ISO 14644-1 & FED-STD-209E

There are two ways to express the air cleanliness of a clean room, for example “class 1000” and “ISO class 6”. These are based on US Federal Standards and ISO respectively. While the US Federal Standard is now obsolete, it is still widely used.

In practice, it is sometimes explained that Class 1000 in the US Federal Standards is equivalent to Class 6 in the ISO standards, but the actual definition is slightly different. This article briefly summarizes the background. We will explain in order of US Federal Standards FED-STD-209 Original, FED-STD-209E, and ISO14644-1.

The original US Federal Standard (FED-STD-209) defines a cleanroom class as the number of particles with a diameter of 0.5 µm or greater per cubic foot of air, as measured using an optical particle counter. A Class 100 cleanroom means no more than 100 particles larger than 0.5 µm per cubic foot.

A subsequent revision of FED-STD-209, FED-STD-209E, specifies particle concentrations of 0.1 µm, 0.2 µm, 0.3 µm, 0.5 µm, 1.0 µm, and 5.0 µm, depending on the cleanroom class. . It now includes particles smaller than 0.5 µm because it takes a long measurement time to measure a statistically significant number of 0.5 µm particles when the cleanliness is very high.

In addition to changing the sample volume to metric, ISO14644-01 adds ISO classes 1, 2 and 9 not covered by the FED-STD-209E equivalent. By the way, Class 1 in the FED-STD-209E standard is equivalent to Class 3 in ISO. The ISO 14644 classification defines a class as the index of the number of particles with a diameter of 0.1 μm or larger per cubic meter of air expressed in powers of 10.