Background
Automated Optical Inspection (AOI) of assembled printed circuit boards has been
around for over 15 years. In the last 7 years, the PCB assembly industry has
had an increasing interest the AOI process. AOI is now accepted as a viable test
method with many tangible benefits. However, several facts about the inspection
process remain the same. Printed circuit boards will continue in the foreseeable
future to use integrated circuits and discrete components that are attached to a
substrate using solder as their electrical connection. The true challenge for
automated optical inspection systems has been optimizing the implementation of
the available technologies for lighting, cameras and image processing to
duplicate the capabilities human eye and mind. The use of the various
technologies and combining them to perform the function of inspection has been
a wildly pursued adventure with varied results. This article will review the various
technologies and their combinations to illustrate and highlight the benefits of
combined color imaging technologies.
Cameras
Camera technology is the part of the process that attempts to capture visual
information as well as the human eye. In fact, cameras have additional
capabilities that the human eye does not. Cameras have the ability to magnify
the image as would a human using a magnification lens. Most cameras operate
on a similar image capturing method by using Charge Coupled Devices (CCD).
Black and white or gray scale cameras have been the most popular camera
technology to be implemented. Gray scale cameras provide many benefits that
make them attractive to OEMs who manufacture AOI systems. From a simplicity
viewpoint, the gray scale camera provides only one CCD array. Because of the
wide industrial use of the gray scale camera, its cost has been significantly
reduced while the number of manufacturers available who can supply the
cameras has increased. However, these features only benefit the OEM and not
the application or the end user. Gray scale cameras have a limitation to their
sensitivity. There is a limitation of 256 shades of gray that gray scale cameras
can use to discriminate features of what they are imaging. This then limits the
amount of data available to discriminate images of the PCB which causes lower
accuracy.
In contrast to gray scale cameras, color cameras have been increasing in their
use and popularity in the inspection industry. The color camera offers several
pros and cons to its use. Color cameras use either one CCD array or multiple
CCDs. Single CCD arrays use filters on the individual elements to differentiate
color between red, blue and green. Multiple CCD array color cameras use three
separate CCD arrays, one for each color, red, green and blue. Multiple CCD
array cameras can be perceived as having inherently inaccurate components
that if assembled and calibrated improperly can cause inaccuracies in the
complete inspection process. However, as stated by several manufacturers of
color CCD cameras, their quality manufacturing and assembly processes
eliminates any variability of aligning the multiple CCD elements. Color cameras
are important to inspection for the same reason that the human eye can see
color to differentiate objects and help make decisions. Wouldn’t it make sense to
use the same technique when inspecting a PCB?
Pixels are the basic element of a camera sensor. Each pixel, or CCD element,
detects the amount of light being reflected by an object. Combine a group of
pixels and an array is formed. The larger the array then the larger the area that
can be imaged with consistent sharpness and clarity. The larger the area that
can be imaged by a camera at one time improves the speed at which a PCB can
be inspected. A leading trend in industry has been the adaptation of CCD arrays
measuring 1024 pixels by 1024 pixels. This is almost four times as large as
previous camera models offering 480 pixels by 640 pixels. A good comparison is
to the sensors on the retina of the human eye. The more pixels available the
better resolution an image can have at a given magnification. Larger CCD arrays
offer greater inspection area coverage at higher magnifications and improves
inspection speed.
Magnification improves the details of what is being imaged and inspected. The
greater the magnification the more detail that can be imaged. Magnification can
either be performed optically or digitally. Each method offers advantages but
only one method offers a benefit to accurate inspection. Digital magnification can
increase the magnification of an image using software techniques. However,
image sharpness and detail is lost using this method. Unfortunately, for the
digital zoom technique, image sharpness and detail are critical to PCB
inspection. A good analogy is why we use a magnifying glass for seeing small
objects. This technique improves our optical magnification. We can see the
image in full detail at a greater resolution. This is why optical zoom is preferred
for the inspection process. Optical zoom enables the image to appear larger and
in more detail without sacrificing clarity. The greater the optical zoom then the
better small components such as 0201 chips can be inspected accurately for
solderability as well as placement.
Illumination
Illumination is the key and cornerstone of inspection. When the human eye
inspects an object we adjust the lighting to create the greatest contrast between
the features that we want to see and the features that are not important.
However, when we use both of our eyes we see a feature that a camera cannot
and that is stereo vision or the ability to see depth. It is for this reason that
illumination is used in various ways to convey three dimensional information from
a two dimensional image. Illumination can be performed in a variety of methods.
However, only some methods provide the optimum image for analysis.
A white strobe light can be employed for several purposes. First the benefit of
bright illumination covers a wide area and creates strong shadows for a gray
scale imaging system. Second, strobe lighting can be used to reduce the effects
of unstable camera mounting systems. Due to the quick on-off cycle rate of a
strobe, the flash reduces the amount of camera jitter that occurs when the
camera is being moved or if a fixed mounted camera is vibrated. |
