Clean Room Primer - Newsletter
EXCERPTS FROM The Clean Room Primer
Cleanrooms have been in existence for approximately fifty years and since their inception, much has been written regarding this concept of cleanliness relative to creating dust-free atmospheric areas wherever required. However, a problem that must be resolved is that printed information is expressed in highly technical terms and an advanced degree in engineering is generally required to facilitate comprehension of technical data concerning this specific field. This guide would be devoid of all technical jargon - written in layman's language, enabling anyone to comprehend - as the contents would be informative, simple, and enjoyable to read.
What is particulate matter?
Particulate matter consists of small bits or particles, usually of microscopic dimensions. It can be any material, organic or inorganic, and may be found in gases, liquids, or solids as either suspended or settled material. When suspended in a gas, it's known as an aerosol: when in liquid, a suspension. When settled at the bottom of a liquid, it's known as silt. And when suspended in a solid, it's called included matter. When you're trying to determine the source, you can take size, shape, and hardness into account. And since particulate matter is three-dimensional, you can also describe it by volume, cross-sectional area perpendicular to the line of sight, and by its longest dimension. Since an infinite number of sizes and shapes are possible, microscopic techniques are often needed. Surface markings and characteristics such as transparency, translucency, and opacity, as well as color and occasional markings and other discontinuities, can also be helpful. But perhaps the most important factor is size, because often it's the size of the particulate matter, more than anything else, that determines the degree of the potential problem it may cause.
How is the size of particulate matter measured?
The conventional unit of measurement for fine particles is the micron, which is 1,000,000th of a meter, or 0.00003937 inch (25,400 microns equal one inch). Molecules are about 0.001 micron in diameter; human hair is usually between 30 and 200. Airborne particles are usually from 0.01 microns to 1,000 microns. The size of the particles is of utmost importance, because that's the characteristic tied in most directly with its ability to contaminate.
What's meant by contamination?
Contamination is any foreign substance that can have a detrimental effect on whatever you're trying to accomplish. Specifically, airborne contamination is anything that can be distributed in the air in the form of fine particles or fibers.
Are all fine particles sources of contamination then?
No. Some of the smaller particles can remain suspended in the air indefinitely. But the grosser ones - sand, dirt, human skin, hair, and lint, for example - eventually settle out and cause problems. In other words, all contamination consists of fine particles. But not all fine particles are contaminants.
When does a fine particle become a contaminant?
When it can cause problems. The particle must have the physical properties that will produce damage, and it must be able to migrate to, or be in place at the vulnerable area. And there must be a significant number of them.
How many different classifications of contaminants are there? Basically, there are three big categories. Airborne contamination, which is carried by air currents; fluid contaminants, which are carried by fluids (such as in servovalves); and transfer contaminants, which are picked up inadvertently by personnel and brought to critical areas. Any of these three kinds can be prompt-action or delayed-action contaminants.
What's the difference?
Once they come in contact with an object, prompt-action contaminants can immediately cause problems. Hard particles are capable of damaging the surface of the component; they do this usually through a grinding action, or by becoming embedded in the surface. Because of lower tensile strength than the component material, softer particles usually do not cause surface damage, but may still interfere with the operation of the device. Delayed-action contaminants, on the other hand, cause no harm until another process takes place. They need something else to push them across the contamination threshold.
Oxygen, sulfur dioxide and trioxide, for example, can produce oxides or salts of the metal base. These salts can then grow in size by a process known as nucleation - the absorption of water vapor - until their size becomes a problem. Also, pressure and/or heat may cause the particles to be formed into the surface of the component, where they form an alloy or compound, resulting in a serious loss of structural strength.
How do these particles migrate from uncritical to critical zones?
There are several ways. They may be thrown off rotating parts, from non-critical to critical areas, blown or wiped from one point to another, or moved by electrostatic, gravitational, and inertial forces.
How many sources of contamination are there?
There are many. But some of the most common include soldering, brazing, welding, adhesives, wire drawing, grinding, fitting, handling, and chips from machinery operations. In addition, contamination can also include such things as casting inclusions within the component, such as air bubbles, sand grains, dissolved impurities that will recrystallize in the metal, glass, or plastic when the casting cools, cleaning fluids, when they evaporate, may leave a contamination residue. Drawers and sliding door cabinets can produce plenty of tiny particles. Electrical devices such as arcs may produce metal oxides, which grow by nucleation and coalescence (combining). Contamination can also result from shipping. The constant vibration of movement can cause particle migration. During storage - which can last from a few hours to several years - gravitational settling and electrostatic collection can cause contaminants to accumulate. Contamination can even come from the very containers or covers.
Padded containers may trap particles that are not released until the stored device causes a deformation of the padding. Because of their normal electrostatic charge, plastic containers may pick particles from the air and permit them to transfer onto the device.
In fact, the act of cleaning itself can be a source of contaminants. Lack of a thorough job is the culprit here. Often the covers or containers are improperly cleaned. But if during cleaning the bond between the contaminant particles and the device is thoroughly broken, the particles carefully removed and not allowed to resettle, the problem will be avoided.
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What about personnel?
They are prime sources of contamination. They shed skin and hair, give off perspiration and dandruff, and emit oral and nasal emissions, which can produce from 100,000 particles per minute (PPM) to 3 million of 0.5 micron size and larger. In fact, all normal body exudations are potential contaminants.
To what extent can personnel emissions be a serious problem?
- Particles of 0.3 microns or larger are emitted from each person per minute unless properly garbed.
Are there any other important sources of contaminants?
