|Clean Room Primer|
Clean Room Primer
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 recrystalize 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.
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?
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.
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 forstarting 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.