Cleanrooms are designed to minimise the ingress of airborne particles (achieved through HEPA or ULPA filters) and to control what happens to particles generated within the cleanroom. Good air flow design — such as turbulent flow — helps to prevent particles from being deposited onto surfaces (particles settle by two primary mechanisms: gravitational sedimentation and turbulent deposition)1. The removal of these particles is achieved through the extraction of room air with the addition of clean air into the room (air exchange rates). The flow of particles in air from a less clean area can also be blocked from entering an area of a higher cleanliness level through positive pressure differentials.
Once a cleanroom is built and activated, constant monitoring and maintenance are required. Equipment, tools, furniture, raw materials, outside air, people and even the type of garment worn by them will have to be examined for contamination risk before being allowed to enter. Specific conductivity material, antistatic characteristics, out gassing properties or even antimicrobial aspects may be required.
The Cleanroom
The purpose of every cleanroom is to control an environment by limiting the presence of sub-micron particles and modifying inadequate environmental conditions. The level to which the environment is controlled is what separates one cleanroom from another. For instance, a Class 10 cleanroom has 10 particles of size 0.5 microns per cubic foot of air, where as a Class 100,000 cleanroom has 100,000 particles of size 0.5 micron per cubic foot of air.
Let us compare the numbers of airborne particle or micro organism in cleanroom with our surrounding environment. The numbers of particles in general environment vary from time to time so that any fixed number cannot be determined, but roughly classified as shown in the diagram to the right. From this figure, you will see that such a clean condition in the highest class cleanroom cannot be found in the natural world, even in the upper area of stratosphere. Also, in the center of Pacific Ocean, the cleanliness level of the air is lower than that of middle class of cleanroom. In other words, cleanroom is an ultra clean space where airborne particles or micro organism are been eliminated, as we can never experience in our normal environment.
The dust and particles controlled in cleanroom are very small and invisible as shown below. Visible article is generally larger than 10 μm, which is removed out at the pre-filter stage. The cleanroom sub-micron particle, smaller than 1 μm, is the subject to control.
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