A well-designed clean room (which we will be discussing shortly) won't permit 0.5 micron particles to enter. However, a faulty filter installation or damaged air filters will. Silica, rubber, spores, seeds, microbes, fungi, oil droplets are just a few of the contaminants that can be pulled in from the outside environment. Other contaminants, such as asbestos, cellulose, and glass fiber may also be produced by the faulty filter itself.
Are there different types of air filtration equipment?
Most air-cleaning equipment, using gravitational and inertial methods such as filtration, washing, and electrostatic precipitation, will remove a good portion of the particulate matter in a given volume of air, yet it will not remove all of it. The deficiency is vitally important, because in a clean room, an above-average amount of cleanliness must be achieved and maintained. Air filters must be used.
How do air filters work?
An air filter is made up of many interstices larger than the diameter of the particle to be removed. If they were smaller, the surface would soon become covered with contaminants that would block the air flow. Thus, the filtration depends not so much on the blockage as the particles adhering to the filter.
Not always. When it concerns assembly work, most of the problems come from particles less than 0.1 micron in size. These are less than 1 % of all particles by weight in the air; but on a particle-count basis, they account for 65% of the number of particles. To deal with these sub-micron problem-causers, a High Efficiency Particulate Air Filter (HEPA), also known as a "super interception" or "absolute" filter, 99.97 to 99.999% efficient at .3µm and larger, via DOP test method, is a must or the ULPA (ultra low particulate air) is 99.9999% efficient, down to .12µm laser tested.
How is a HEPA filter constructed?
A HEPA filter contains glass (filter), media separators, adhesives, and gasketing material. In the older style; the glass filter media was pleated accordion fashion, and a separator is inserted in each fold, creating a channel through which the air flowed into the pleats and through the filter medium. In the case of the mini-pleats, currently being used, or cassette HEPAS, as they are known in the industry, the aluminum separator is eliminated and the filter pack banded together with either latex coated string or tyvek. Mini Pleat is now a standard in the industry.
The efficient filtration lies in the filtration media containing submicron size glass fibers evenly distributed throughout the media. A filter 3" deep and 111¼2" square will contain 6.94 square feet of filtering surface. These filters are known for their long life. Although the life expectancy depends on the amount of particulate matter being filtered and prefilter efficiencies, the average life is approximately 8,000 to 20,000 hours.
How is the quality of the air filter maintained?
Through periodic testing. The ultimate test for particulate filtering was the air generated aerosol challenge and aerosol photometer-downstream scan test method. The method is a derivative of the DOP or Dioctyl Phthalate Smoke test developed by the Chemical Warfare Service in World War II as the definitive test for the particulate filtering respirator canisters. Laser is now "State-of-the-Art".
How does it work?
The test is performed by introducing DOP aerosol (or specified substitute) upstream of a filter and searching for leaks by scanning the downstream side of the filter with the photometer probe. Verify that the design airflow velocity has been set, prior to performing the filter installation leak test. Introduce the aerosol immediately upstream of the filter in question and measure the upstream concentration using either a linear or logarithmic photometer scale. Care must be exercised to assure uniform distribution of the challenge aerosol.
For linear readout photometers (graduated 0-100), the upstream concentration should be established to produce an upstream concentration of approximately 20 to 60 micrograms of DOP per liter of air. The photometer should be adjusted to read 100 percent.
For logarithmic readout photometers, the upstream concentration should be adjusted, using the instrument calibration curve, to give a concentration of 1.0 x 104 above that concentration required to give a reading of one scale division. The filter face should be scanned by passing the probe in slightly overlapping strokes so that the entire area of the filter is sampled. The probe should be held approximately 1 inch (25 mm) from the area to be tested during scanning. Separate passes should be made around the entire periphery of the filter, at a traverse rate of not more than 10 feet per minute (0.05 m/s). Report all leaks which exceed the following: Linear readout photometer: a reading greater than 0.01 percent of the upstream challenge aerosol concentration.
A reading greater than one scale division.
What about less expensive approaches?
One approach is to inject a pre-measured amount of dust in front of the filter, where it mingles with dust coming through the filter, then weigh the total amount and subtract the known quantity.
Another, known as the AFI (Air Filters Institute) Code Test, is to aspirate a known weight of dust into the air stream ahead of the filter being tested. The entire air stream beyond the filter is passed through a glass mat filter approximately 80 - 83% efficient on a particle- count basis. If it's assumed that all the dust passing the test filter is captured by the glass mat, the weight gain of the filter is a good measure of the degree of penetration of the filter.
Still another method, the ASHRAE (American Society of Heating, Refrigeration, and Air Conditioning Engineers) Test, is to inject a known quantity of a prepared dust into the air supplied to filters.The quantity of dust in the cleaned air is determined by extracting - by filtration through a porous crucible - the dust from a known quantity of air and weighing it.
There is also a method called "jet impingement." Here, cleaned and uncleaned air is pumped in turn through a series of nozzles in which the air reaches progressively higher velocities and then "impinges" on plates coated with sticky material. The higher the velocity, the finer the particles that are captured. Unfortunately, this is a laborious procedure since the particles must be counted with a microscope.
How should HEPA filters be shipped?
Care must be exercised in shipping these filters. The pleated folds should be kept vertical to prevent the sagging of the filter medium that can result from mild hairline cracks, and the inevitable moisture that is absorbed during shipping.
What about testing the filter for defects?
Examination of a filter for flaws that might impede its performance begins when the delivery reaches the purchaser - and while it is still aboard the carrier. The first thing that should be done is to check the carton for external damage and improper positioning in the cargo space. If the carton is damaged, or has a dented corner, it should be set aside for a thorough inspection.
