Two Definitions:

(a) Federal Standard 209 Defines a Clean Room

"A Clean Room is an enclosed area employing control over the particulate matter in air with temperature, humidity and pressure control as required. To meet the requirements of a 'Clean Room' as defined by this standard, all Clean Rooms must not exceed a particulate count as specified in the air cleanliness class." This standard, first issued in 1963, has been used in the current version 209B amendment 1 since 1976. It is shortly to be re-issued, entitled "Airborne Particulate Cleanliness Classes for Clean Rooms and Clean Zones".

(b) BS 5295 Definition

"Environmental cleanliness in enclosed spaces". "A Clean Room is a room with environmental control of particulate contamination, temperature and humidity, constructed and used in such a way as to minimize the introduction, generation and retention of particles inside the room." This standard dates from 1976 and falls into 3 parts.

Part 1: Specification for controlled environment Clean Rooms, workstations and clean air devices.

Part 2: Guide to the construction and installation of Clean Rooms, workstations and clean air devices.

Part 3: Guide to operational procedures and disciplines applicable to clean rooms, workstations and clean air devices.

Why are they needed?
To reduce contamination levels in the product or services performed.

Contamination Control Technology

Contamination control technology concerns itself with every single thing that happens within the Clean Room. How items are handled, cleaned and stored, what equipment is used and the method of its use. What materials are used in the process and what changes take place to that material, what are the contaminants, where do they come from and how can they be eliminated. Clean Room technology is the part of the contamination control that concerns itself with the design facilities, air handling equipment and construction techniques. As such it deals with the removal of particles from the air and the levels of air cleanliness that can be achieved.

How are they classified?
The determination of how clean an area is, depends on the classification that it has been designed to. The classification of air cleanliness in FED standard 209 is very simple as the class is directly related to the number of particles per cubic foot of air equal to or greater than 0.5µ (micron). The present British Standard system designates 4 classes of environment as class 1, 2, 3 and 4. Class 1 being the highest class of cleanliness and in fact this is a slightly tighter specification than class 100 as 3000 particles/m3 at 0.5um converts to about 85 particles/Ft³.

BS 5295

Class 1: The particle counts shall not exceed a total of 3000 particles/m³ of a size of 0.5µ (micron) or greater. The greatest particle present in any sample shall not exceed 5µ (micron).

Class 2: The particle count shall not exceed a total of 300,000 particles/ m³ of a size 0.5µ (micron) or greater: 2000 particles/m³ of a size 5µ (micron) or greater: 30 particles of a size 10µ (micron) or greater.

Class 3: The particle count shall not exceed 1,000,000 particles of a size of 1 micron or greater: 20,000 particles/m³ of a size 5µ (micron) or greater: 4000 particles/m³ of a size 10µ (micron) or greater; 300 particles/m³ of a size 25µ (micron) or greater.

Class 4: The particle count shall not exceed a total of 200,000 particles/m³ of a size 5µ (micron) or greater: 40,000 particles/m³ of a size 10µ (micron) or greater: 4000 particles/m³ of a size 25µ (micron) or greater.

209B Defines four classes of air cleanliness

1. Class 100,000: Particle count not to exceed a total of 100,000 particles per cubic foot of a size 0.5µ (micron) and larger or 700 particles per cubic foot of a size 5.0µ (micron) and larger.

2. Class 10.000: Particle count not to exceed a total or 10,000 particles per cubic foot of a size 0.5µ (micron) and larger or 65 particles per cubic foot of a size 5.0µ (micron) and larger.

3. Class 1,000: Particle count not to exceed a total of 1000 particles per cubic foot of a size 0.5µ (micron) and larger or 10 particles per cubic foot of a size 5.0µ (micron) and larger.

4. Class 100: Particle count not to exceed a total of 100 particles per cubic foot of a size 0.5µ (micron) and larger.

Remember - The Clean Room class is generally achieved in the "at rest" state when there are no people in the room!

Where do particles come from?

Clean Room Air

The task of the Clean Room filter is to ventilate the atmosphere with microbial and particulate free air. They create a positive pressure, so that any air-borne contaminants present are from within the room.

• The source of micro-organisms is from people.

• The source of particulates are from people and processes.

Microbes

Microbes are dispersed from skin cells, and a human body sheds the outermost layer of skin every 24 hours.

• 1 BILLION SKIN FLAKES EVERY 24 HOURS.

A skin flake is typically 33 microns - 44 microns. They break down to typically 20µ (micron) but 7-10% are less than 10µ (micron). The equivalent diameter of bacteria carrying particles is 12-14µ (micron). These settle by gravity at 0.37 meters per second.

