Choices for Cleanliness
Pages: S-2 - S-9; March,
If cleaning is a step in your manufacturing
process, cleanliness verification should be a part of your
day-to-day operations. Why? Without a means to measure cleanliness
you cannot assess if your cleaning process is performing as it
should. You may be producing products that may fail later on down
the line. Or you could be expending too much effort – and money — on
There are a myriad of analytical options available.
In this special section, we look at a few of them. This overview
defines some general processes and technologies. You can get an
in-depth look at particle counting, surface tension measurement, and
contact angle measurement.
Review the case study on grazing-angle reflectance
FTIR, which discusses how the Navy uses this instrument in the field
to verify the cleanliness of aircraft. The supplier directory
provides resources, grouped by category, for the system that is
best-suited for your operation.
Whether you are first contemplating a test method
or already have one in place, we hope the information in this
section helps you maximize your cleaning process efforts and
maintain your leading edge.
The level of cleaning obtained by a given process
is determined at the substrate surface, upon completion of all
cleaning, rinsing, and drying procedures. The cleanliness
verification system used is determined by the quality objectives for
the method and the finished product.
Cleanliness verification techniques can be
classified into three main categories: gross verification,
semi-precise verification, and precise verification. (They can also
be more broadly classified as direct and indirect methods.) As some
overlap may exist among the techniques in these categories, the test
of choice is dependent upon the needs of the specific
verification means that there is no visible contamination, but
this is not quantified. These techniques are totally qualitative
(you can almost see the dirt - 0.1 gram/square inch and above).
Among the applications that use gross verification are auto
parts/engine cleaning, basic metal cleaning, firearms, and heavy
Examples of techniques used for gross verification
include the dissecting microscope, nonvolatile residue (nvr), Scotch
tape test, ultraviolet (uv) fluorescence, water break test, and
white glove test.
verification is highly qualitative or quantitative to a moderate
level of precision (0.001 - 0.1 gram/square inch). Semi-precise
verification is used for high-quality finishing/plating, subcritical
electronics assembly, avionics, some instrumentation, electrical,
subcritical aerospace, and automotive control parts (fuel injectors,
Contact angle measurement, the Millipore test, and
the optical microscope are examples of semi-precise verification
Precise verification is quantitative to
extremely low levels of measure or ultra qualitative (0.001
gram/square inch down to absolute zero). Semiconductors, disk
drives, critical aerospace, oxygen lines, and medical devices use
Tests that fall under this category include Auger
electron spectroscopy (AES), carbon coulometry, electron
spectroscopy for chemical analysis (ESCA), Fourier transform
infrared (FTIR), fluorometer, gas chromatography/mass
spectrophotometry (GC/MS), ion chromatography, optically stimulated
electron emission (OSEE), particle counting, scanning electron
microscope (SEM), and secondary ion mass spectroscopy (SIMS).
A description of some of the tests noted for the
three main categories of cleanliness verification follows.
Gross Verification Tests
Nonvolatile Residue (NVR)The NVR test
requires extraction of contamination from a dirty part into a
volatile solvent, evaporating off the solvent, and measuring the
weight of the remaining residue using an analytical balance. Almost
any clean volatile solvent can be used.
Scotch Tape Test
For polished or lapped
parts, a common test is known as the Scotch tape test. A strip of
transparent tape is affixed to the surface in question with firm
pressure. The tape is removed and placed on a clean, white sheet of
paper. The surface should appear as white as the original sheet of
Ultraviolet (UV) Fluorescence
can provide a visual indication of where contamination remains on a
surface. (Contaminants will fluoresce in the presence of UV light.)
The intensity of the radiation can also be measured via a registered
signal on an instrument, which dictates the degree of contamination
on a surface. This form of analysis is useful for locating
contamination, but it does not identify it.
Water Break Test
In the water break test, if
water beads, the surface is considered to be contaminated with a
hydrophobic substance (oil/grease). If the water breaks or sheets
off, the surface is considered clean.