The filter itself must be removed carefully from the carton. Haul up by the frame, lest one's fingers inadvertently puncture the soft filter medium. Each channel should be inspected, then the adhesive seal encircling the filter unit face. The corner joints of the frame should be checked for adhesive sealant and tightness. The gasket strips should be checked for decompression, then for full adhesion to the frame.
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Should filters be stored in any special way?
They should be stored no more than three filters high. Excessive heat, cold, or dampness, or rapidly changing temperatures, should be avoided. Also, the pile should be inverted every 6 months to equalize the strain between the opposing adhesive seals which bond the filter pack to the frame.
Mechanical warehousing equipment is recommended in handling the filters. A flat bed is advised, and if a forklift is utilized, a pallet should be included. Chains, slings and hooks should be stringently avoided, and the cartons must not be dropped or jarred.
When lifting them up, a strap equipped with a handle and slide fastener should be used. In affixing the handles, never use nails. Screws should not be pounded for starting; it is recommended that a drill be utilized for starting them. Also note, that great care must be taken to avoid penetration of filter. Filters transferred from one area to another should be kept in original shipping carton.
What chief precaution must be taken in installing HEPA filters?
At all times, the installation crew must be kept aware of the delicate nature of the filter packs. The packs must be installed in such a manner that the chances of air slipping past is minimized.
When should a filter be replaced?
When tests indicate a loss of efficiency, or when there is visible damage or rupture, excessive build-up of lint or combustible particulate matter on the filter unit from environment, a change in production method is recommended. Frequently, a good prefilter, if utilized, can prolong the life of a filter considerably.
What is a pre-filter?
As the name implies, a pre-filter precedes the main filter. Its purpose is to extend life to the main filter by removing the larger particles. However, because of its nature, replacement or cleaning of the pre-filter is required more frequently than replacement of the high-efficiency unit.
Can filters present a fire hazard?
Due to the nature of the filter - its multi-layered construction - fire can present a problem. Once ignited, fire progresses rapidly through the depth of the filter pack and spreads laterally until the entire pack is consumed. When the units are banked, there is a quick spread due to the explosive force of ignition.
The important thing to remember, in dealing with such a fire, is not to damper off the air flow completely; it is needed to remove explosive or combustible gases. Water with a wetting agent is the only effective extinguishing material, though occasionally, a fine spray of plain water can control the lateral spread of fire within the filter unit, even though the main brunt of the fire will still burn through the pack.
What about glass fiber filter mediums?
Here a different story exists. Should fires occur, they are much easier to combat. Although the fire rapidly melts and ruptures the glass filter medium, once the source of the ignition is removed, the fire ceases. As in the other filters, however, air should not be dampered off completely. If necessary, precautions should be taken against collection of explosive or combustible gases.
However, the benefits of filters far outweigh this potential hazard, because, if they were not available, clean rooms would not exist.
What exactly is a "Clean Room"?
A clean room is any room or area where an attempt is made to limit, control, and eliminate the amount of airborne contamination. The word "attempt" is important, because, as you will see shortly, there is no such thing as a totally clean room, i.e., a room with absolutely no contamination. There are only degrees of cleanliness, but more often than not, these less-than-perfect conditions will suffice for the purpose at hand.
Why are clean rooms necessary?
The performance of electronics, aircraft, missile equipment, food processing, the purity of most drugs and chemicals, and the success of most research laboratories and hospital operations are often limited by the presence of undesired bacteria, viruses, sub-micron particles and inadequate environmental control conditions. Microminiature apparatus of any type, especially, is sensitive to impurities of about 0.5 microns in size, as well as variations of a tenth of a degree in environmental changes. The clean room is basically a tool to enable industry to manufacture, assemble, clean, preserve, inspect and measure precision products economically. This is accomplished by controlling the pressure, temperature, humidity and contamination level.
Like all tools however, the clean room will not function effectively, unless it is placed in the hands of competent technicians. Contamination generated by the technician and his working habits can be controlled only by the technician.
How did the clean room originate?
The first "clean rooms" were operating rooms, and originated in the late 1800's. The Civil War, with its need for extensive surgery, played an important role in their development. With the introduction of anesthesia, the physicians could concentrate on improving cleanliness instead of on how to best restrain the screaming patient in order to complete the operation successfully.
The operating room evolved principally as a means of lessening the chances of infection to the patient, and also as a means of isolating him from other patients. The first operating rooms also concentrated on providing a place for the physician to "scrub up," and an area that could be closed down for 24 to 48 hours after an operation for decontamination purposes.
Is there a difference between the clean room in surgery and the clean room used in industry?
Yes. In industrial clean rooms, the quantity of particulate matter is of the utmost importance. Due to lack of bacteriological food, lack of bacterial colonies, and low humidities, industrial clean rooms have little difficulty with bacterial growth. In the operating room, however, the quantity is unimportant so long as the particulate matter itself is sterile.
How did the industrial clean room develop?
Like the operating room, the development of the industrial clean room was inextricably tied to wars - but for a different reason. During World War I, many of the small bearings and gears used in the first aircraft instruments chronically malfunctioned. During the Korean War, over a million replacement electronic parts were needed for only 160,000 units of equipment. Radar was inoperative 84% of the time; sonar, 48%. Sixty-five to 70% of the Army's electronic equipment was inoperative, and the maintenance bill for the Navy was close to twice the price of the original equipment. However, that was insignificant compared to the maintenance bill of the Air Force: over a 5 year period, it amounted to 10 times the initial cost!
For a long time, nobody knew what the enemy was. After Korea, it eventually came to light that particulate matter was causing the problem - and that only by restricting the amount of particulate matter was the problem ever going to be remedied. The only way to do this was to develop a Liberty Clean Room environment - in other words, a "clean room."
