Silicone Contamination, Part II
Barbara Kanegsberg and Ed Kanegsberg, May 2004
Preventing or identifying the source of silicone contamination requires an
holistic approach. You have to be aware of the entire operation, not just your
own part of the process. This implies awareness of oils, lubricants, and other
materials used not only in the product but also in support materials such as
vacuum pumps, transfer lines, automation, as well as materials handling equipment
and even storage containers. Storage containers can be a contamination source;
silicone is commonly associated with surface agents used during manufacture
of plastic items.
Researchers at the Stanford linear accelerator (SLAC), investigating
aging of wires used for gas detectors, found aging to be related
to contamination
from auxiliary equipment, a gas bubbler.1 Silicone oil from the bubbler,
at levels of 18 ppb in the gas phase, was sufficient to produce aging.
Non-silicone
oil did not affect aging.
People are also a potential source of silicone contamination. Many creams,
cosmetics, hair care products, antiperspirants and some eye-glass cleaning
tissues contain silicones. Careful education and training helps; production
personnel need to understand why procedures and constraints are in place.
The source of airborne silicone contamination may be remote from the
fabrication facility. It is prudent to visually observe the plant
layout as well as neighboring
operations. In one instance, exterior rooftop outlet vents located near
air intake vents, directed contaminated air from a non-critical process
back
into the cleanroom.2 Even if your facility has somehow excluded silicones,
the coating
facility down the block might impact your operation.
It may not be desirable or feasible to exclude silicone from all manufacturing
processes. If silicones are anticipated, contamination can be minimized
during equipment and process design. Masks or sleeves may be needed to
protect critical
components from silicones during and after a manufacturing process. In
electronic contact manufacturing, sometimes a conductive polymer can
be applied during
manufacture that will act as a barrier to deposition of silicone.
Detection
Prevention is the ideal. Once contamination is strongly suspected, through
visual or performance indication, a number of analytical tools can
be used for confirmation. Some analytical instruments are available as
compact,
portable systems. This takes detection from the analytical laboratory
to
the fabrication
facility.
If the exact silicone compound does not have to be identified, the
presence of silicon can be determined relatively rapidly by elemental
analysis,
e.g. atomic adsorption or inductively coupled plasma (ICP). To
localize the contaminant,
a two dimensional technique such as scanning electron microscopy
with energy dispersive X-ray analysis (SEM/EDX) might be used. The
SEM provides
a characteristic
visual observation of a very small area (e.g. a coating defect
or a particle). EDX elemental analysis of that area identifying carbon,
oxygen, and silicon
can be used as evidence that some sort of silicone is the culprit.
Silicone can be detected by Fourier transform infrared spectroscopy
(FTIR), with some degree of molecular identification by comparison
of the suspect
contaminant with a known material. The technique can be performed
on specific areas of
the surface, such as suspect particles. Especially if mixed soils
may be present, it is advisable to have a sample of the suspect
silicone-based
contaminant to run for comparison.
Other techniques provide more definitive identification of silicone
contaminants. One is electron spectroscopy for chemical analysis
(ESCA), which is also
referred to as X-Ray photoelectron spectroscopy (XPS). Another
is secondary ion mass
spectrometer (SIMS).3
For volatile silicone contaminants, thermal desorption gas
chromatography mass spectroscopy (TD-GC-MS) can be used.
The technique was used
to detect outgassing
of silicones materials used in a hydrocarbon sensor.4 It
might be pointed out that silicones have value in GC instrumentation.
Gas chromatography
is actually
gas-liquid chromatography, and the liquid portion is often
a silicone.
Where contamination of a complex object is suspected, extractive
techniques are preferable. In most cases, the sample will
be shipped to an analytical
facility. It is important to take care that the extraction
process itself does not introduce spurious indications
of silicone contamination
(through
cosmetics,
perhaps). It is helpful to prepare, ship and analyze an
additional sample where silicone contamination is known to be absent.
Once identification of the specific silicone contaminant
has been made, the question of how to remove the contaminant
arises.
Some
answers
will be provided
in our next column.
References
1 J. Va’vra. SLAC-Pub-5207 (1990).
2 B. Kanegsberg, M. Chawla. M. A2C2 Magazine (March,
2001).
3 B. Kanegsberg, E. Kanegsberg. A2C2 Magazine (December,
2003).
4 E. Butrym. “Scientific Instrument Services application note,” http://www.sisweb.com/referenc/applnote/app-88.htm
Barbara Kanegsberg and Ed Kanegsberg are independent consultants in critical cleaning, precision cleaning, surface preparation, and contamination control. They are the editors of “Handbook for Critical Cleaning,” CRC Press. Contact them at BFK Solutions LLC., 310-459-3614; info@bfksolutions.com; www.bfksolutions.com.
