Minute Components Challenge AOI
by Tom Lecklider, Senior Technical Editor
One of the major changes to affect automatic
optical inspection (AOI) equipment used during PCB assembly and
inspection is the greatly reduced size of the latest components.
As stated in an October 2007 press release from Hitachi
High-Technologies, "In recent years, the size of resistors,
capacitors, and other electronic parts has generally fallen from
0402 (1.0 mm x 0.5 mm) to 0201 (0.6 mm x 0.3 mm) with the need
to cope with size 01005 parts (0.4 mm x 0.2 mm) on the horizon."
After describing how component sizes have decreased, the Hitachi
press release announced
the development of the GXH-3 Component Mounter capable of
processing 95,000 chips/h.
MIRTEC Model M7 AOI System
To put the size of a 01005 part in
perspective, it is smaller than the dot on this lower case i.
Because these components are so small, several AOI machines
feature a choice of magnification. It's a trade-off: Greater
magnification reduces pixel size to allow accurate inspection of
01005 parts, but it also can decrease the machine's throughput.
High magnification often means that a smaller
area is inspected per unit time. To achieve high throughput on a
machine with programmable pixel size, the magnification needs to
be kept low for as much of the inspection process as possible.
This consideration applies to all types of
AOI applications although it usually is associated with the
multi-megapixel cameras used to inspect component placement
before and after solder reflow. Nevertheless, it also relates to
bare-board and paste-print inspection for which line scan
cameras often are used. For example, the Viscom S3054QS Paste
Print Inspection Machine uses an 8-kpixel line scan camera but
offers either 22-µm or 10-µm pixel size. More scans are required
to cover the same area with the higher-resolution option.
The very small parts also affect 3-D solder
paste inspection machines. Many industry sources agree that
solder volume is a critical factor in determining a good solder
joint. Accurately measuring solder volume is especially
important for 01005 components because so little solder is
involved. Laser triangulation is a common technique used to
acquire 3-D information in these machines.
As the use of 01005 components increases and
the cost comes down, AOI rates will need to increase if today's
throughput is to be maintained. This is not improbable based on
recent performance improvements being delivered in real
production environments by newer AOI machines.
Commenting on changes to components and AOI
within the last 10 years, Jeff Bishop, product marketing
engineer at Agilent Technologies, said, "During that time,
optics resolution has gone from around 30 µm/pixel to about 18
µm. System inspection speeds have increased from 1 in.2/s
to about 5 in.2/s today. And cameras have gone from 1-Mpixel analog to 4-Mpixel digital.
"In addition, machines are much more
flexible, allowing their use for paste-print inspection,
pre-reflow inspection, a mix of pre-reflow and paste,
post-reflow, post-wave, and in some cases final assembly." Mr.
Bishop continued, "This flexibility, along with very attractive
return on investment, positions AOI as a popular production
tool. Today, equipment is being used to measure placement
accuracy and for volumetric paste analysis, component part
identification, and tombstone and general solder-joint defect
detection."
YESTech Model B3 Benchtop AOI System
Aside from purely technical considerations,
the companies are changing their organizations, as well as their
AOI machines to address a global redistribution of contract
manufacturing services. This was the view of Lyle Sherwood, vice
president and technology director at Landrex Technologies:
"Local and regional low-cost markets are developing in Eastern
Europe, and other both historical and new nontraditional markets
are on the rise. Success in these areas will require more
local-language GUIs as well as renewed focus on speed and ease
of programming, inspection of smaller parts, and flexibility of
the inspection process.
"In addition," he continued, "customer and
product support strategies will need to become more creative to
effectively support a wide-spread installed base. Time to
travel, language, and new technological cultures will play a
larger role in sales and support than in the past."
Brian D'Amico, president of MIRTEC, expanded
upon the effects of these market changes. "Over the past few
years, the electronics manufacturing industry has become
increasingly polarized. There are the North American and Western
European markets in which an emphasis on low- to medium-volume
production will provide a competitive edge. In contrast, there
are the Asian, Mexican, and Eastern European markets with a
predominant focus on higher-volume production.
"An AOI machine that is quick and easy to
program is ideal for low- to medium-volume manufacturing. On the
other hand, high-volume production requires high-speed
inspection," he explained. "Fast programming also is a benefit
but is far outweighed by high-speed inspection capability."
Once you understand why AOI machine
capabilities are valued as they are by different users, the
various models and feature mixes make more sense. Nevertheless,
one machine by itself may not solve your problems no matter
which machine you choose.
Don Miller, president of YESTech, proposed
using several machines collaboratively, especially when working
with 01005 components: "For assemblies using 01005 chip
components, the post-paste print AOI system serves mostly as a
process monitoring tool because there really is no effective
inline repair mechanism for random paste defects. Any manual
manipulation tends to introduce more problems than it can
correct.