Semi-precise Verification Tests
See article at: Contact Angle Measurement
The patch test, also known as
the Millipore test, consists of spraying a representative number of
parts with filtered hexane, isopropyl alcohol, or trichlorethylene
at a pressure of 60 to 80 psi through a filter jet nozzle with a
1.2-micron membrane filter. The spray is collected and vacuum
filtered onto a clean filter membrane, and the membrane is inspected
for contaminants (placed under a microscope to measure — in microns
— and count the number of dirt particles remaining). Weighing the
membrane pad determines the total contaminant (in milligrams) that
has been left behind
Optical microscopes use a
beam of light and lenses to magnify objects. There are simple,
high-power, and optical microscopes. Optical methods are an
excellent way to perform simple quality control checks or verify
certain types of cleanliness quickly and efficiently.
Simple microscopes, or magnifiers, consist of a
single lens or a set of lenses that provide direct
High-power compound microscopes are used in
evaluating the cleanliness of critical components like circuit
boards. These instruments are typically delicate and expensive,
requiring a variable degree of operator skill, training, and
patience in order to maximize their potential.
Optical microscopes are ideal for viewing residual
oils and greases, flux residues, certain particles, and surface
Precise Verification Tests
Auger Electron Spectroscopy (AES)AES is
used for compositional analysis or determining which atoms are
present on a surface. Electrons are directed toward the surface,
ionizing surface atoms by causing the removal of an electron from
the atom’s inner shell. The atom now becomes excited and must
release energy to "relax" and return to its original state. This is
done by transferring the extra energy to an electron that can leave
the atom. The exiting electron is known as the auger electron. The
AES method of analysis measures the energy of the auger electron,
which is unique to each particular atom. AES is used in the
semiconductor field for corrosion and failure analyses, and
The technique employs
in-situ direct oxidation of surface carbon to carbon dioxide (CO2),
followed by automatic CO2 coulometric detection.
Electron Spectroscopy for Chemical Analysis
ESCA is a spectrophotometric technique in which X-ray
bombardment of a surface results in the emission of an electron from
a given atom. Knowing the energy of the X-ray and measuring the
energy of the emitted electron can determine the binding energy of
the electron. ESCA methods reveal chemical structure, bonding, and
oxidation state. ESCA has the potential to be very useful in
identifying organic compounds.
See article at: Fourier Transform Infrared
Gas Chromatography/Mass Spectrophotometry
GC/MS is used to identify surface contamination by
extracting contaminants into solvent and analyzing them. Organic
compounds are separated via GC and are then identified, based on
molecular weight, by MS.
separates, identifies, and quantifies ions. The analysis begins with
a sample, typically a water matrix containing ions of interest. A
portion is injected into the system and combined with an eluent
stream that carries the sample to the analytical column. The
analytical column separates the ions of interest in the sample into
narrow bands within the stream of the eluent.
The eluent then sweeps these groups of ions into
the suppressor device, which electrolytically transforms the eluent
into pure water, leaving just the ions of interest in pure water to
be swept downstream to the conductivity detector. The detector
detects the ions based on their conductivity relative to the water
eluent. At this point all interfering ions have been removed and the
detector’s sensitivity has been maximized, allowing for detection of
very low part-per-billion levels of ions.
Optically Stimulated Electron Emission
When high energy UV light hits a surface, electrons
will be emitted, and the reflected current can be measured. A clean
surface will give the highest return current, so any drop in current
represents contamination. This method is good for seeing low levels
of contamination (both ionic and nonionic). It can detect
contamination, but it cannot identify it.
See article at: Particle Counting
Scanning Electron Microscope (SEM)
utilizes a beam of electrons that is passed over a very small area
of a surface. The beam scatters when it strikes the surface,
outlining the topography of the surface. The back-scattering carried
in by the return beam of electrons is measured by the microscope.
The result is a finely detailed, 3-D image of the surface being
Scanning microscopes are capable of magnifying an
image to more than 100,000 times its original size. This method is
well suited for identifying particulate and potentially nonuniform
or thick films of contaminant.
Secondary Ion Mass Spectroscopy (SIMS)
identifies elements but does not identify bonding characteristics.
An incident high-energy ion beam strikes the surface and blows atoms
and molecules off the surface. The atoms and ions flying off the
surface enter a mass spectrometer and can be identified by the mass
and charge ratios. SIMS is sensitive to all elements and their
isotopes and can be used to quantify part-per-billion levels in
Generally, SIMS is considered a surface analysis
method with a spatial resolution laterally on the order as small as
1 micron in certain cases. Though SIMS is used most in the
semiconductor field, it has made a major impact in other thin-film
subjects, and a variation of the process, called Time of Flight
SIMS, is now used on many organic bio-molecules and inorganic