 The
process of attaching surface mount components entails a series of mechanical
(solder stenciling and component placement) and metallurgical (solder fusion)
operations. Consequently, it is possible for components to be misaligned
or missing upon completion of the process. While good process control can
reduce the incidence of these problems, some form of inspection is still
required. In a high-mix manufacturing environment, the availability of good
statistical data is limited since the entire output for a particular circuit
board often is assembled in one lot. Therefore, the use of Automated Optical
Inspection (AOI) systems is a very efficient way to supplement process control
and improve quality levels. |
| The
advantages of AOI systems include: |
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AOI
systems can identify process problems resulting in missing and misplaced
components. AOI Systems are ideally suited for inspecting passive components
(1206, 0805, 0603, and 0402 package types), which generally have the highest
defect rate. Use of an AOI system to inspect such components can improve
first-pass yields by 20 to 25 percent. |
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AOI
systems can point out equipment problems such as misaligned or insufficient
board supports and bent nozzles. AOI data can reveal patterns of problems
that can be traced to particular pick-and-place machines, indicating maintenance
needs. |
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AOI
data indicate areas for process improvement. For example, based on measurement
information obtained from an AOI system, pick-and-place machines can be
recalibrated to bring component placements closer to computer-aided design
(CAD) data. |
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Use
of AOI systems can result in process enhancements that reduce the time circuit
boards spend in inspection and minimize the volume of product requiring
repair. Using component failure information obtained from an AOI system,
operators can more quickly track missing parts, offset errors and polarity
defects. Also, if quality levels drop below a prescribed percentage or if
multiple errors of the same type are found on consecutive boards, the manufacturing
process can be halted immediately and corrective action taken. |
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AOI
data can be used to establish a capability baseline for the process. A typical
baseline is defined by determining the X, Y, and Z (theta) movement after
reflow for: 1206, 0805, 0603, and 0402 passive components; 50 mil pitch
small outline integrated circuits (SOICs); and 20 mil pitch quad flat packs
(QFPs). These package types can then be placed on a test capability outline
board (48 are required for a full DOE) and inspected to create a capability
baseline. |
| Mr.
Tech Dweeb Tech Tip |
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The
acronym “DOE” is often used in place of Design of Experiments.
DOE is a structured methodology for establishing process variability
and capability by tabulating the data from successive passes
through the process. Engineers use this data to determine whether
the process can meet the customer’s requirements. |
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| Selecting
the AOI System Right for You |
| When
choosing an automated optical inspection (AOI) system, it is important to
first define and understand your company’s manufacturing process needs.
The system that you select should help to identify deficiencies in your
process and result in improved product quality. It is best to begin with
a survey of your current process. Focus on determining the most common types
of component placement defects resulting from your process. When comparing
AOI systems, evaluate which system has the imaging technology and features
best suited to identify and remedy problems with your company’s manufacturing
process. |
| Implementation
of an AOI system should focus on: |
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improving
assembly line process control, |
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ensuring
high first pass yields and |
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establishing
measurable benchmarks across all assembly lines. |
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The level of inspection
accuracy and repeatability provided by the AOI system must be suited to
your manufacturing process. Inspection accuracy for resistor and capacitor
chips is usually not as critical as for leaded devices, and different
systems offer different capabilities in this regard. For example, some
systems attempt to identify defects based on the use of a “golden board.”
These systems compare the circuit board that is being inspected to the
“golden board” and identify all differences, regardless of whether the
differences represent real defects or just harmless variations. In contrast,
AOI systems that provide position measurement data offer superior defect
detection and process control for component placement. In addition, it
is easier to verify that the performance of these AOI machines meets industry
measurement standards (i.e., IPC-A-610c and J Standard 001).
|
| A
critical aspect of any inspection system is its robustness. Designs often
have 500 or more components per board side. If the inspection process results
in too many false calls, line operators will have to re-check the results
too often and will quickly lose confidence in the accuracy of the results.
Poor robustness in the inspection of component placement is often caused
by cosmetic variations in the board or its components. An AOI system that
is sensitive to changes in the color or finish of a circuit board, or that
relies too heavily on special lighting techniques, may have problems with
variations in the board finish, lighting, and board or component color.
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False calls include defects identified as good placements and
good placements identified as defects. |
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At a minimum, an AOI
system should measure the position of each component along its X, Y and
theta dimensions, and should check that the component’s polarity is correct.
Actual component positions should be compared to computer-aided design
(CAD) data to see whether each component position is within acceptable
tolerances. Components positioned outside of tolerances should be identified
and the measurements should be used to update statistical process control
(SPC) charts.
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Inspection Technology |
| There
are two main types of technology for performing automated optical inspection
(AOI) for component placement: |
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camera-based
systems and |
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laser-based
systems. |
| Most
systems use either gray-scale or color charge-coupled device (CCD) cameras.
The cameras collect images of the circuit board, and the images are analyzed
to determine whether there are any defects in each area of the board. Camera-based
systems can be very fast, but because they rely on the brightness of light
reflected from the board, they can be sensitive to changes in lighting conditions
and materials. Most systems that rely on cameras for image collection have
a programmable lighting feature for creating optimal images of each site
or component. However, as board complexity increases, problems with lighting
contrast or shadowing may arise. As images become more complex, the image
processing becomes more difficult and inspection cycle times can drop. |
| Laser-based
inspection systems for component placement use a laser scanner to create
a 3-D image of the circuit board. This 3-D image is based on the height
of the board surface and its components, and is much less sensitive to changes
in component color. Laser scanning systems also can create a 2-D gray-scale
image, similar to the image from a CCD camera. This image can be used to
identify objects where there is little height contrast, such as board fiducials,
and to detect component leads in solder paste. Laser scanning provides accurate
position measurements of components, resulting in fewer false calls and
the type of information needed for optimal process control. |
| Mr.
Tech Dweeb Tech Tip |
|
dsi
uses AOI to inspect all SMT assemblies. On a typical day over
250,000 components are scanned for acceptance. |
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