Is it a Cleaning Step?
By Barbara Kanegsberg
Identifying the cleaning steps is not necessarily obvious, especially in
multi-step processes. In fact, when we consult with our clients, particularly
in troubleshooting projects, one of our first bits of detective work involves
working our way through the build process and noting the cleaning steps.
It is important to identify each activity that is actually serving as a cleaning
process, because, if such a process is unwittingly removed, surface quality
and product quality can suffer.
Sometimes a cleaning step is not termed cleaning. Perhaps the original
process designer thought it was more elegant to avoid the word cleaning.
Perhaps
the step is thought of as having a function other than cleaning, but it
actually serves as a cleaning step as well. In a more general sense,
identifying the
cleaning steps involves understanding the function and value (positive
or negative) of a particular step to the overall build process. The
following
examples of cleaning euphemisms may inspire you to revisit the build process
to determine if a step actually constitutes cleaning. You may also find
the terms helpful optimizing a process, troubleshooting, and achieving
lean manufacturing.
Surface Preparation; Removal
In metal working, fabrication or finishing, acid pickling is thought of
in terms of surface preparation or surface stabilization. However, the
process
can also serve to remove a variety of contaminants, particularly inorganic
contaminants.
Surface preparation means something very different to those in microelectronics
and wafer fabrication. Often, surface preparation is used as a substitute
for cleaning.
The term “removal” is
also used as a substitute for cleaning. Photoresist removal is actually
a cleaning process. As such, we have
to consider not only the efficacy of removal but also the potential
for residue and impact
on the surface.
The problem with avoiding the term cleaning is that it is far too easy to forget the principles of cleaning and surface preparation.
“de’s”
Then, there are the “de” processes, such as defluxing,
depainting, decoating, or deblocking. Most people recognize
flux removal as a cleaning
process. Depainting and decoating are also cleaning processes;
the paint or coating has become a soil (soil being matter
out of place).
Deblocking as used in optics processes is more complex. Prior
to polishing, optics may be securely embedded in any of
a variety of blocking agents
to hold them in place. Deblocking involves removal of pitches,
waxes, plastics, even nail polish. Undesirable residues
of polishing compound
are often
also
present. The blocking compound (at this point it has become
a soil) must be completely removed without modifying the surface
and with minimal
residue.
All too often, the deblocking process is surrounded in
mystery, or at least in inexplicable habit. Because of this, it becomes
all too
easy to ignore
incipient problems and to discount innovative, solutions.
Prior to deblocking, a very specific, desirable finish has been achieved. The challenge is to remove the soil without damaging the product.
Force
This concept includes a diffuse array of processes that
can be a simple as air, an inert gas, carbon dioxide,
or water. Properly
used, they
can remove
particles. However, if the air supply is contaminated,
oil, particles, and debris from welding operations
may be deposited. If CO2 snow
is used, even
if the CO2 is from a very clean source, temperature
changes (cooling) may result in water condensate. Condensate can
carry particles
onto the part
and the water itself may be a problem with sensitive
substrates.
Media Blast or Media Finishing or Polishing
Most people carefully separate media blast or media
finishing or final polishing from the concept of
cleaning. Media
blast can be accomplished
dry with a
dizzying array of plastics, chemical powders, natural
bio-based products (including walnut shells), metals, and minerals.
The media
can itself
remove thin film soils and particles. Carriers may
be used, such a water or water
containing surfactants. These carriers can enhance
the cleaning function. However, if media and/or
cleaning agent residue dries on
the part,
adherent, undesirable residue can remain. This
is sometimes referred to as a film
or a “white rust.” It is not unknown
for a polishing compound to combine with the metal
surface
to leave a specific
finish or a desirable
residue.
Supplementary information from BFK Solutions: [David:
can you make these references hyperlinks to the
article itself?
I don’t
think we have the 3rd one on the site since that
was before we started the column]
“
Microbes and Boulders” (Contamination, Biomedical Gases), B. Kanegsberg,
E. Kanegsberg, R. Baier and P. Nagathan, Center for
Biosurfaces, SUNY Buffalo, and T. Brown, Praxair, A2C2 Magazine, August,
2004.
“ Beneficial Contamination, B.Kanegsberg and M. Chawla, A2C2 Magazine,
January, 2002.
"
Critical Cleaning by Abrasive Impact," E. Kanegsberg and B. Kanegsberg,
A2C2 Magazine, May, 2000.
