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OC White
Microscope Information
Introduction
Careful visual inspection is still the primary means of ensuring quality, and discovering defects,
in electronic assembly. Indeed, most defects, such as misaligned components, solder bridging, coplanarity
problems, soldering defects, and surface board damage (as well as component damage, such as 'popcorning'
of plastic packages) as well as solder ball problems can be detected by visual inspection.
Certainly, there are defect types that can only be discovered by more sophisticated techniques such as
X-ray imaging. But visual inspection still is the primary means of finding defects in circuit assemblies and
correcting them before they reach later stages of test and make it out into the field as delivered product.
Good visual inspection requires two basic elements: a skilled and trained inspector, and the appropriate
equipment to aid the inspector in his or her job. Given that we have the former, equipped with good vision,
we turn to the latter, since the unaided or naked eye is insufficient in itself to closely inspect today's miniaturized
parts and assemblies.
Magnification and Illumination
The human eye discerns details carried by reflected light. Therefore, it is not only necessary to
magnify details, but to properly illuminate them. The reasons for this go beyond the role of discerning
difficult to spot defects such as cracks in solder joints; operator comfort is a major factor in good inspection.
Can a fatigued worker perform a detail-oriented task well? No, and an inspector with eye fatigue is likely to
miss defects that would otherwise be detected. Therefore, it is the role of the manufacturing supervisor to
ensure that his inspectors are using equipment that minimizes the possibility of eye fatigue. Appropriate
levels of magnification for the job, as well as proper illumination, as well as ergonomic function of the
equipment (such as adjustability and positionability) are critical.
Microscopes and Illumination
The highest level of optical inspection magnification and clarity is provided by microscopes.
There are many quality microscopes available on the market, and it is not the purpose of this paper
to delve into a discussion or evaluation of the many types and options available. Obviously, virtually all
microscopes in use today are the stereo type, providing a very high quality image and to a certain degree,
some depth perception for the operator. High-magnification CCD cameras are also used in optical inspection,
whereby the image is projected onto a color monitor screen. Although there may be slightly less detail with
this approach, one advantage of this equipment is that multiple individuals can view the same image
simultaneously, for consensus, analysis, or instruction. It is also easy to capture digitized images for
transmission over the Internet, or for training videos, etc. Either way, proper lighting is critical.
Microscope/CCD camera illumination takes several forms. One type mounts on the microscope itself,
consisting of a hood enclosing a circular fluorescent bulb. The circular shape is designed to distribute
illumination evenly. Fluorescent lamps have long been favored because they supply "cool" light; however,
in the past, the greenish spectrum of light emitted, plus annoying flickering, shadows, and humming caused
by the ballast, annoyed operators and contributed to eye fatigue. The advent of solid state technology and
advances in fluorescent bulb science now make it possible for these bulbs to provide higher intensities of cool,
white light across the entire spectrum of light. Without flickering or shadows, and with silent operation.
Furthermore, something that was once impossible - the ability to dim a fluorescent bulb - is now easily
done through sophisticated microelectronic controls.
Another type of microscope light provides illumination generated by an external power unit, and supplied
via a flexible, metal-sheathed fiberoptic cable to an attachment on the microscope. This attachment will
provide light in an even, circular area through an annular ring, possibly with an iris diaphragm that can be
adjusted to maintain constant Kelvin color temperature. At the generator end, the intensity of the light can
be adjusted, giving the operator a high degree of control over the volume and intensity of light distributed on
the inspection area.
In order to provide sufficient light to the point of illumination, light of sufficient intensity must be generated
at the point of origin. In the past, there were problems with fiberoptics being burned, and intensity-adjustment
rheostats heating up and burning out. Today's generating units IR-filter the light to prevent damage to fiberoptics
and keep the light "cool". Solid state intensity controls have replaced rheostats. Slimmer profile annular light
guides facilitate connections to microscopes, and quick-connect designs make them simple to adapt to CCD
cameras.
These fiberoptic systems are also supplied, in lieu of annular rings that attach to the microscope, dual
point illuminators with focusable lenses. These give the operator the ability to adjust lighting in a number of
different ways from an infinite variety of different angles, for complete and total flexibility of illumination into
tight PCB architecture. The ability to focus the lenses adds to the operator's ability to "tailor" the illumination
as well as direct the angle and source. Depending on the unit, it may be supplied with mini-annulars or an
8-spot annular (for longer working distances), an iris diaphragm, and a color filter kit.
The Role of Color
Maintaining constant Kelvin color temperature is important in some applications because…
Discuss yellow bulbs (for inspecting photoresist), and ultraviolet (black light) bulb for inspection of
conformal coatings, plating voids and other hard to detect flaws.
Illuminated Magnifiers
Microscope-based inspection can be tiring for the operator. In applications where lower levels of magnification
are sufficient, illuminated magnifiers are simpler and easier to use than microscopes. They provide a wide
field of vision, and are less tiring to use, adjust, focus, etc. than a microscope. Often, a whole assembly
can be inspected in a single view. An illuminated magnifier is essentially a large magnifying lens mounted
in a frame with built-in illumination, all attached to the end of an adjustable arm that clamps to the workbench.
That's the simplest description, but the illuminated magnifier is much more than that in sum.
To begin with, there are different levels of glass, and these affect the clarity of the lens and the quality of
reflected light allowed through. For example, many illuminated magnifiers use grade B green lenses, but
this type of glass restricts light transmission and has been associated with long term eye fatigue. The
best lenses for illuminated magnifiers are crystal clear crown white lenses that have the highest level of
light transmission.
Different levels of magnification are referred to in terms of "Diopter", which is basically a unit of measurement
of the refractive power of lenses equal to the reciprocal of the focal length in meters. For example, a typical 5"
diameter lens may be rated a 3 diopter, with a magnification of l .75X. Same lens in 5 diopter would have a
2.25X magnification.
Light sources for illuminated magnifiers continue to be fluorescent, and these products have also benefited
from advances in fluorescent lamp technology. Usually, lamps are placed on either side, or in a "U" shape
around the lens, to provide as even and shadow-free lighting as possible. Solid state microelectronics have
made fluorescent dimmability possible, and have, as mentioned earlier, eliminated flickering and humming.
In addition, a new "glare-free" bulb has been developed. Glare contributes to operator eye fatigue and can
mask defects that would otherwise be clearly seen. A new tri-blended phosphor that differs from the familiar
cool white phosphors that were designed to generate white light, but also have the drawback of creating
reflective bounce-back and glare. This new phosphor blend eliminates the light that bounces back, thus
reducing glare by 60%. It allows the operator eyes to better absorbed and more efficiently used the light
generated by the bulb. The result is enhanced comfort, better operator performance and efficiency, and a
better quality product.
One cannot stress enough the breakthrough that fluorescent dimmability has been for illuminated
magnified inspection. The ability to dim a fluorescent bulb from 100% down to 25% is significant because
it allows highly reflective surfaces to be seen easily, whereas in the past, glare would be a significant problem.
Conclusion
A thorough knowledge of the technology available today for visual inspection, as well as the particular needs of one's
inspection application, can help the manufacturer choose the most appropriate inspection equipment for the job. Minimizing
operator fatigue - eye fatigue or otherwise - is essential to maintaining good inspection procedures, practices, and operator
effectiveness. Optical inspection equipment is not as simple as it may seem, and ongoing technological innovations continue
to improve both the quality and functionality of visual inspection equipment.
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