• 12-14µ (micron) size particles will settle in wounds of hospital patients and aseptically filled containers in pharmaceutical applications by gravity.

Inert Particles

Particles from people are dispersed:

1. From their skin

2. From their normal outdoor clothing

3. From their Clean Room clothing (both through it and from the surface).

The mixture of skin flakes and fabric fibers fragment into smaller pieces, so that the total number of particles is:

• 10 BILLION EVERY 24 HOURS. This figure is dependent on activity rate, work activity, more particles.

Demonstrations have indicated that typically this means:

• 1 MILLION PARTICLES EQUAL TO OR GREATER THAN 0.5 MICRONS ARE DISPERSED EVERY MINUTE.

They disperse into the air from exposed skin, through the apparel fabric and also out through the neck, waist, trouser opening and wrists.

Other Particle Sources

From the table it can be seen that roughly 50% of particles are caused by people. In "other" and "cleaning materials" will lie the content attributable to "yesterday's dirt" from the floor, walls, worktop and equipment surfaces - which is why some high-tech companies continually janitorially clean the Clean Room.

What are the objectives in cleaning a Clean Room?
The objectives in cleaning a room can vary between industries but the types of contamination that should be removed may be one, two or all of the following types:

A Particles and Fibers
B Chemicals
C Bacteria
D Electrostatic Charges

In all types of Clean Rooms particles and fibers are undesirable although the minimum size of particle which is undesirable will vary. In the electronics and optical industries, very small particles will cause malfunction of the product and in the pharmaceutical and medical devices industries it is thought unwise to introduce particles into the patient.

In all Clean Rooms, spillages of chemicals from one part of the process may be undesirable in another part of the process. The pharmaceutical and medical devices industries are very concerned with bacteria because of the danger to patients but the microelectronics industry is also concerned with bacteria because of their high ion content.

The microelectronics industry is concerned with static electricity because the discharge of very small voltages can destroy silicon chips. Many of the surfaces of Clean Rooms are poor conductors, i.e. plastics, and can develop and retain a high electrostatic charge. It may be necessary to clean these surfaces in such a way as to leave a film of antistatic agent to conduct the charge away.

The "Critical" and "General" areas of a Clean Room
A Clean Room may be divided into two areas, i.e. the "critical" and "general" area. The "critical" area is the area around the point of production where contamination can gain direct access to the process. This is the area often protected by localized laminar-flow clean benches and workstations. The "general" area is the rest of the Clean Room where contamination will not gain direct entry into the product but should be kept clean because of the transfer of contamination into the critical area.

It is necessary that the "critical" area be cleaned most often with the best cleaning wipes along with solutions which have the best cleaning ability without introducing contamination. The "general" area may be cleaned with less stringency. Cost effectiveness in relation to requirements can best be achieved by considering the Clean Room in the "critical" and "general" zones.

Are there standard codes of practice for Clean Room operatives?
Every Clean Room user should establish a code of practice commensurate with the standard of cleanliness required of the product or service carried out therein.

Factors to consider include:

Equipment in the Clean Room

Assess each unit permanently located there for contamination including particle emissions.

People and Clean Room Consumables

Clean Room Clothing
Balance the need for a complete barrier, with the need for wearer comfort.

Criteria:

• Equivalent pore density (µ)

• Air permeability (ml/cm2/sec @ 1 cm water gauge)

• Particle penetration (>/= 0.5µ to >/= 5.0µ spectrum)

• Anti-static properties (rating 106-1012 ohms per square)

• Effects of laundering on pore size.

Clean Room Gloves

Criteria:

• Particle shedding (hence "class 100" cleaning)

• Chemical/Ionic contamination

Wipes

Criteria:

• Particle shedding (Dry & Wet Flex Tests)

• Absorption Rates (seconds)

• Contamination from inherent extractables (Extraction in DI-H2O % gm/m2 & TCE)

Cleaning Materials

Criteria:

• Filtered (0.2µ)

• Low ionic contents (earth elements measured ppm)

• Low toxicity and high flash points (TLV values)

Conclusion
There is no doubt that the technology changes required, driven by the demand for lower contamination levels in the food, pharmaceutical and now well established "High Tech" industries like Semiconductors have brought Clean Room technology into the public eye.

Originally spawned by the space race of the early sixties, the pace of change is speeding up. There are now controlled contamination requirements in all these areas:

Some have emerged only in the last three years.
 

With acknowledgements to:
Dennis Lambert of Motorola. Dr Bill White of Glasgow University and Norman Donaldson of Union Chemical.