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X-RAYS
X-ray are a form of Electromagnetic radiation, having essentially
the same nature as light rays, but having a relatively shorter
wavelength. X-rays have a wavelength of less than 100 angstrom
units as compared to a wavelength of 10,000 angstrom units with
visible light. Because of the short wavelength of x-rays and
gamma rays they are capable of penetrating materials that reflect
or absorb light rays. Unlike light years, However x-rays are
not refracted to any easily measured degree by glass and therefore
it is impractical to focus them.
X-rays are generated using an x-rays tube. When electrons traveling
at high speed collide with some material or when they have an
abrupt change of direction, x-rays are generated. In an x-ray
tube a high voltage on the target(anode) of the tube results
in electrons being “boiled”off the filament of the tube (cathode)
and striking the target material. When the electrons strike
the target they are stopped abruptly, resulting in the production
of x-rays.
X-RAY TUBE
 As
previously stated, x-rays are generated using a high voltage
source and an x-ray tube. Figure #1 shows the component of a
typical x-ray tube used in the micro focus system. This tube
uses a tungsten target(anode).
The anode or target area must be cooled on this particular type
of tube because most of the energy is dissipated in the form
of heat when the electrons strike the tungsten target.
The filament of the tube is heated by current passing through
it of several amperes from a low voltage source.
This results in electrons being boiled off the filament. The
greater the current passing through the filament, the greater
the number of electrons emitted from the filament. This electron
emission is called the tube current.
X-ray output will be proportional to tube current.
The electron optics of the x-ray tube in the x-ray systems are
designed to focus these electrons down into a very small beam,
which is, in turn, focused onto the tungsten target.
A MICROFOCUS TUBE HAS A GREATER BEAM
DENSITY, REQUIRING LESS EXPOSURE TIME TO GET A GOOD IMAGE
Because the electrons are striking a smaller target, more x-rays
will be emitted at the smaller focal point, Creating greater
beam density.
Micro-focus X-ray tube refers to the way the electron beam in
an x-ray tube Is focused down to a very small area on the target
of the x-ray tube, because The electrons are focused down to
this small size, the diameter of the area On the target emitting
X-rays is smaller. This area is called the focal spot. If the
focal spot is small, in the case of the X-ray systems 10 microns,
the System is able to resolve items at least to that size.
( 15 microns would be About 6 mils). micron is equal to about
.000059 inch, therefore 10 microns would equal about .00039
inch or about 4 mils.
THE RANGE OF THE HIGH VOLTAGE WILL DETERMINE
THE RANGE OF THE WAVELENGTHS OF THE X-RAYS?
As the High Voltage is increased, shorter wavelengths are added
to the X-ray frequencies. Shorter wavelengths penetrate denser
samples.
PENETRATION
When x-rays are generated from the x-ray tube, they are radiated
in all directions. Because of the beryllium Window and lead
casing around the x-ray tube only The x-rays passing through
the window penetrate the samples. When the x-rays penetrate
the sample some of the x-rays are absorbed in the sample and
some Are passed through the sample creating the shadow image
of the sample. The amount of absorption is dependent upon the
sample thickness,the Density of the material and the atomic
number of the material.
Shorter wavelengths of x-rays will penetrate denser samples.
X-RAY BEAM INTENSITY
The x-ray beam intensity is the function of magnitude and energy
of the x-ray. Both increased tube current and increased tube
voltage will result in an increase In x-ray beam intensity.
X-RAY BEAM DENSITY
X-ray beam density is a function of area and tube current. This
refers to the number of x-rays passing through an area at any
one time. If the tube current (mill ampere) is increased, the
number of electrons striking the focal spot of the tungsten
anode increases. This in turn increases the number of x-rays
being generated at that point in time.
Greater beam density allows for a shorter exposure time for
obtaining a radiographic image. High beam density of the Micro
focus systems enhances viewing a sample in real time.
SHORTER WAVELENGTHS HAVE MORE PENETRATING
POWER
At lower voltage levels, x-rays of longer wavelengths are produced.
At Higher voltage levels both long wavelength x-rays and shorter
wavelength x-rays are produced.
This means that at higher voltages, there will be more penetration
power.