"The best inspection strategy for 01005
assemblies uses multiple AOI machines at several points in the
SMT process. By combining the defect reports from the upstream
AOI machines, the post-reflow machine can ensure the defect will
not escape detection.
Viscom Model S3088-II AOI System
"In an ideal situation," he continued, "there
are three AOI machines on the line. The post-paste system should
be tuned to detect stencil misalignment and insufficient solder,
the pre-reflow machine can detect component defects such as
missing parts or incorrect polarity, and the post-reflow system
should detect bridging and tombstone defects. Dividing the tasks
and combining results provide optimal detection."
Magnification
You can change the size of a pixel by
changing the magnification of the camera's lens. However, with
components as small as 01005 parts, maintaining a single
magnification may be impractical. On the one hand, you need to
image more than some minimum number of pixels to define the
smallest dimension of a part or marking. On the other hand, if
an imaging system is designed to acquire sufficient pixels to
inspect even 01005 components, its resolution will be an
overkill for larger parts.
One way to cope with this situation is to
change the magnification to suit the inspection job. For
example, the Agilent Medalist sj5000 Automated Optical
Inspection System provides 19-µm/pixel resolution scalable from
21 to 12 µm/pixel. The datasheet's 44.7-mm x 32.8-mm field of
view (FOV) corresponds to a 4-Mpixel monochrome camera at the
nominal 19-µm/pixel resolution.
The 0.1-mm height of a 01005 component is
equivalent to only five 19-µm pixels. That quantity may not be
sufficient for reliable inspection if you need to identify cases
of slight tombstoning, for example. At the 12-µm/pixel
resolution, the same measurement would be based on 8 or 9
pixels.
However, the camera's FOV will scale in
direct proportion to the pixel size. So, for a 12-µm pixel, the
FOV is 28.2 x 20.7 mm or an area reduction of 60%. The FOV for
12-µm pixels is only 40% that for 19-µm pixels. This means that
the inspection speed changes significantly if the magnification
is altered to cope with smaller parts.
Pixel size in the sj5000 can be changed to
any value between 21 µm and 12 µm but must remain constant
throughout the inspection process. The machine's software takes
into account the actual pixel size when measuring components.
One approach that avoids changes to the FOV
uses multiple cameras. In the latest Viscom AOI machines, four
4.5-Mpixel cameras are used to provide a 57.6-mm x 43.5-mm FOV
and either 23.4-µm/pixel or 11.7-µm/pixel resolution. The
23.4-µm size corresponds to about 4.5 Mpixels for the given FOV
and results from running all four cameras in a low-resolution
mode. When the cameras are switched to high-resolution mode
maintaining the same FOV, four times the basic resolution
results, equivalent to 2x magnification.
It may be that processing four times as many
pixels takes longer, but no change to the optical system is
involved. Viscom calls the feature OnDemandHR-Operation. It
offers selective rather than fixed high-resolution imaging.
Agilent Technologies Medalist sj5000 AOI System
YESTech's new M1 AOI uses a 3-Mpixel camera
to provide a 40.96-mm x 30.72-mm FOV with a 20-µm pixel size.
The lens can be changed to give greater magnification equivalent
to a 12.5-µm pixel size with a corresponding reduction in FOV to
25.6 mm x 19.2 mm. The M Series Machines are intended for
high-volume applications, and you choose the more appropriate
pixel size when developing the inspection program. Either 20-µm
or 12.5-µm pixels must be used throughout an inspection routine.
In contrast, YESTech's F Series Machines
feature two cameras with different magnifications. You can use
one or the other programmatically within an inspection routine.
The standard resolution camera produces a 25-µm pixel size and a
32.0-mm x 25.6-mm FOV. The optional high-magnification camera
gives 12-µm pixels and a 15.36-mm x 12.29-mm FOV. The Model B3
Benchtop AOI Machine also is available.
An interesting aspect of the M1 is its
telecentric lens. Although in theory any AOI machine could use a
telecentric lens, many don't. The benefit of having one is
constant magnification independent of object position. Of
course, there are limits to how far an object can be displaced
from its ideal position, but a telecentric lens generally gives
the same size image even if it may be slightly out of focus. An
ordinary lens changes magnification depending on the object
distance.
Using a telecentric lens in an AOI machine
may reduce the need to correct PCB warpage, for example. As long
as the board isn't warped too badly, the components being imaged
will measure the same size as though the board was not warped at
all. Object displacement in the direction of the lens axis does
not affect the image size because a telecentric lens only works
with light rays parallel to its axis.
