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Aluminum Oxide Particles
on a Silicone Wafer |
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After Blowing with
Dry Nitrogen |
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After Blowing with
SNO GUN™II |
. . . Because particles in the 1-2 micron size range
and smaller are difficult if not impossible to remove
by conventional blowing with compressed air or dry nitrogen
gas.
Such micro-particles are held to the surface not only
by electrostatic attraction, but also by bonding forces
in the surface layers of absorbed water and organics
that are typically integral to the substrate surface.
Small particles have a larger percentage of their total
surface area in contact with these surface layers, thus
giving them greater adhesion. Less of their surface
is exposed to any aerodynamic drag forces generated
by a nitrogen gun, DI water gun, or other flow of gas
or liquid.
Aside from these adhesive forces, small surface contaminants
are difficult to remove due to the boundary layer phenomena:
a fluid flow (gas or liquid) is only effective at some
finite distance away from the surface being cleaned.
At the surface itself, the flow velocity must be zero,
thus reducing the chance that small particles in the
boundary layer will feel the full force of the flow
and may not be removed.
Dry Ice Cleaning—Sandblasting
Without the Mess!
One way to remove these very small contaminants is to
introduce a mass into the cleaning flow, thereby penetrating
the boundary layer, transferring energy to the contaminant
and knocking it off the surface. The challenge is to
perform this sandblasting without damaging the delicate
part or generating further contaminant residue.
This is precisely what a dry ice cleaning system does.
Frozen carbon dioxide particles meet all the requirements
for high precision cleaning of delicate components:
they are soft and leave no residue, when supplied with
precision-grade CO2.
When liquid CO2 is released from a room
temperature cylinder to atmospheric pressure, it passes
through its triple point—the state at which some
portion exists in solid, liquid, and vapor phases. The
liquid and vapor quickly dissipate with no net effect
on the environment, but the solid dry ice "snow" is
directed toward the surface to remove the particles.
Nozzles supplied with SNO GUN™ II are used to adjust
the dry ice particle size and the shape of the spray
pattern to fit a wide variety of applications.
Effective as a Hailstorm or
as a Gentle Snow
Typical dry ice particle sizes are in the 5 micron range,
which produce a rather aggressive "hail storm," but
they can be adjusted in size to about 0.5 cm where they
produce a very gentle snowfall effect for cleaning the
most delicate surfaces.
At higher velocities (up to 1000 fps) the CO2 not only removes particles, but also dissolves
thin organic layers of oils and fluxes—even fingerprints.
This solvent action is due to a thin layer of liquid
CO2 that forms at the collision interface
between a dry ice particle and the surface. Liquid CO2 is an excellent solvent for organics. The very
short duration of a liquid film at the interface dissolves
the organic film contaminants, which are then carried
away in the flow of CO2 snow and vapor. The
liquid state of the CO2 also reduces the
impact stress on the surface and permits thorough cleaning
without destructive forces.
At lower velocities, larger flakes settle on contaminants
and lift them off the surface in the process of sublimation
to the vapor state—without disturbing delicate
substrates. As the dry ice particle sublimes, it creates
a micro-explosive expansion that dislodges contaminants.
Dry Ice Cleaning and Thermal
Shock
Although dry ice has a temperature of -109.3°F (-78.5°C)
at atmospheric pressure, it rarely poses a threat, even
to extremely sensitive micro-components.
This is because the sublimation process occurs so
quickly, and the mass of the CO2 particles
is so small relative to that of the components being
cleaned, that brief spraying will not cause significant
drop in surface temperature. At high ambient humidity,
however, even relatively small temperature drops can
lead to condensation of water vapor. In these applications,
a dry nitrogen environment can prevent moisture-related
damage.
Preventing Moisture Contamination
If cleaning causes the temperature of the parts to drop
beneath the dew point of the ambient air, moisture will
collect on the parts being cleaned. In many applications,
this condensation could lead to oxidation or other contamination
problems.
For these applications, Terra recommends cleaning in
a nitrogen-purged dry box. At the much lower dew point
inside such a chamber, parts will remain dry even if
dry ice cleaning does cause small, temporary temperature
drops.
Protecting Against ESD (Electro-Static Discharge)
Although CO2 is itself non-conductive, the
dry ice particles can generate friction as they leave
the nozzle of the SNO GUN™ II, leading to potentially
hazardous static charge build-ups.
One solution to this problem is grounding ESD-sensitive
parts. Another is to direct a convergent stream of ions
at any static-sensitive surface. Terra’s Ionizing
Air Gun provides this ESD protection, and can be configured
to work in tandem with the SNO GUN™ II. |
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