How does a clean room control contamination?
A clean room deals with contamination in three ways:
- By filtering the air entering the room, it prevents the entry of particulate matter.
- The air-handling system changes the air in the room, effectively purging the air of particulate matter generated within the room.
- It also provides an area for cleaning parts and personnel, and specifies special clothing made of "limited linting" fiber, as well as floors chosen for resistance to particle generation.
Are there any set standards for clean room operation?
There are several. The first document accepted as a workable clean room document was AIR FORCE TECHNICAL ORDER 00-25-203, entitled "Standard Functional Criteria for the Design and Operation of Clean Rooms." It was revised, as of July 1, 1963, into "Standards and Guidance for the Design and Operation of Clean Rooms and Clean Work Stations." Its purpose was to help standardize clean rooms for military use. During its compilation, groups made several extended tours of industry and military facilities to gather data. The first draft was finished in early 1960, but engineers of the Special Projects Branch, Industrial Engineering, Directorate of Maintenance, Olmsted Air Force Base in Tennessee, led to its revision and updating in 1963-65.
The Navy office concerned with clean rooms is the BUREAU OF NAVAL WEAPONS. Its first clean rooms were used for bearing repair and closely paralleled Air Force Developments. Federal Standard 209, "Clean Room and Work Station Requirements, Controlled Environment" (1963), was prepared through joint contributions of industry and government, with chairmanship of formulating committee held by the Sandia Corporation, in New Mexico. The standard was revised a number of times until the latest revision to Federal Standard 209E titled "Airborne Particulate Cleanliness Classes in Cleanrooms and Clean Zones" was released September 11, 1992. This document establishes a framework of standard clean room classifications and provides definition of terms. This will be discussed further later.
The objectives of the familiar Federal Standard 209E are also net with the International Organization for Standardization (ISO) ISO Standard 14644-1, entitled "Cleanrooms and associated controlled environments-Part 1: Classification of air cleanliness." This document was published May 01, 1999. This international standard assigns ISO classification levels for the specification of air cleanliness in the same manner as Federal Standard 209E.
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Are there any other government agencies involved in clean room classification?
Four of the most prominent are the Nuclear Regulatory Commission, the National Aeronautics and Space Administration, the National Bureau of Standards, and the U.S. Public Health Service.
How many kinds of clean rooms are there?
There are two broad categories: conventional clean rooms, and laminar flow clean rooms.
What is meant by a "conventional clean room"?
A conventional clean room is any clean room with an air handling system that does not produce laminar flow (which will be discussed later), and in which a high degree of cleanliness is achieved/maintained by controlling the generation of particulate matter inside the room. The cleanliness of such a room is dependent on the ability of the air handling system to purge the room of contaminants. There are four important factors in bring about this end: 1) the effectiveness of the filter; 2) the number of air changes per hour; and 3) the distribution of air within the room; 4) creation of an adequate pressure.
What is the most important consideration relating to efficiency?
The key to efficiency is low velocities of air entering through large areas - low velocities, because higher velocities may stir up contamination already on the floor. This is usually accomplished by having a wide center ceiling plexus with perforated diffusers, and wall exhausts at the floor parallel to it. The air is changed at least 20 times per hour - a standard measure. Studies indicate that beyond that, air currents will stir up more dust then they will purge.
Are HEPA filters to be utilized?
Most certainly. Also, they should be placed on the discharge side of the system rather than the suction side. Otherwise, leakage of atmospheric dust through faulty ductwork or improper gasketing may contaminate the air supply. However, if the filter is on the discharge side, the leaking will be outward, if any exists.
What kind of floors are utilized in a conventional clean room?
It has been estimated that 150 million particles of micron size can be generated from a floor space the size of a postage stamp! For that reason, a floor with a long life/high resistance to breakdown is a must. When sheet vinyl is used, elastic sheer force is considerably lessened. Other good materials include troweled epoxy, or polyurethane.
What about the walls?
They should be strictly non-flaking, preferably made of stainless steel, with porcelain finish or particle board with plastic laminate finish. Or they may be gypsum board with good quality hardboard material, or epoxy. They can also be fiberboard (such as Masonite), with a baked-on epoxy finish.
And the ceilings?
They can be any material that does not produce or collect dust and is easily cleaned, since there's no danger from impaction by foreign objects. We recommend a ceiling panel, vinyl coated, embossed aluminum, bonded to the panel substrate. The lights should be fluorescent, made so they can be cleaned from above. Also, it is recommended that each work station have a fluorescent light.
Are there any other component parts to a conventional clean room?
Yes. Some conventional clean rooms contain a pass- thru, such as Liberty Model H-100 Series, or an opening in the wall to introduce objects without requiring the movement of personnel into the room. These pass-thru windows should be stainless steel or plastic laminated so that they can resist abrasion and rough usage. They should consist of a double-door, turntable arrangement, air vent to purge the area of contaminants, and may also include an intercom, voice diaphragm, or speaking tube.
Air locks, used to maintain pressurization in clean rooms, are also frequently used. Their size depends on the number of people passing through in a given time span.
Air Showers (such as Liberty Model AS-200, 200E, AS-200SS, HVAS-2, and AS-400), are also an integral part. Personnel standing in the showers, with arms lifted, are subjected to jets of air while they turn slowly 360 degrees. This helps remove gross contamination. Unfortunately, if not properly designed, the air showers often leave much to be desired: the air velocities may be too low, or the exhaust inefficient.
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Air showers are usually built into air locks and designed so that they won't open until the cleaning process isCR Primer finished (unless an emergency button is pushed). Or they may be designed so that an alarm will sound, should a person attempt to leave before the cleaning is completed. To be effective, an Air Shower must have two characteristics:
- It must be finely filtered to prevent personnel being impinged with contaminants.