FOR MOST CASES, WE HAVE FOUND THAT X-RAY
WAVELENGTHS PRODUCED WITH 100KV AND A TUNGSTEN ANODE ARE ABLE
TO PENETRATE MOST SAMPLE IN THE ELECTRONICS INDUSTRY X-RAY TERMS
The following terms describe some of the major characteristics
of x-ray generation.
TUBE VOLTAGE
This is the voltage that is applied to the anode (target) of
the x-ray tube. The amount of tube voltage is stated in terms
of kilovolts. An increase in tube voltage results in x-rays
of a shorter wavelength x-rays being generated.
The advantage of shorter wavelength x-rays being generated is
an increase in the ability to penetrate thicker and denser samples.
There are disadvantages to increased tube voltage. Increased
voltage can cause “with-out” where no detail is detected because
the x-rays are of a wavelength which completely penetrated the
sample. Higher tube voltage also results in more heat being
generated on the anode of the x-ray tube and greater x-ray shielding
requirements.
HIGH VOLTAGE
The high voltage applied to the x-ray tube will determine the
speed with which the electrons will travel to the target. Are
considered to be low voltage x-ray machines using an 80-120kv
x-ray tubes. Because of the small sizes of the focal spot a
much lower tube current is required for the required x-ray beam
density because the sample can be moved close to the source.
An increase in the high voltage applied to the x-ray tube will
also have a relative effect on increasing the intensity and
penetration of the X-ray beam.
Produced at 120 kilovolts contain all the wavelengths that would
be produced at 80 kilovolts, however, x-rays produced at 120
kilovolts Would also contain some shorter wavelengths that would
not be present in the 80 kilovolt x-rays. Therefore, there would
be a relative increase in the penetration of the x-rays because
of the shorter wavelengths present at the higher kilo voltage.
TUBE CURRENT
Tube current refers to the flow of electrons present in the
electron beam in the x-ray tube. Tube current is stated In terms
a mill ampere When tube current is Increased there is a relative
increase in the number of electrons Striking the tube target.
Therefore, there will be a resulting increase in x-ray beam
density improving the quality of the x-ray image by giving a
wider ranger of grays scale in a shorter exposure time.
SPOT SIZE
The spot size is generally measured in microns “one micron equals
approximately. 000039”. In the micro focus systems, the electron
beam is focused Down to a very small focal spot (approximately
10 microns ). This results In being able to resolve very small
detail. The smaller the spot size, the smaller the detail that
can be resolved.
RADIOGRAPHY
The Micro product family employs micro-focus real-time radiography
to accomplish non-destructive testing of electronic components,
hybrid circuitry, multi-layer circuit boards and surface mounted
device assemblies.
Radiography is essentially, the process used to create and image
an x-ray or gamma ray sensitive surface.
Using the highly penetrating X-rays or gamma rays which are
non-destructive, an image can be created showing the physical
internal parts of an object. The information this image or radiograph
supplies can be used to determine hidden defects or internal
irregularities.
 MILLIAMPERE
The amount of radiation emitted by an x-ray tube is determined
by the amount of current passing through
the tube, the kilo voltage and the time the tube is energized.
When the current(mill ampere)is increased there will be a corresponding
increase in the intensity of the emitted x-rays.
There is no increase in x-ray penetration because no additional
wavelengths are added to the beams shown in Figure 4.
GEOMETRIC PRINCIPLES
X-ray and Gamma rays exhibit properties which are similar to
light rays. This is especially true when we think of the radiograph
as a shadow picture of the sample produced by x-rays. It is
therefore important to be familiar with the principles of shadow
formation. The geometric principles will be explained using
light rays as being analogous to x-rays, but it should be noted
that x-rays and gamma rays Have other properties which will
affect imaging such as the Scattered radiation that light rays
do not have. These principles of Shadow formation are, enlargement,
sharpness and distortion
 The
target material of an x-ray tube is selected because of the
melting point of the material and the atomic number.
The high melting point of tungsten is desirable because of the
large number of electrons striking the target and the resultant
transformation of heat at the focal spot. The efficiency of
the material to generate x-rays is dependent on the atomic number,
therefore tungsten is doubly desirable as a target material
because of its high atomic number.
