Hunger strikes! After exploring the nearly-empty fridge, you slather all the peanut butter over the last slice of bread; and promptly drop the whole thing on the floor, peanut butter side down (naturally). As your tummy grumbles, you wonder: Does the five-second rule hold?
While you ponder the possible demise of your snack, let us consider the implications for contamination control and critical cleaning. In the case of contamination control, the issue is in attempting to relate Cleanroom Classification with deposition of particles on the surface of the product. A part or component arrives for final assembly in a cleanroom. Perhaps the parts are received appropriately pre-cleaned relative to a specified level of cleanliness, sealed in packaging that meets specification. There may be initial receiving inspection or some other non-destructive test. This means opening the package and exposing the component to the surrounding environment; as soon as this happens, there is the potential for contamination. Can you assume that for a given exposure time the number of particles deposited on the part can be directly calculated from the class of the cleanroom?
Suppose you were doing receiving inspection in cleanroom A; from years of experience, you could leave the part sitting out for one minute. You have the opportunity to do receiving inspection in cleanroom B, where cleanroom B has one tenth the concentration of particles in the air. Does that mean you could allow the part to be exposed for 10 minutes in cleanroom B? And by the way, why don’t standards indicating levels of particles on the part mesh with levels of particles in the air and with AMC (Airborne Molecular Contamination)?
Can you set up the equivalent of a five-second rule for product exposure based solely on the class of the cleanroom? The short answer is: no! Cleanrooms are a tool to help lessen product contamination. A cleanroom in and of itself does not correct a contaminated product nor does use of a cleanroom prevent all sources of contamination. So, given that it is not productive to stare at the component through the protective packaging, how do you develop an approach to managing product exposure to the environment, even to a controlled environment?
Fallout prediction
It is possible to use fallout prediction to develop a rational, defendable estimate of allowable product exposure to a given environment. However, realistic fallout prediction is data-based, and it is situational.
Information about fallout prediction is available in the literature. Parasuraman, et al.1 use experimental modeling employing a number of standard test dusts and an aerosol dust generator along with witness samples to characterize particle fallout in a cleanroom.
The data varies significantly from a theoretical model of Hamberg2, derived from the terminal velocity of particles in still air. Hamberg’s equation, using metric units, is:
ṅ = 4.8 105 Nc0.773
where
ṅ = particle fallout rate (number of particles >5 μm settled/m2/24 hr and
Nc = airborne particle level (number of particles >5 μm/m3 of air).
Fallout measured by Parasuramen also differs depending on the location within a room. Any activity within the room contributes to air turbulence. For very small particles, the terminal velocity will be less than the velocity of air turbulence so a model that assumes still air breaks down.
Mackler3 points out that particle fallout is proportional to size, and that the effectiveness of cleaning is also proportional to particle size. The IEST-STD-1246D standard for cleanliness of product surfaces (as well as ISO 14644-09 for particles on cleanroom surfaces) assumes that the surface has been cleaned. Therefore the IEST standard shows a steeper slope for particle number vs. particle size than does experimental data from witness samples.
A more complete picture
Fallout prediction tends to focus on particles. You have to be concerned about other sources of airborne contamination. As Tribble, et al.4 point out, concern with airborne particles alone is not sufficient.
Outgassing from product, from auxiliary equipment, emissions from nearby processes and neighborhood air quality can contribute airborne molecular contaminants (AMC) that deposit on surfaces or even interact with surfaces.5 AMC can contaminate or even modify surfaces, thus impacting performance.
Witness samples6 can be located at strategic areas throughout the cleanroom to provide an estimate of the overall impact of particles and AMC on the product. Designing witness samples that emulate the product requires a bit of thought.
If your product resembles peanut butter in that it tends to “hold” soils, a highly-polished, inert surface may not be predictive of susceptibility of your product to contamination.
Process
Every time a part of component is exposed to the surrounding environment, there is the potential for harmful interaction with that environment. Unless you plan to keep the part in its appropriately-qualified protective packaging and simply peer intently at the product during receiving inspection, and unless you intend to do no further assembly, treatment or modification of the product, you need to establish some equivalent of the “five-second rule.” Published approaches can help in setting guidance for your particular work area.
However, even in fully-automated facilities, any version of the five-second rule is only part of the picture. Consider work habits, policies, and practices. Establishing the right practices involves training, monitoring, and accountability.
Contamination control and monitoring do not substitute for critical cleaning. Critical cleaning includes the cleaning method and the points in the process where critical cleaning occurs.
If there is receiving inspection or testing, is additional cleaning required? If you receive components that meet required cleanliness levels, and then you do further assembly, is the assembled product still clean? Those are the questions you need to answer.
References
1. Parasuraman, D.K., et al, “Prediction Model for Particle Fallout in Cleanrooms,” Journal of the IEST, Vol. 55, No. 1, 2012.
2. Hamberg, O. “Particle Fallout Predictions for Clean Rooms,” The Journal of Environmental Sciences, 25: 15-20, 1982.
3. Mackler, S. “Point of View: Facilities Cleanliness Requirements,” Controlled Environments, May 2010. http://www.cemag.us/articles/2010/05/point-view-facilities-cleanliness-requirements
4. Tribble, A. C., et al., “Contamination Control Engineering Design Guidelines for the Aerospace Community,” NASA Contractor Report 4740, May 1996.
5. Kanegsberg, B., and E. Kanegsberg. “Airborne Molecular Contamination” (3-part series), Controlled Environments, June 2009, July/Aug. 2009, Sept. 2009.
6. Kanegsberg, B. “Detecting Contamination with Witness Samples,” Clean Source, the BFK Solutions Newsletter, February 2016. http://bfksolutions.com/detecting-contamination-with-witness-samples/
Barbara Kanegsberg and Ed Kanegsberg (the Cleaning Lady and the Rocket Scientist) are experienced consultants and educators in critical and precision cleaning, surface preparation, and contamination control. Their diverse projects include medical device manufacturing, microelectronics, optics, and aerospace. They can be reached at [email protected].
This article appeared in the May/June 2016 issue of Controlled Environments.
