Common Causes of Cleanroom Contamination
Once a cleanroom is built and activated, constant monitoring and maintenance are required. Equipment, tools, furniture, raw materials, outside air, people, and even the type of garment worn by them will have to be examined for contamination risk before being allowed to enter. Specific conductivity material, antistatic characteristics, out-gassing properties, or even antimicrobial aspects may be required.
The objectives of environmentally controlled Cleanrooms are to provide contamination-free space in which to test and/or manufacture contamination-free products. Nonetheless, contamination has a way of unexpectedly occurring without any indication of its origin.
The following are only basic contamination causes. A regular evaluation by a Little Brothers Construction contamination control engineer is recommended.
People: Improper cleanroom garments, gowning protocol, insufficient gowning for a particular class, or infrequent change-out intervals of suits, smocks, or other garments and floor mats. This particular source is more than 80% of all infusions of contamination in most cleanrooms. Most of the airborne particles emitted from people migrate up through their cleanroom garment collars or down their legs when walking. Colognes, deodorant, lotions, makeup, and perfumes are definitely not allowed in cleanrooms. Proper training of all personnel that may enter the cleanroom - even once - is imperative. The speed at which they walk through the cleanroom may not only triple the particles they emit but may also change the fluid dynamics of the room temporarily by traveling faster than the natural laminar airflow. Human skin usually falls between .4 to 10 um and hair from 70 to 100 um. Below are figures for personal particle generation:
|Particulate Propagation Rate ≥ 0.3um / min.|
|Motionless - laying down, sitting or standing||100,000|
|Motion - head, neck or arm||500,000|
|Motion - head, neck, arm and foot||1,000,000|
|Motion - standing up or sitting down||2,500,000|
|Walking at 2.0 mph||5,000,000|
|Walking at 3.5 mph||7,500,000|
|Walking at 5.0 mph||10,000,000|
Paper, pencils, and retractable pens: A major cause of particle contamination is writing supplies. Special cleanroom writing materials or electronic documentation are recommended. If non-approved cleanroom paper is used it must be enclosed in sealed plastic (anti-static plastic may be required in certain applications).
Processing equipment and tools: The introduction of equipment into the cleanroom should begin before the equipment arrives. It is imperative that a proper evaluation of all materials used in manufacturing the equipment and any chemicals used internally by the manufacturer. Once approval for the introduction of the equipment is made, the supplier must conform to proper packaging and shipping methods. There are two basic ways the equipment will arrive. The following are basic acceptance guidelines for handling equipment deemed to enter the cleanroom:
- If the vendor has cleanroom environment resources, a standard approved procedure should be in place for packaging and shipping. In cleanroom packaging, the product should be double, or triple-wrapped depending on cleanliness requirements. Once received, unwrap the first layer in a cleanroom staging area or gowning room. The inner wrap is to be removed in the main cleanroom.
- For equipment received from unapproved packaging methods, the cleaning solutions, and tools to perform hard-to-access areas should be in place before arrival.
Equipment must be cleaned thoroughly with approved solutions. Specialized cleaning methods are used for difficult access surfaces. It is imperative that all items brought into the cleanroom are examined and cleaned. An MSDS may be required for all materials used to manufacture tools or equipment. Generally, everything brought into the Cleanroom, including construction materials used to build the facility, should have MSDS approval. Some tools and equipment generate gases, volatiles, and solid airborne particulates during use and are sources of contamination. They should be isolated and controlled from contamination from other processes or products. Equipment emissions of airborne particles may be minimized or eliminated with unidirectional airflow, de-ionization, baffles, vinyl, and/or enclosures.
Raw material and product: Before any raw material or product is brought into the cleanroom it must be cleaned. The extent of cleaning depends upon the class of cleanroom, the destination of the raw material or product, and the possibility of any cross-contamination with other processes. Isolation of material with transport tunnels, hanging shields, nitrogen-purged processing, vacuum processing, SMIF, laminar flow stations, or isolation carriers can dramatically reduce contamination.
Cleaning products: Regular household or industrial cleaning products are not acceptable for cleanroom maintenance. Cleanroom wipes and mops must be used for all cleaning needs in the Cleanroom. Cleanroom vacuums must have a classification rating equal to or better than the cleanroom it is used in. Many vacuums claim they have a HEPA filter in them; however, they emit particles, and the filter has only an efficiency rating of 99.96%.
Make-up air: Depending on the design of the cleanroom, make-up air, (additional air supply to maintain positive pressurization) either from HVAC or other air sources contains a wide variety of airborne contaminants. Even if the air is passed through a HEPA filter many airborne particles and volatiles pass through. Some of these are nitrites from farming, carbon dioxide from traffic, smoke (forest fire, coal, oil, all ranging from .01um-1.0 um in size), pollen, bacteria, mold, and dust (at .03 um and up to 100 um).
If you have any comments or for a free evaluation of your facility, please contact us.