Surface World May 2020 Surface World May 2020 | Page 55

TESTING & MEASUREMENT
(e.g. liquid nitrogen) and they have much better resolution (120eV –
200eV) when compared to proportional counter detectors. As a
result, they are better able to resolve similar alloys when performing
elemental analysis, much thinner coatings can be measured, trace
analysis is more accurate, elements of lower atomic weight and
measurement of multiple layers is far better than with proportional
counter detectors.
The above components are essential for correct functioning of the
instruments; however, some instruments can possess multiples of the
same component. For example, instruments do exist where multiple
X-ray generators have been fitted to use different primary excitation
beams to preferentially excite different atoms in a sample. Multiple
detectors can also be used to speed up measurements or isolate
specific responses from atoms in a sample.
Instrument Types
There is a wide variety of XRF instruments on the market currently and
they generally fall into 4 categories:
Chambered – Beam down systems:
These are typical XRF systems on the market. The X-ray beam is
generated above the sample; thus, these instruments tend to have
larger chambers, tables for sample support and potentially XY
motorised stages which permit automated measurements of many
samples at once, increasing throughput. Focusing of the instrument
can be manual or automatic and they can accommodate complex
component geometries.
Chambered – Beam up systems:
These instruments offer a smaller footprint than the “beam down”
systems, but normally have a smaller chamber. They unfortunately do
not handle complex geometries as their focus is fixed to a window
onto which a sample is placed. This limits the type of sample that can
be tested to components with flat sides. The benefits of having a
system of this type is that any sample that needs to be tested is
immediately in focus once placed on the window.
Portable instruments:
These instruments are miniaturised versions of the above instruments.
They are primarily used for large components and/or Positive Material
Identification (PMI) however, certain manufacturers have designed
these instruments to be able to measure multiple layered coatings.
This makes these instruments highly versatile, however, because they
lack a chamber, they require extra engineering and operator vigilance
to be operated safely. Some manufacturers offer these instruments
with a chamber for added versatility and safety.
Special projects:
These instruments tend to be more complex than the off the shelf
instruments above. They can include, but are not limited to,
instruments which operate under a vacuum or helium purge [for the
analysis of extremely light elements], reel-to-reel systems which
automatically analyse strip plated components and offer feedback
control to the plating line [for thickness testing of coatings on strip
plated connectors], Wafer handling systems which incorporate into
clean room environments for automated measurements [used for
semiconductor wafer manufacture]. These systems tend to allow more
automated control of the systems that they are integrated with.
Sample Preparation and Parameters
Affecting Readings
XRF tends not to require much sample preparation at all. Once a
component is manufactured to its final specification or indeed any
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intermediate steps, the sample can be analysed by XRF. This makes
XRF an extremely versatile method for testing that samples meet
manufacturers specifications. Not only are solid samples able to be
tested but XRF can be utilised for testing liquids for metal content,
powders and slurries for trace element analysis, plastics or rubber
for RoHS applications to name but a few.
There are six factors that influence the trueness and precision of X-ray
fluorescence measurements:
Measuring Distance:
The measuring distance is one of the first parameters that needs to be
addressed when a sample needs to be tested. The signal at the
detector (count rate) can be summarised by the equation
Thus, as the measuring distance increases, the count rate drops as a
function of the equation above. Additionally, the repeatability of the
precision can be written as:
This suggests that to increase the count rate at the detector and the
precision repeatability of the readings, one must decrease the
distance from the sample surface to be measured. This may not be
possible with complex component geometries however, pioneering
methods such as the patented Distance Controlled Measurements
(DCM) method offered by Helmut Fischer GmbH seek to overcome
this obstacle by correcting the readings in relation to the measuring
distance. This allows one to measure precisely and truly at suboptimal
distances. Without DCM, acquisition times would need to be much
longer than what is practical for everyday use.
Collimator size:
The size of the collimator selected for the application is the next factor
that influences the trueness and precision of the readings. The
diameter of the collimator and thus the size of the X-ray beam (spot)
directly affects the count rate.
This suggests that the more X-ray energy that is delivered to
the sample the more that the detector can detect. In practice,
a smaller measurement spot allows the best determination of the
inhomogeneities in a sample. Additionally, the measurement times
tend to be longer to improve trueness and precision. Conversely,
a larger spot size is influenced less by the inhomogeneity within the
sample; the acquisition times tend to be lower with larger collimators.
Measurement time:
The measurement times tend to increase repeatability as more signal
is offered to the detector and the instrument can perform statistical
calculation on more data.
By this relationship a four time longer measuring time improves the
repeatability precision by a factor of 2. Often a very long measurement
time (> 4 minutes) is not feasible, thus it is recommended to
perform several shorter measurements and average them.
Sample thickness:
The thickness of a coating influences the measurement uncertainty.
Typically, all elements have a maximum thickness at which the
uncertainty of the readings is far higher than the possible thickness of
the sample. This is dictated by the density of the coating that is being
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