and founder of Nexlogic Technologies and can be reached at
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