- It must be sufficient to establish a high velocity of air to cause fluttering of close turbulent conditions, with an effective air removal system in the floor of the unit removing air at the same volume supplied.
What about support rooms?
Generally, most conventional clean rooms include such rooms. Usually, they consist of: locker rooms, rest rooms, ante rooms, wash rooms and air shower. They may also include special shoe and sole cleaners (Liberty Model 2001TB, 2010SC or 100FJ Shoe Cleaner and Sole Cleaners - Models 6000M10 and 8000 Series, trade name TACKY MATS®), placed at entrances to remove gross contamination from the soles of shoes and wheels of utilized equipment.
What specific requirements should a clean room meet prior to installation of equipment?
It should resist generation of particles by chipping, flaking, oxidizing and other deleterious influences. If any object may be rapped frequently, jolted or abraded, it should possess a tough, resilient low particle-generating surface. For example: stainless steel, plastic, epoxy, vinyl type material, plastic laminate, enamel, PVC, polyethylene, etc. All Liberty Clean Room Accessories meet or exceed these rigid cleanroom standards.
What about fire protection?
All clean rooms - by law and common sense - must have emergency exits. During routine tests, state officials may at times violate clean room standards due to the necessity of being required to test these exits. However, this is a small price to pay for ensuring the safety of clean room personnel in the event of a fire.
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What's the most important step in keeping a clean room clean?
Aside from efficient construction and a set of good quality HEPA filters, keeping a conventional clean room clean is dependent on employee awareness. Employees must be trained not only about entering contamination, but to limit contamination generated in the room.
How is this accomplished?
The first step is training employees in personal hygiene. All personnel should be examined, to ascertain no profuse nasal discharge exists, or skin conditions that can result in above-normal skin shedding. Examinations should also include the possibility of the existence of dandruff, skin flaking or high amounts of acid found in the moisture of hands, or severe nervous conditions; such as itching, scratching or claustrophobia.
In addition, all personnel should receive periodic re-indoctrination. They should be encouraged to bathe frequently and shampoo weekly to discourage dandruff, wear clean clothes and avoid scratching or rubbing of exposed areas. The wearing of gloves is advocated, should their hands be severely chapped and daily shaving (or utilization of masks) should be encouraged. Should long hair be prevalent, the usage of caps or hoods should be encouraged.
In addition, the following instructions should be adapted to achieve the maximum potential from a Clean Room facility. It is not intended that these instructions be utilized in all Clean Rooms under all circumstances. We suggest that they should be utilized as a guide in total or part, as required to achieve the end results of individual Clean Room specifications.
- The Clean Room shall be considered a restricted area. Only authorized personnel will be allowed to enter. Visitors must be approved by the Clean Room Supervisor.
- All entry and egress will be via air locks and air showers (Liberty Model AS-200) except in an emergency.
- Clean Room garments will be worn by all personnel and visitors in the Clean Room.
- Clean Room garments will be discarded when leaving Clean Room at the end of the shifts, lunch periods, etc. New Clean Room laundered garments will be utilized when returning to work.
- No smoking will be permitted within any room in the Clean Room facility.
- No paper products will be allowed in the Clean Room unless they are suitably sealed in plastic containers. This includes books, manuals, paper pads, towels, etc.
- All writing will be done with ball point pens; no pencils or erasers will be allowed.
- Personnel tool boxes will not be allowed in the Clean Room.
- Clothing that will produce lint shall be avoided even if covered with Clean Room garments.
- Operations which will contaminate the Clean Room, grinding, sawing, filing and etc., will be prohibited unless they are done within an approved exhaust device.
- The use of abrasive sandpaper, emery cloth, etc., will be prohibited unless they are used within an approved exhaust device.
- Personnel with excessive skin disorders, sunburn, rashes, etc., will use approved lanolin base skin lotions to control viable particulate contamination.
- No cosmetics will be worn in the Clean Room.
- Waste Baskets must be covered at all times. They shall be emptied daily and vacuumed.
- No flaking or corrosive materials will be allowed in the Clean Room.
- All material parts, and containers, will be cleaned prior to transfer to the Clean Room.
- Personnel shall have clean hands and fingernails.
- Personnel shall be clean shaven and their hair shall not be excessively long.
- Personnel with dandruff problems shall wash their hair at least once a week, utilizing suitable lotions to control this problem (Doctor recommended).
- Personnel with head colds or other ailments which will cause excessive excretions from coughing or sneezing shall be given work in an area that does not require contamination control.
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How should clean room personnel be selected?
This booklet is useful for training clean room personnel. The ideal clean room worker is not only motivated to achieve and maintain cleanliness, but also knowledgeable in contamination control principles and aware of the consequences of each action or inaction. They should be willing to keep their hands, fingernails, and face clean, and refrain from combing their hair in the room, or wearing fingernail polish or jewelry. Also, they should be careful to never leave exposed parts on their workbench and to keep surplus parts in their appropriate containers. Most importantly, they must be willing to assume responsibility for cleaning up their assigned work station at periodic intervals.
How can personnel be made more aware of their responsibilities in a conventional clean room?
Psychology can be helpful in maintaining a clean room environment. It is important that employees be made constantly aware of the job they are performing, maintain as strict a discipline as possible, and are constantly reminded of the nature of the work they are accomplishing. This can be accomplished by constant repetition and by establishing a group identity, a sense of group unity, to unite all the workers in a single purpose - the maintenance of a clean environment.
What are the major weaknesses of the conventional clean room?
