Assembly processes for devices used in the electronics, medical and military sectors have strict requirements for clean environments. Clean rooms and safety cabinets have to control the concentration of dust and particles of less than 0.5 micron diameter in the surrounding air.
The human eye can only see particles greater than 10 microns in diameter. But a clean environment needs to control dust and dirt where the particles are less than 0.5 microns in diameter. Such particles are only visible under a microscope. The standard procedure is to pass incoming air through a high efficiency particulate air (HEPA) filter that collects more than 99.97 percent of particles of 0.5 microns diameter or larger. Particles of this size can be oil mist, cigarette smoke and smog, but these are only visible when water vapour in the air condenses around them. Super clean rooms for the semiconductor industry use ultra low penetration air (ULPA) filters that halt 99.99 percent of particles of 0.1 micron diameter.
Cleaner Than Nature
The human body is one of the biggest sources of dust from skin particles. An immobile body can release about 100,000 particles of 0.3 microns diameter or larger per minute. This can rise to 30 million particles during vigorous exercise. The density of particles in the highest class of clean room or laminar air flow cabinet is 10 particles of 0.1 microns or more in one cubic metre of air. This standard cannot be replicated in nature. The upper stratosphere has 100 times this particle density. The particle count in any clean system is measured at three different levels: as built, when it is empty of any equipment or workers, at rest, when equipment has been moved in and operational, when equipment and workers are present. The particulate count changes at each level.
There are four basic principles for clean room design. Firstly, no dust should be brought into the room by people or equipment. Secondly, operators in the clean room should not generate any dust and wear dust-free garments. Thirdly, dust should not be allowed to accumulate in a spare space. Finally, all dust should be removed quickly. This can be achieved by increasing the air change rate in the clean room or laminar air flow cabinet. The greater the number of filters in the room or cabinet, the higher the air change rate. This air change rate should be increased as the number of operators using the room increases as they bring in more body heat and contamination.
Manufacturing processes can change and expand. Rebuilding a larger clean room from scratch can be expensive. A modular design enables the clean room to be expanded as needs increase or decrease. Clean rooms can also be built on castors so that the clean space can be moved around a factory or laboratory floor.
The biggest source of contamination in any clean room is the worker conducting experiments, operations or assembling parts of any device. To maintain product integrity and high operational standards, company managements should consult clean room specialists who can recommend strict working procedures to avoid unnecessary contamination.
Julia Adams is a clean room consultant and works on the design of specialist microbiological safety laminar air flow cabinet features.