 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).
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| 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. |
| 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 |
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dsi uses AOI to inspect
all SMT assemblies. On a typical day over 250,000 components
are scanned for acceptance. |
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