[Table of Contents]People old and young enjoy waxing
nostalgic about and learning some of the history of early electronics. Popular Electronics was published from October
1954 through April 1985. All copyrights (if any) are hereby
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 hole.
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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
Anode of this gigantic x-ray unit gets one million
volts. G-E built it for Sutter Hospital, Sacramento, California.
As 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
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.
Posted October 14, 2013