MIRTEC's MV-7 Machines come in three sizes:
inspection areas of 250 x 350 mm, 400 x 500 mm, and 510 x 660
mm. You also have several camera and magnification options
available. A 2-Mpixel or 4-Mpixel digital color camera can be
chosen as well as 13.4-µm or 18.2-µm pixel size for either
camera. For each of the four combinations, the datasheet lists
the resulting FOV as well as the inspection speed. These machines process images at a
rate of about four per second, indicating that handling data
from twice as many pixels doesn't present a bottleneck.
On the other hand, the actual area inspected
per second does vary greatly because of the different FOVs. A
2-Mpixel camera with 13.4-µm pixel size has the smallest FOV of
21.4 mm x 16.0 mm and processes only 1,556 mm2/s. At
the other extreme, a 4-Mpixel camera with 18.2-µm pixel size has
the largest FOV of 37.2 mm x 37.2 mm and inspects 4,940 mm2/s.
Like the YESTech M Series, the magnification is constant
throughout an inspection routine.
What throughput figures do the manufacturers
quote? For Viscom, 20 cm2/s to 40 cm2/s is
listed. Agilent notes that inspection speed may vary but lists 5
in.2/s or 32.3 cm2/s—midway between
Viscom's extremes. MIRTEC speeds range from 15.56 cm2/s
to 49.4 cm2/s. The YesTech datasheet specifies > 5.5
in.2/s or >35.5 cm2/s.
The Landrex Optima II 7300 AOI System uses
one orthogonal and four angled cameras to provide imaging at
typically 8 in.2/s or 51.6 cm2/s. This
rate is as high as it is for two reasons. First, the orthogonal
camera has only 300 kpixels but runs at up to 120 frames/s in
the Model 7310.
A second reason is the relatively coarse
pixel size: 27.9 µm for 0201 components and 38.1 µm for 0204
parts, corresponding to FOVs of 1.0 cm2 and 2.25 cm2
respectively. Including images from the angled cameras, 10
images of each field of view are captured under different
structured lighting conditions. The machine's FOV is altered by
changing the camera head assembly.
Side-Viewing Cameras
Manufacturers have different terms for
side-viewing cameras. MIRTEC uses Side Viewer® Cameras, YESTech side viewing,
Viscom angled-viewing module, and Landrex angled cameras.
Agilent doesn't have side-viewing cameras, instead creating
multiple views of a component under different structured
lighting conditions.
One purpose of side-viewing cameras is to
identify lifted leads, especially on very fine-pitch components.
These cameras also provide revealing images of lifted components
and aid solder-joint inspection. In addition, side-viewing
cameras can image pins that the component itself hides from the
view of a camera directly overhead.
Lighting
Regardless of how many cameras are used and
their relative positions, the resulting image quality depends
entirely on the lighting you provide, and several approaches are
possible. For example, Viscom's 8M Color Camera Module uses
white LED lighting, which means that colored areas in images can
be compared to expected values. Color can be used in addition to
size and orientation as an indication that the component is the
right one and mounted as intended.
In contrast, Agilent's sj5000 uses a single
4-Mpixel monochrome camera but a structured lighting system
described by the company as a "multiple-color, multiple-angle,
multiple-segment LED lighting head with auto calibration." The
camera can run at a 60-frame/s rate so as many as eight images
of a component can be acquired with varied lighting. The sj5000
algorithms use the structured light angle and direction when
combining data from related images.
Agilent's Mr. Bishop commented that the new
sj5000 has been optimized as a cost-effective post-reflow
inspection solution. Although the algorithms used in conjunction
with structured lighting give the machine a degree of 3-D
capability, the company's Series 3 SP50 Solder Paste Inspection
Machine with laser triangulation is much more accurate for that
purpose. By swapping the optical head, a Series 3 Machine can
perform either solder-paste inspection as an SP50 or post-reflow
solder-joint inspection as an SJ50 Machine.
Landrex also uses structured lighting in the
Model 7310 provided by an array of 400 individually controlled
LEDs. According to the product datasheet, "The LEDs can be
programmed to provide the optimal texture, angle, direction, and
intensity of light for each defect type the cameras see."
Summary
Many types of AOI machines are available,
from large freestanding production models to bench top units
suitable for smaller batch inspection. In all cases, however,
you must determine the role a machine is to play in your
manufacturing strategy.
If you are going to use just one AOI machine
post-reflow to catch component and solder errors, you're only
getting part of the available benefit. Many machines are capable
of measuring more accurately than required simply to detect
gross errors.
Making use of AOI data as part of a
process-control system supports continued quality improvement.
Rather than simply finding mistakes, trends can be identified
before errors occur. Once trends are recognized, their causes
can be addressed through better stencil and board design or by
adjustments to upstream paste or placement machines.
The use of the latest 0201 and 01005
components and consumer demand for high-quality products have
made AOI machines an essential part of a PCB manufacturing line, not just a
desirable one.