Clean Air Philosophy

Clean air is a difficult subject to comprehend. The levels of cleanliness and mathematical probabilities of contamination from any source spoiling production runs tend to be an emotive issue.

In order to put a perspective on the subject, Clean Air "The Total Issue" should be thought of as an iceberg. The section of the iceberg above water is the cleanroom or clean air cabinet. The section below water is the clean standard to which we wish to aspire. Clean problems, like icebergs, also tend to roll over and, as one problem is resolved a new issue emerges requiring a different solution. In order to tackle the subject, one needs to understand the sources of contamination and resolve these to zero wherever practically possible.

As with all processes, the education of the personnel is the most important. The particles that contaminate are not likely to be visible to the naked eye. Unless all the staff involved, including those outside the clean area, such as purchasing, fully understand the clean procedures, it is unlikely that a correct solution or selection of equipment or clothing will be arrived at let alone be achieved.

It is not a discipline that one can pay lip service to; either one wishes to work to operate at a standard of quality or not. Whilst pursuing the requirements, costs will be a major consideration.

The high value product will receive fewer objections to spending to prevent the loss of that product whilst low cost items on higher quantity outputs are less likely to find the financial backing, as the items are less able to carry the added cost.

However, with volume outputs that are experiencing 12-20% rejection rates due to airborne contamination, reducing these to 1-5% can significantly assist in reducing the product cost, and the added cost can be borne.

Clean Air:

This is the easy part. Modern HEPA and ULPA filters have very high efficiency and it is practically possible using a U15 filter at 99.9998% at 0.12 microns to remove virtually all particles down to 0.03 micron size (1 micron = 0.000039 inches and a human hair average diameter 100 microns or 0.004 inches). Therefore, the particles that are being filtered out of the air are 200,000 times smaller than the cross section of a human hair.

We can therefore, assume that the contamination will not be present in the incoming filtered air. The cleanroom designer should therefore (providing he or she understands the product process), be able to design the area or device to ensure that sufficient volume of clean air is available to entrain any stray or liberated particles, and hold in suspension until they can land on a filter. All particles will eventually gravitate and the need for low-level extraction is obvious.

With the room empty, and the equipment not being used, the room can be physically cleaned and the re-circulating airflow should maintain a low background count. Note: Cleaning will liberate particles into the air stream. On completion of cleaning any large particles, say above 0.5 microns can then be attributed to either a process action or the personnel.

Personnel:

It is well known that the majority of cleanroom problems walk in on two legs!

Dress:

Cleanroom clothing correctly worn and laundered to cleanroom standards will trap the release of human debris within the suit. The largest output of human debris is from the head and mouth.

All these facts we are aware of, but we brush them aside. It is most important therefore, no matter what other standard of dress is worn to have the head, neck and mouth covered by any cleanroom, or indeed clean air cabinet, operators. This is needed if any attempt is made to achieve a standard of cleanliness and indeed, quality. The part covering of personnel will depend on the process and contamination control required. One must decide for them selves if a smock, which does not cover outdoor clothing that is probably contaminated with cat and / or dog hairs and exhaust fumes, would be acceptable in a cleanroom.

The training of all personnel in associated disciplines is most important. The training must start from basics and cover the complete subject, otherwise it will appear no more that a conjurer's trick which the operator will marvel at rather than aspire to.

Static:

No matter how clean the room, processes that create static will attract the liberated particles that exist in the airflow. There are several solutions:

  1. Totally encapsulate the process in localised clean air;
  2. Earthing out;
  3. Provide an ionising system to combat any static production;
  4. A combination of all of the above

These contamination problems need to be analysed to provide the correct or viable answer.

Cleaning:

Cleanrooms need to be cleaned each day. Good installed vacuum points are the best approach. This paragraph may be short but the time, care and devotion to this subject is lengthy.

Cleanrooms can be checked by the use of a UV light source. With the cleanroom lights switched off a UV light will detect the areas of biological (human mainly) particle build up. Remember there are visible particles that should not even be present in an ISO Class 7 (Old Class 10,000 M5.5/Class 'J') area.

Where this contamination exists, one can assume that sub micron build up will also be present.

Cleanroom and Clean Air Equipment:

This philosophy has assumed from the start that the clean equipment will meet the standards one needs to achieve. Most manufacturing can be achieved in a good Class 7 (10,000) or sometimes Class 6 (1000) area with, if needed, localised Class 5 (100) or Class 4 (10) environments. A good designer should explain his or her ideas. However, most will be obvious.

Conclusion: