Cleanrooms

Cleanroom scientists wearing shoe coversCleanrooms (or clean rooms) are used in virtually every industry where small particles can adversely affect the manufacturing process.  Typically located in scientific research or manufacturing settings, a cleanroom is a controlled environment that has a controlled level of contamination (pollutants such as dust, airborne microbes, chemical vapors, and aerosol particles) that is specified by the number of particles per cubic meter (m3) or per cubic foot (ft3) at a specified particle size.  Believe it or not, the ambient air outside in a typical city environment contains about 35,000,000 particles per m3, 0.5 μm and larger in diameter, which corresponds to an ISO 9 cleanroom.  At the other end of the spectrum, an ISO 1 cleanroom allows no particles in that size range and only 12 particles per m3 of 0.3 μm and smaller.

Cleanroom Overview

A cleanroom is any given contained space where provisions are made to reduce particulate contamination and control other environmental parameters such as pressure, temperature, and humidity. The key component is the HEPA (High Efficiency Particulate Air) filter that is used to trap 99.97% of particles that are 0.3 microns and larger in size. All of the air delivered to a cleanroom passes through HEPA filters, and in cases where more stringent cleanliness performance is necessary, ULPA (Ultra Low Particulate Air) filters are employed.

The use of multi-layer adhesive mats for cleanrooms is almost universal.  Matting can vary in size, color, placement, and number based on the characteristics and logistics of each individual cleanroom.

Personnel who work in cleanrooms go through extensive training in contamination control theory, practices and procedures. They enter and exit the cleanroom through air showers, airlocks, and/or gowning rooms, and they must wear special clothing designed to trap contaminants that are naturally generated by our bodies.

Depending on the room classification or function, personnel gowning may be as limited as lab coats and hairnets/beard covers, or as extensive as being fully enveloped in multiple layered bunny suits with self-contained breathing apparatus.  The cleanroom clothing itself must not release fibers or particles to prevent contamination of the environment.

Cleanroom garments include things such as boots, shoes, shoe covers, beard covers, hairnets, bouffant caps, facemasks, coveralls, aprons, frocks/lab coats, gowns, glove and finger cots, hoods, and sleeves. The type of cleanroom garments used reflects the cleanroom classification and product specifications. For example, Class 10,000/ISO 7 cleanrooms may use simple smocks, head covers, and shoe covers.  On the other hand, careful gown wearing procedures with a zipped coverall, boots, gloves and complete respirator enclosure are required for Class 10/ISO 4 cleanrooms.

Cleanroom technicians wearing shoe covers
Cleanroom technicians wearing shoe covers

Air Flow Principles for Cleanrooms

Cleanrooms maintain particulate-free air through the use of either HEPA or ULPA filters employing laminar or turbulent air flow principles. Laminar, or unidirectional, airflow systems direct filtered air downward in a constant stream. Laminar airflow systems are typically employed across 80% to 100% of the ceiling to maintain constant air processing and unidirectional flow. Laminar flow criteria are mandated in ISO 1 through ISO 4 cleanrooms.  Turbulent, or non-unidirectional, air flow uses both laminar air flow hoods and non-specific velocity filters to keep cleanroom air in constant motion, although not all in the same direction. The rough air seeks to trap particles that may be in the air and drive them towards the floor, where they enter filters and leave the controlled environment.

Proper cleanroom design encompasses the entire air distribution system, including provisions for adequate downstream air returns. In horizontal flow applications, this involves the use of air returns at the downstream boundary of the process.  In vertical flow rooms, it requires the use of low wall air returns around the perimeter of the zone.  It should be noted that the use of ceiling mounted air returns is contradictory to proper cleanroom system design.

Cleanroom Classifications

Cleanrooms are classified by how clean the air is according to the number and size of particles permitted per volume of air. Federal Standard 209E is used here in the U.S.  The newer standard is TC 209 from the ISO (International Standards Organization).  Both standards classify a cleanroom by the number of particles found in the laboratory’s air. The cleanroom classification standards 209E and ISO 14644-1 require specific particle count measurements and calculations to classify the cleanliness level of a cleanroom or clean area.

Large numbers like Class 1,000 or Class 100,000 refer to FS 209E, and denote the number of particles of size 0.5 µm or larger permitted per ftof air. The standard also allows interpolation, so it is possible to describe other classes such as Class 2,000.

Small numbers refer to ISO 14644-1 standards, which specify the decimal logarithm of the number of particles 0.1 µm or larger permitted per mof air.  For example, an ISO 4 cleanroom has at most 104 = 10,000 particles per m³.

Both FS 209E and ISO 14644-1 assume log-log relationships between particle size and particle concentration. For that reason, there is no such thing as zero particle concentration.

US FED STD 209E cleanroom standards

Class Maximum particles/ft3 ISO equivalent
≥0.1 µm ≥0.2 µm ≥0.3 µm ≥0.5 µm ≥5 µm
1 35 7.5 3 1 0.007 ISO 3
10 350 75 30 10 0.07 ISO 4
100 3,500 750 300 100 0.7 ISO 5
1,000 35,000 7,500 3,000 1,000 7 ISO 6
10,000 350,000 75,000 30,000 10,000 70 ISO 7
100,000 3.5 x 106 750,000 300,000 100,000 700 ISO 8

ISO 14644-1 cleanroom standards

Class Maximum particles/m3 STD 209E equivalent
≥0.1 µm ≥0.2 µm ≥0.3 µm ≥0.5 µm ≥1 µm ≥5 µm
ISO 1 10 2.37 1.02 0.35 0.083 0.0029
ISO 2 100 23.7 10.2 3.5 0.83 0.029
ISO 3 1,000 237 102 35 8.3 0.29 Class 1
ISO 4 10,000 2,370 1,020 352 83 2.9 Class 10
ISO 5 100,000 23,700 10,200 3,520 832 29 Class 100
ISO 6 1.0 x 106 237,000 102,000 35,200 8,320 293 Class 1,000
ISO 7 1.0 x 107 2.37 x 106 1,020,000 352,000 83,200 2,930 Class 10,000
ISO 8 1.0 x 108 2.37 x 107 1.02 x 107 3,520,000 832,000 29,300 Class 100,000
ISO 9 1.0 x 109 2.37 x 108 1.02 x 108 35,200,000 8,320,000 293,000 Room air

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