 COOLING
When the electrons strike the target (anode) of the x-ray tube
most of the energy is transformed Into heat. To prolong the
life of the target material, the heat must be dissipated from
the target area. This is accomplished on the X-ray systems by
circulating air through the inside of the x-ray tube enclosure
to reduce the temperature to a suitable operating level.
FOCAL SPOT
The x-ray system normally use a micro focus x-ray source or
a “point source’ x-ray tube. This refers to the size of he area
onto which the electron stream is being focused and the size
of the effective focal spot. The target of the x-ray tube is
placed at an angle of 20’ to the electron beam as shown in Figure
7.
This creates an effective focal spot much smaller than the actual
area of the focal spot on the target. The focal area shown in
Figure 5 is of a line-focus tube to better illustrate this cripple.
The X-ray systems use a spot-focus tube which would result In
a similar smaller “effective” spot size.
X-RAY TUBE CURRENT AFFECTS BEAM DENSITY
AND THEREFORE, IMAGE CONTRAST
As the tube current increases, the number of electrons hitting
a specific Target area on the x-ray tube anode Increase This
means that more X- Rays are being generated per spot, or there
is increase in beam density.
THE MICRO SYSTEMS HAVE A FIXED TUBE
CURRENT. APPROXIMATELY 0L.125 MILLIAMPS AT 80KV / POWER(WATTS)
EQUALS CURRENT TIMES VOLTAGE
The Micro Focus penetrate at 10 watts of power. Therefore: 10
Watts = 80,000 volts times 0.125 milliamps. ( P=VI )
THE HIGHER THE X-RAY TUBE TARGET VOLTAGE,
THE GREATER THE PROBLEM WITH SCATTERED RADIATION
Scattered radiation causes a degradation of the x-ray image.
SEALED X-RAY TUBES SUCH AS THE ONE USED
IN THE MICROFOCUS SYSTEMS, AVOID THE CONTINUED MAINTENACE OF
THE VACUUM PUMP SYSTEM USED ON THE VACUUM-PUMP TYPE TUBES. HIGH
ATOMIC NUMBERED ENCAPSULATIONS PREVENT THE IDENTIFICATION OF
LESS DENSE INTERNAL COMPONENTS
Because the x-ray image is on time of the density of a component,
and The density of an object is directly related to its atomic
number, Covering a component with a material that is of a higher
atomic Number than the internal parts of a component will prevent
the x-rays from detailing the internal component.
THEREFORE : OUR 10 MICRON SPOT SIZE EQUALS APPROXIMATELY
0.0004 INCHES
10 X 0.03937 = 0.00039 = 0.0004 INCH
 IMAGE
QUALITY
Because of the short wavelength of X-rays, they are able to
penetrate solid object. X-rays are both absorbed by the objects
and pass through the objects. When the x-rays pass through an
object they are projected onto an x-rays sensitive surface and
a “shadow” image of the object can be obtained.
When the image is projected onto x-ray sensitive film it is
called a radiograph. There are a number of factors which will
affect the quality of the recorded image or radiograph. The
amount of x-rays emitted from the source, the size of the effective
focal spot, the amount of radiation which is absorbed by the
sample, the proportion of the radiation which passes through
the sample and the affect of the recording surface or intensifying
screen.
The intensity of the x-ray beam in the X-Ray tube will be determined
by the setting of the Kilovolts control.
Therefore, the penetration of the radiation and intensity are
increased when the Kilo voltage setting is increased.
IF THE FOCAL SPOT STAYED THE SAME SIZE,
AN INCREASE IN HIGH VOLTAGE WOULD RESULT IN MUCH MORE HEAT BEING
GENERATED ONTHE TARGET OF THE X-RAY TUBE
Heat causes degradation of the target area of an x-ray tube.
Most of the Power is dissipated in the form of heat on the target
of the tube.
THE SMALLER THE FOCAL SPOT SIZE, THE
BETTER THE RESOLUTION OF THE SYSTEM
If the focal spot size is 10 microns, as with the micro focus
X-ray systems, Then you should be able to resolve detail to
at least 4 mils or less.