Almost all conventional clean rooms do not have self-cleanup capabilities to offset contamination brought into the room by personnel and equipment. Most contaminants introduced in this fashion settle to the floor or other horizontal surfaces and may be introduced into the air by changes in air currents, or by activity in the room. This contamination must be removed by manual cleaning. In addition, the air-flow patterns in the conventional clean room are generally not uniform, nor are they directed in a manner that carries particulate matter away from critical work areas. In addition, they do not remove airborne contamination from the room as quickly as it is brought in, and since all personnel, despite their efforts to the contrary, are going to contribute to the quantity of contamination, rigid personnel controls are required.
CONVENTIONAL CLEAN ROOM "Turbulent Flow"
What kind of effect do these liabilities have on operations that must be as free of contamination as possible?
Plenty. These deficiencies were brought home to the Sandia Corporation in New Mexico, who due to their involvement in the manufacture of nuclear weapons, required extreme miniaturization and close tolerances that could be offset by the smallest amount of contamination. If they were unable to think of a better method, the work would have to be performed in inconvenient vacuum hoods. Therefore, they decided to propose an entirely new concept in clean room technology.
The new concept would have to fulfill the following criteria:
- It would have the best ambient air filtration system economically feasible.
- It would have a self-cleanup capability which would offset both contamination brought into the room and contamination generated within it.
- Its air-flow patterns would have to carry airborne contamination away from the work area.
- It would reduce personnel restrictions. The solution they came up with, the revolutionary new concept in clean room technology, was laminar flow.
What is laminar flow?
Laminar flow is produced when air is introduced uniformly at low velocities into a space confined on four sides and through an opening equal to the cross-sectional area of the confined space. This stratifies the air so that minimum cross-stream contamination occurs. Little or no transfer of energy from one streamline to an- other occurs. Even when there are objects in the room, tests indicate that the streamline will reform once it is detoured around them. Also, if some of them theoretically were to break up, other adjacent streams would capture the contamination.
What are the advantages of laminar flow?
If a HEPA filter is utilized, one will achieve the lowest contamination level presently possible. The cleanliness of the atmosphere created is almost independent of the operation or activity in the room. The cost of achieving or maintaining extremely low levels of contamination is cheaper, and the levels of contamination achieved are considerably below that required by any device currently either contemplated or in production. In addition, there is no transfer of contaminants by random air flow between work stations. Each work station is totally isolated from every other, encapsulated in its own stream of air.
How many different kinds of laminar flow rooms are there?
There are three basic kinds: laminar flow between ceiling and floor (vertical); between two walls (horizontal); and a cross flow arrangement in which air flows from one room into an adjacent one, then back to the first, in a cyclical motion.
What are the advantages and disadvantages of a vertical laminar flow room?
The advantages are that every operation is isolated from every other one by a private streamline of laminar-flow air. The down-flow room also produces the shortest distance from contaminant generation to contaminant removal. The chief disadvantage of the down-flow room is the cost.
What about the cross-flow (horizontal) room?
The cross-flow room has the disadvantage that the arrangement, by its very nature, will not isolate the operations from each other, and downstream operations will be in a dirtier atmosphere. Thus, "staging," or placing most critical operations closest to the filter is necessary. There can also be a heat build-up at the point where the air exits. To correct this, it is essential to attempt to have heat-generating consoles located outside of the clean room with only the controls visible within it.
The twin-room can be constructed in a low ceiling area, where it would be difficult or impossible to place a over- head duct system. What generally occurs is that a wide room is divided into two by a partition. The air, once it flows through one room, goes through a filter and emerges in the room next to it, destined to return to the first room when it reaches the other side of the second room, etc. This type of arrangement presents less significant heat build-up problems.
Can the laminar flow principle also be applied to operating rooms?
Yes. The rooms thus provided will be much cleaner than the most modern operating room. Down-flow would be ideal, as bacteria would be carried from the patient downward and through the floor, eliminating the need of closing the room to undergo an hourly germicidal scrub. For hospital patients with low resistance to infection, a bedroom would function as well as, or more efficiently than a plastic tent isolator. Additionally, patient comfort would be maximized and housekeeping requirements would be minimized.
What conditions would have to be met to adopt laminar flow to an operating room?
The patient and immediate attending personnel would have to be positioned within the laminar flow, in order that it will produce the minimum contamination at the surgical area. The design of the system would not in any manner limit or interfere with the entry of personnel and equipment. Additionally, the floor should be designed to facilitate cleaning.
How is a clean room monitored to make sure the job is being accomplished?
There are several methods that may be employed. The most effective is the "manual method," which traps contaminants on a membrane filter and then employs a microscope of at least 100x power for counting and sizing particulate matter five microns and larger. Automated methods may be utilized, but primarily to supplement, but not substitute, for manual methods.
Counting by microscopy is the oldest, simplest, most effective method known. Unfortunately, this procedure is tedious and time consuming and depends on operator technique, visual acuity and judgment. Specific characteristics like shape, density, color and frequency of particle type cannot be observed by any other single method.
When is automatic monitoring justified?
When other techniques are either inaccurate, inadequate, inefficient or costly, or where greater reliability, repeatability and accuracy are required. They are generally utilized in situations where the transient contamination level must be known, or where smaller particles that cannot be adequately monitored by manual counting techniques will effect the production or operation of the product.
The most commercially successful automatic particle counter designed for optical detection of submicron and micron particles is based on the theory of light scattering.
What other type of automatic monitoring devices are being utilized?
