April 1955 Popular Electronics
Table of Contents
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inspection capability is a vital tool not just in medicine but in
industry. During my tenure with a major cellphone power amplifier
company as an RF Applications Engineer, I relied heavily on x-ray
imaging for many of the teardown reports that I wrote on competitors'
products. It included everything from determining metal layers in
integrated circuits to deducing multi-layer ceramic PCB routing
lines and distributed components to just getting a quick look inside
a shielded enclosure without needing to disassemble it. A skilled
x-ray technician can make a huge difference in the quality of information
that can be gleaned from the images since angle, power level, and
focusing takes a deft touch. My first experience with using x-rays
for inspection was while doing automated test system design for
production base station equipment companies. Very stringent PIM
(passive intermodulation) specifications were required for high
power filters in order to minimize 3rd and higher level intermods.
The N-type connectors ended up being the Achilles' heel of the switching
filter matrix, and we had ordered a large quantity from one highly
qualified manufacturer. Quite a few needed to be returned to the
factory for rework because of failed intermod tests. The relationship
got a bit contentious after a while. One problem ended up being
that the connectors were supposed to have staking pins to guarantee
that the connector body could not turn during torqueing. The vendor
swore to me that their inspection process was so perfect that there
was no way any could have been missed. Fortunately for me, the company
I worked for also happened to manufacture x-ray tubes and had an
x-ray machine for testing the tubes. I had the production test technician
slide one of my suspect filters into the machine and sure enough,
the filters that had the stainless steel pins installed showed in
high contrast to the aluminum filter cases. I then had a way to
verify every filter, which was not possible with only a visual inspection
because an epoxy backfill material was placed into the staking pin
Penetrating powers of x-rays help industry probe
secrets of materials and new products.
X-rays are similar
to radio waves in that they are electronically produced, are invisible,
and travel at the speed of light. X-rays, however, are in the extremely
high frequency range and have very short wave-lengths. Even the
shortest of radio waves, the so-called "microwaves" - are gigantic
by comparison. A typical wavelength of x-ray radiation is 0.0000000001
meter (one ten-billionth of a meter). These extremely short wavelengths
have great penetrating power, and can pass through substances which
light cannot penetrate.
Besides their well-known use in dental
and medical examinations, x-rays have many industrial applications.
For example, x-ray apparatus is used to detect internal flaws in
metal castings, check packaged foods for presence of foreign particles,
inspect welds, check the alignment of elements in electron tubes,
check the centering of the wire in insulated cables, etc.
Anode of this gigantic x-ray unit gets one million volts. G-E
built it for Sutter Hospital, Sacramento, California.
shown in the diagram, the x-ray tube is basically a diode. Electrons
emitted from the filament are attracted down the length of the tube
to the copper anode. The anode contains a tungsten insert which
acts as a target for the electrons. Traveling at a very high speed,
the electrons strike the target, producing the x-rays. To give the
electron stream its high velocity, a large amount of voltage must
be applied to the tube. Plate voltages as high as two million volts
have been used in commercial x-ray units.
Electrons from filament hit anode at high speed; resultant
radiation creates x-rays.
The object to be x-rayed is placed between the x-ray tube and a
sheet of photosensitive film. The x-rays penetrate the object and
strike the film. This action produces a shadow image of the internal
structure of the object. For example, if the object being examined
is a pulley belt with internal reinforcement wires, the x-rays will
pass easily through the rubber portion of the belt, but will be
obstructed by the wires. The developed film will therefore show
an image of the wires (see photograph above).
In some applications,
speed of inspection is an important feature and the time required
to develop the film introduces an objectionable delay. In these
cases, a fluorescent screen is used instead of the film. Such screens
glow where they are exposed to x-rays, and thus produce an immediate
image. An installation of this type is known as a fluoroscope.
October 14, 2013