[Originally published July, 2005]
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.
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.
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.