Thermal precipitation, in which particles are drawn through two closely spaced hot/cold surfaces causing them to collect on a cold place is sometimes utilized. So is impaction, in which the particulate matter is forced through a small orifice at near-sonic velocity and onto the gathering surface. The force of the impaction and the sudden direction change of air stream, separate the particles from the air stream to the flat surface, where they are retained by tacky material or electrostatic force.
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What types of garments are utilized in a clean room?
A clean room garment is not a piece of modesty clothing. It is a personnel filter designed to filter the employee out of the clean room environment.
The garment should produce little or no particle emission of itself. This requires the fabric or material to be stable, possessing a high ability to resist breakdown. Occasionally this characteristic is expressed as good wearability.
Synthetic fibers woven into fabrics of various types have the most desirable qualities. Nylon is suitable for garments wherever acid and static are not a major factor.
Nylon makes garments that are crisp and firm, have a silk-like luster, are very durable and are stain resistant. This does not imply that nylon garments cannot be stained; only that very few substances will penetrate nylon to such an extent that they cannot be removed. Nylon is one of the most washable fabrics known, as it washes easily and is extremely quick drying, because it absorbs only 4 to 5% moisture. It usually requires a light pressing.
Dacron fabrics are more opaque than nylon fabrics of comparable strength and weight. Dacron is different from nylon in both physical and chemical senses. Dacron is a polyester fiber, where nylon is a synthetic hydrophobic fiber. Dacron has definite advantages over nylon. It is crisp, but softer; drapes more smoothly and has a finer feel. It has a better initial whiteness and retains the whiteness if washed properly. Nylon, on the other hand, has a tendency to yellow with age under improperly controlled wash conditions.
Dacron is one of the most wrinkle-resistant fabrics known. Unless subjected to excessive heat, dacron will retain its original shape, regardless of how it is wrinkled. Conversely, excessive heat applied to an already-wrinkled garment means permanent wrinkles.
Dacron polyester melts rather than flames when exposed to fire. It should be utilized instead of nylon when a high acid resistance is essential for the garments. Dacron is less moisture-absorbent than nylon, absorbing 0.2 to 0.8% moisture.
Where synthetic fabrics are a necessity, along with low electrostatic properties, a blend of continuous filament yarns is recommended. An example of a blend fabric is a dacron and rayon weave. When this type of weave is utilized in clean room garments, it is with the knowledge that it has a shorter life. Rayon is a short fiber which does not tend to spring loose from the weave (particularly under high drying temperatures. Due to its desirable surface conductance static charge) build-up is reduced on these garments.
Dacron garments are being favored more and more as clean room garments. They appear to be a compromise garment giving good wear and low static charges. When further treated with anti-static agents, the surface conductivity of the dacron increases somewhat comparable to that of cotton garments.
Recently, conductive garment material has been introduced for applications wherever electrostatic sensitive devices are being manufactured in the clean room.
What other types of fibers may be utilized in clean room garments?
Cotton garments, although having low static problems, do produce large amounts of throw-off particles. The amounts may be reduced by heavy starching, achieved with some of the new synthetic starching components prepared for domestic use. However, starches are basically exterior coatings on the fabrics and/or fibers which, when dry, shed great amounts of the material. Cottons are not desirable for critical clean room operations.
Herringbone twill weaves have the greatest strength to resist runs in a fabric. They are closely woven, which is desirable, and have been selected for the majority of clean room garments in use at the present time.
Taffeta weaves are beginning to become more popular, not only because of a tighter weave than the herringbone twill, but they also appear to feel lighter and cooler. Taffeta does have less resistance to runs, but actually this is of little consequence.
Clean room garments become soiled quicker on the inside than on the outside. The reason is simple; the inside of the clean room garment is in direct contact with the surfaces of street clothes.
Garment changing for employees is a necessary function due to the amount of physical activity required of each person during performance of his tasks. Light activity will allow the garment to be worn for a longer period of time before laundering is required, whereas large amounts of activity will saturate the garment, increasing its potential to allow more particles through the weave. Garments brushing against surfaces will quickly collect contamination on their outer surfaces. It should be stressed however, that a specified contamination level of the garment is not the sole criterion for garment changes. The criterion for garment changes is a function of the garment's emission effect on the clean room contamination level. Normally, most activities change on the average of two to three times weekly.
How are clean room garments designed?
Garments made from synthetic fabrics for utilization in non-linting areas must have these basis characteristics:
- A minimum of seams. The seams must firmly envelop the raw edge of the material.
- Loose fitting to eliminate abrasion against clothes worn underneath.
- Absence of pockets, belts and pleated or tucked areas.
- The individual filaments of the fabric must be of the strongest available material, to avoid easy breakage.
- Tight fabric weave minimizes passage of dust and lint from the individual's body and inner clothing to the exterior.
- The fabric must reduce the generation of static electricity as much as compatible with its ability to remain non-linting.
- Sewing threads should be of monofilament materials and resist linting.
- Garments should cover as much of the person as feasible, particularly around the neck, since usually the person bends over his work and such exposure could create possibly harmful conditions.
What types of garments are used in clean rooms?
Hoods and Caps. The hood-type garment is preferred for very clean applications. This garment covers the entire head, ears, neck and tucks into the coverall or frock. This should fit fairly snug around the face. Another feature is that these are fashioned with various adjustments for sizes, etc. Long sideburns, moustaches and long hair, may be necessary to furnish specially-designed hoods, which will adequately cover all hair on or about the face (eyebrows excepted). For additional cleanliness, the surgeon's type cap may be utilized. This also should cover the ears as much as possible. It should be noted that caps worn in this manner may gap at the ear which may allow dandruff, hair and other particles to fall down from under the cap.