THE SMALL FOCAL SPOT SIZE PERMITS REAL-TIME
MAGNIFICATION OF THE SAMPLE
Because the parallax problem is minimized due to the small spot
size, The sample does not have to be in contact with the x-ray
film or the Image camera. Therefore, the distance of the sample
to the camera can Be varied resulting In magnification of the
video image.
X-RAYS REACT A MANNER SIMILAR TO LIGHT
RAYS EXCEPT, PRACTICALLY SPEAKING, THEY CANNOT BE FOCUSED
The image created by the x-rays penetrating a sample is similar
to a Shadow.
The image will not be three dimensional, just as a shadow will
Not be three dimensional.
The shadow principles can be applied to x-ray Images as well
as light images.
 The
larger the effective focal spot or area of the source the further
the source must be from the sample to Minimize the parallax
effect. This is illustrated in Figure 9.
It is also important in the formation of shadows that the sample
and the surface upon which the shadow is Projected be as parallel
to each other as is possible and that the center of the light
ray beam or x-ray beam be perpendicular to the sample and recording
surface. This will ensure that the shadow is a true image representation
of the sample.
ENLARGEMENT
It is possible to magnify the sample image on the X-Ray systems
by moving the image camera of film away from the sample and
source. It is also possible to determine the degree of Magnification
by using the following formula:
Where B is the Sample diameter, S is the shadow diameter, D0
is the distance from the source to the sample and D1 is the
distance from the source to the shadow. Figure 9 illustrates
this relationship.
SHARPNESS
Another characteristic of shadow formation is the feature of
sharpness of the image. As illustrated in Figure 10, the size
of the light source,(or the effective focal spot on the target),
will affect the sharpness of the shadow image. If the source
is more than a single spot, there will be multiple shadows cast
of a single sample which will be slightly displace d, creating
an image area which is not sharp, the penumbra.
This condition is referred to as a parallax condition. The larger
the effective focal spot, the greater the penumbra area. This
unsharpness is also affected by the distance of the source from
the sample as shown in Figure 10.
CONTRAST
When a sample is placed in the path of the X-ray beam, some
of the x-rays will be absorbed by the sample and some of the
x-rays will be transmitted through the sample. The x-rays which
pass through the sample are projected onto a recording surface
which is sensitive to the x-rays. In the case of the X-ray systems,
the x-rays are projected onto a phosphor target which in turn
projects visible light photons onto the surface of a Image Intensifier
camera tube. There are a number of factors which determine how
many x-rays will pass through the sample. First, the wavelength
of the x-rays will determine their penetration, shorter wave-length
X-ray are able to penetrate denser sample. As the Kilo voltage
is increased on the Anode of the X-ray tube, shorter wavelengths
are added to the continuous spectrum of the x-rays, increasing
the effective penetration of the X-rays.
Another factor which influences the penetration of the x-rays
is the relative density of the sample. Sample that Have a high
atomic number tend to absorb more of the x-rays, therefore,
fewer x-rays will strike the recording surface.
Finally, the exposure time will have an effect on the recorded
image or radiograph. The greater the exposure time, the more
x-rays that will be accumulated on the recording surface. Therefore,
an extremely dense sample can still have a radiograph done of
it by increasing the exposure time. The advantage of the and
is that the micro focus tube gives a beam density that, with
most sample materials inspected, is sufficient to generate a
radiographic Image in real-time.
The contrast of the radiographic image is related to the amount
of x-rays which penetrate each thickness of the sample. The
density differences in the sample are displayed as gray level
difference on the radiograph.
At the lower kilo voltage, fewer x-rays penetrate the two thick
nesses or densities is large. However, when the kilo voltage
is increased, adding x-rays penetrating the thick and thin sections
effect decrease the contrast between the two thick nesses.
 SENSITIVITY
When a sample is being inspected with X-rays and the sample
has minute variations of densities, The lowest possible voltage
should be used to increase the contrast between slight density
differences.
When a sample which is extremely dense is inspected, it may
be necessary to use a higher voltage to penetrate The sample,
thus, minute variations of density within the sample would not
be detectable. It would be possible however to increase the
exposure time of the sample to detect minor flaws. |
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