Non-chemical cleaning

Ed and Barbara Kanegsberg, of BFK Solutions, get back to basics with methods of cleaning without liquids, reviewing the mode of action, application and benefits of different technologies.

Mention the word “cleaning” and manufacturers envisage processes involving water or organic solvents. Indeed, the majority of cleaning processes, both for general cleaning and for critical cleaning, involve liquids. However, there are non-liquid cleaning processes¹ and many are either inherently suited to, or can be adapted for, cleanroom use. They are generally termed “non-chemical” processes even though some involve chemical interactions.

For cleanroom applications, a subset of non-chemical processes are used. In fact, non-chemical processes cover the gamut of applications, ranging from de-painting aircraft and removal of grime from old buildings to very critical cleaning during fabrication of wafers and precision optics.

For cleanroom applications, any heavy soils must be removed before a part enters a cleanroom, so in this context, any cleaning should be for light soil or particle removal.

Soil removal

The mechanism for soil removal from most non-liquid processes involves the momentum of impact. These processes include abrasive action of particles such as grit blasting with hard pellets of silicon carbide or bombardment with much softer particles of baking soda or solid carbon dioxide. However, not all impact cleaning involves solid particles. Examples include high pressure steam (sometimes considered a non-chemical process even though water is involved) and ions in a plasma.

Additional physical and chemical mechanisms play a role in some advanced non-chemical techniques. Carbon dioxide cleaning systems are based on solid (snow or pellets), liquid or supercritical. These systems include both an abrasive action and chemical solvency. Because liquid and supercritical CO₂ are neither aqueous nor organic solvents, both are typically considered to be non-chemical cleaning agents.

Solid CO₂ systems actually have two additional cleaning mechanisms. First, they have a solvency vector because the CO₂ liquefies due to the pressure of impact. This is analogous to the instant liquefaction of water ice beneath an ice skate blade due to the high pressure of contact. Second, when the CO₂ particle impacts, it not only liquefies, but the liquid penetrates the soil layer. Subsequent vaporisation of the CO₂ dislodges the soil and explosively removes the soil layer above (see figure 1).

Two other non-chemical processes, laser cleaning and UV/ozone, involve light. The mechanism for laser light cleaning is primarily from heating associated with absorption of the laser light, causing vaporisation of the soil.

Laser cleaning can be of value for delicate and metallic substrates as the wavelength can be selected so that only the soil absorbs the light, not the substrate; the metal typically reflects the light. UV light creates highly reactive ozone that oxidises organic compounds on a surface; this process converts the soil into something that is more readily removed.

Sometimes a non-chemical process step is not thought of as “cleaning” but has a cleaning action. Examples include non-critical abrasive processes, such as agitated de-burring or buffing. For instance, a manufacturer of coated parts faced yield problems with coating quality. Cleaning was suspected to be the problem, and a considerable amount of effort was expended to optimise the critical cleaning process.

We reviewed the overall manufacturing processes and yield records and discovered that product processed through a buffing step prior to coating showed a very low defect rate. The buffing process was removing soils that the traditional process did not effectively remove. In this case, it was practical to include the non-chemical buffing process for fabrication of all products.

What is cleaning?

Even the term “cleaning” may not adequately convey the processes that are occurring. When asked to define cleaning, we often start with soil as being “matter out of place” and then go on to define cleaning as the removal of “matter out of place”. However, many cleaning processes not only remove the soil, they also modify the underlying surface. In fact, one manufacturer of plasma cleaning systems, considers the term plasma etching to be more descriptive than plasma cleaning because what is really happening is surface modification.³

Of course, in traditional cleaning systems, there is the potential for surface modification, either through force, such as in ultrasonic erosion, or where the cleaning agent interacts with the substrate. This interaction is usually considered to be undesirable and comes under the heading of substrate compatibility issues.

While plasma is typically effective for light soil loading, it is considered to be analogous to an aggressive cleaning solvent.

Cleaning with high velocity particles, whether hard or soft, dry or liquid, can present problems in cleanroom conditions. Processes such as grit blast are, in most cases, incompatible with cleanroom operation since the cleaning media itself is composed of particles. However, processes that do not contain such particles may blast particles off the component being cleaned. This can lead to unacceptable airborne particle levels and product contamination.

Just as the cleanroom itself is a containment vessel to protect components from ambient contaminants, mini-environments provide containment within the cleanroom.⁴ When plasma cleaning is performed inside closed chambers, it is necessary only to avoid contaminants that are generated inside the chamber from escaping into the cleanroom.

Liquid and supercritical CO₂ processes are not atmospheric processes and therefore they inherently contain contaminants. Even processes that are not inherently enclosed can be confined. A glove box or ventilated hood protects the cleanroom from particles generated by a CO₂ snow gun or a steam wand.

CO₂ snow has been employed in somewhat unusual applications – for example, in removal of particles from optics during transport within a cleanroom. A research lab uses CO₂ snow as part of wall cleaning maintenance.⁵ CO₂ has also been used as a pre-cleaning step, prior to introduction of the part into the cleanroom.

Cleaning is a process, more than a cleaning agent or a particular type of equipment. Non-chemical processes are sometimes adopted because of their real or perceived advantages in reducing exposure of workers to chemicals or in eliminating environmental releases.

With any application, it is important to match the cleaning process to the requirements. Non-chemical cleaning is no exception. Whether the cleaning is cosmetic or for critical performance, cleanroom or general factory floor, consider all options, including non-chemical processes.