Coveralls or Frocks. These garments should be made with a snug-fitting neckline, allowing however, slight looseness to avoid possible abrasion of the uniform- /garment. The design should be simple, because including many pleats, darts, pockets, belts, etc., would necessitate a greater amount of cutting and therefore additional lint would be produced. These areas may become receptacles and thus retain lint.
The coverall should be worn wherever the utmost in cleanliness is required. The hood should be tucked into the collar of the coverall, in order that anything falling from the head shall gravitate into the coverall body. The coverall legs should also be tucked into the boot so that possible falling matter will drop directly into the boot, preventing said matter from entering surrounding environment.
Areas with less critical requirements or areas utilizing clean room benches may only require frocks. However, the major disadvantage of a frock is the open bottom allows particulate matter to fall to the floor from an individual's garments. Particles below bench level can be agitated by excessive movement of personnel throughout the clean room.
Foot coverings can be a great problem area. Personnel arriving for work may have traversed through muddy areas, bringing this material into the dressing rooms. Shoe Cleaners and Tacky Mats® would remove almost completely the dirty soil from shoes of involved personnel. It may be reasonably assumed that some minute particles will be carried into the room via interior of the booties and therefore, will become part of the ambient atmosphere. Again, the details of construction of the boots and shoe covers should be carefully controlled.
Hands come directly in contact with the product, or with the tools that are utilized to handle the product. It is essential that these two items be observed carefully. Skin oils, perspiration and fingernail polish can cause havoc under certain conditions in this type of atmosphere. Since we are dealing with people, one must remember that a nonporous glove will become very uncomfortable. Incidentally, when specific products are subject to chemical contamination, rubber must be observed carefully since it contains a great amount of free sulphur. Perspiration moistens fabrics and seams at times quite thoroughly. Examine carefully in relation to the product involved.
Wiping Cloths for clean rooms are available, however, since only monofilament fabrics are utilized for their manufacture, they offer very little absorbency and very limited advantage. They also have a tendency to spread grease and other materials rather than absorb same. Generally, it appears that mechanics use them as mats underneath their various tasks. However, please note that a properly cleaned Class 100 Wiper has many advantages in the Clean Room work area.
How are clean room garments laundered?
Industrial laundries are accustomed to washing things "clean." Now industry is demanding additional services from its uniform suppliers, such as - uniforms are not only to be "cleaned," but also free from lint, particles of matter and other contamination. It is now necessary to distinguish between the "old" standard and the "new" standard. According to the former standards, a garment was to be free from spots, soil and dirt. The new or "super clean" garment must be free from all contamination, etc.
To achieve a "super clean" garment requires special techniques and procedures. Factors that contribute to accomplishing this objective are:
- The proper garment.
- Control of garment handling from pickup to delivery.
- Proper Washing and drying technique in a Class 100 Clean Room.
What is the best way to treat clean room garments?
They should be kept separated from ordinary laundry. This may be accomplished by utilizing the following: heavy plastic bags, nylon or dacron; nylon-lined carts; or stainless steel tote boxes and carts. These bags and carts must be kept closed at all times, particularly after the garments are washed and dried.
While care is necessary before washing, extremely rigid control is imperative after washing. All personnel handling the garments should be attired in Clean Room garments. If a laundry is processing Clean Room garments, said facility must be operated under the same regulations governing a Clean Room.
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What about washing and drying?
In this phase, the degree of control depends on the requirements of the particular Clean Room. As the Clean Room techniques become more rigid, the laundry requirements will become more stringent. The processing cost per garment will also increase accordingly due to ever increasingly stringent requirements. Be realistic; do not exceed your requirements. Laundries should have only one set of machines to be utilized solely for synthetics.
On completion of the wash cycle, the garments should be removed from the machine and dried in clean, filtered, controlled air. Each garment should be packaged in a sealed bag within the controlled laundry area for de- livery to the customer.
The clean room garment is designed to protect the ultimate product from contamination by the worker during the handling of its components parts.
The following laundry precautions are recommended by the manufacturers of clean room garments:
- Rigid inspection and repair of all defects before washing.
- All zippers should be zipped up.
- Smaller loads to decrease mechanical action. Utilize mild alkaline or neutral soaps and detergents.
- If possible, refrain from utilizing bleach. Should bleaching be required, reduce the concentration.
- Maintain the washing temperature below 140°F if at all possible.
- Eliminate extraction or tumbling to reduce or prevent the setting of wrinkles. The usage of plastic hangers will reduce the possibility of rust.
- Ironing should not be required; however, should it become necessary to do so, the lowest possible temperature should be utilized.
- Do not use antistats unless absolutely necessary. Considerable amount of static is developed by overheated drying ovens. Therefore, lower drying temperatures are suggested (120°).
NOTE: Antistatic is a chemical which is dissolved in the rinse water during the last cycle. When the garment is dried, it adheres to the surface of the fibers. It has the property of picking up moisture from the air and thus helps to reduce static electricity. It is a surface agent. The increased utilization of antistat will cause the reduction of additional static electricity. Great care must be observed to avoid too much accumulation and thus prevent shedding during utilization in the Clean Room.
There are however, several special conductive garments currently available that are inherently antistatic. These garments feature stainless steel threads woven into fabric, or are comprised of carbon encapsulated in nylon. These unique antistatic garments are currently available through Liberty Industries.
How to Save Thousands on Your New Cleanroom - Volume 5, No. 1 in a series of publications from Liberty that makes understanding cleanroom design clear and simple. Utilizing a question and answer format like our previous Cleanroom Primer, Air Shower Primer and Maintenance Primer, the design primer simplifies a complicated subject. 56 pages. Don't buy a cleanroom until you've read this publication. It will save you thousands - I promise.
Published by John J. Nappi, Jr.
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