Having a clean room is a great resource for a contract manufacturer (especially one that is focused on electronics). Clean rooms allow for a slew of products to be easily and safely manufactured, products that in a clean room-less company, would be impossible to create. But by building a functional clean room, that is only half the battle. Once the clean room is fully armed and operational (yes, that was a Star Wars reference) the room must be monitored to ensure the environment stays within the ISO specifications that it was built to. One area that must be continually monitored is the positive pressure of the room.
ISO 14644: Clean rooms and associated controlled environments, lists pressure as one of the many parameters that need to be controlled in a clean room environment. But what does “pressure” mean exactly? First, let us start with why the pressure of the room needs to be monitored at all.
Under a clean room’s ISO classification, a certain number of particles of certain sizes are allowed inside the clean room. The smaller and fewer particles allowed, the cleaner the room, and the higher the ISO classification. If too many particles of a particular size or count make their way into the clean room, the ISO classification will not hold, which could lead to a great deal of manufacturing issues. And what is worse, keeping dust and debris out of a clean room can be difficult when people and materials must constantly be ushered into the clean room and finished products must be ushered out.
This is where positive pressure comes into play. Using an HVAC system (Heating, Ventilation and Air Conditioning), a great deal of air can be pushed into the clean room. By pushing more air into the clean room compared to the surrounding area (host room), we create a pressure gradient between the two rooms, similar to how a dam creates a water pressure gradient between the two sides of a river. This is where the term “Positive Pressure” comes from, the HVAC system creates a “positive” gradient of air pressure between two adjacent areas.
Now let us imagine that we are a fluffy little dust bunny hanging out by the door between the clean room and the host room. With the HVAC system running, there is a positive pressure gradient between the two rooms. If the door that we are sitting by is opened, physics tells us the air inside the clean room will flow out in an attempt to balance both rooms’ pressures and reach an equilibrium. And as the air flows out, our fluffy dusty bunny selves are going to be blown away, keeping us out of the clean room and maintaining the particulate cleanliness.
So by maintaining positive pressure, particulates that might be nearby a clean room entryway will be blown away, maintaining the ISO classification and keeping the clean room running as clean as the day it was certified. These gradients should be continually monitored, usually using minihelic or magnehelic gauges, depending on the “sized” air differential that needs to be maintained. If you are maintaining a differential equal to a small creek or stream, a minihelic gauge will get the job done. If you are maintaining a differential more along the lines of the Hoover Dam, a magnehelic would be better suited.
For further information on clean room pressure, I would recommend looking at ISO 14644, specifically “Part 4: Design, construction and start-up”. And remember, stay positive!
