According to ISO 14644-1:2015 a cleanroom can be classified by measuring the average concentration (number/m³) of a particle size between 0.1 and 5 µm a prescribe minimum number of location in a particular state of occupancy (as build, at rest or in operation). The ISO 14644-1:2015 classification table shows the maximum allowable particle concentration for each particle size. This table shows that for statistically insufficient concentrations a smaller particle size should be selected. A second table shows the minimum number of sampling locations for a uniform cleanroom area.
The number of particles ≥ d µm is measured with a light scattering airborne particle counter (LSAPC). (d is the equivalent diameter of the counted particle) When measuring the particle concentration at one or more locations the acquired data cannot be used to state the cleanroom class. In that case, it is called monitoring and a reference should be made to ISO 14644-2.
In a cleanroom the smaller particles that are generated and distributed by people are removed with the cleanroom air handling system. However larger particles will not be removed by the cleanroom installation. These will deposit on all surfaces in a cleanroom. Surface Cleanliness with respect to particles is expressed in the concentration of particles per surface area.
Surface Cleanliness by Particles concentration SCPs described in ISO 14644-9:2012. The surface cleanliness is expressed in the concentration of particles larger or equal to 1 um per m². In case the number of particles ≥ D µm. D is the largest size of the particle (0,1 ≤ D ≤ 500). The number of particles ≥ D µm multiplied by particle size D gives the Surface Cleanliness level for particles ≥ D µm: N1-ND.D. The surface cleanliness class SCP is found by taking the 10log of the surface concentration. Both concentration and class should be treated as monitoring values.
It is also possible to use VDI Guideline 2083-9.1 which is similar to ISO 14644-9. Here the surface cleanliness is expressed in the concentration of particles larger or equal to 1 um per cm². In case the number of particles ≥ D µm. D is the largest size of the particle (0,1 ≤ D ≤ 500). The number of particles ≥ D µm multiplied by particle size D: N1-ND.D gives the Surface Cleanliness level per cm².
For some applications it is more interesting to know the obscuration or area coverage by the particles on a surface. The obscuration by particles is ex-pressed in ppm which is the total particle area µm² per mm² observed surface area. Alternatively Percentage Area Coverage (PAC) is the total particle area mm² per cm² observed surface area. 1 PAC = 10,000 ppm.
Cleaning Efficiency and Effectiveness
By cleaning a surface the concentration of all particles is reduced. In a cleanroom the majority of the particles arises from human contamination and is therefore fluorescent. The way to select and validate a cleaning process is described in ISO 14644-13:2017.
By measuring the surface cleanliness before and after cleaning the cleaning efficiency of the cleaning activity can be determined. By comparing the end result with the required cleanliness the effectiveness of the cleaning activity can be determined.
Particle Deposition Rate
The number of particles deposited on a surface depends on the Particle Deposition Rate (PDR) which can be ex-pressed in the number of particles ≥ 10 µm per m² per hour. The number of particles deposited is just like the surface cleanliness proportional to the particle size. The PDR determines the change of surface cleanliness.
The particle deposition rate PDR shows the relation between air cleanliness for all particle sizes and surface cleanliness.
The particle deposition rate of particles ≥ D µm RD is determined by the concentration CD and deposition velocity uD: RD=CD.uD or R = C.u
In line with ISO 14644-9 the particle deposition rate level or class is expressed (according to VCCN Guideline 9:2015) as 10log RD.D. Therefore RD.D is often expressed as PDR. The likelihood of particles ≥ D µm depositing on a product with critical area A during exposure of T hours is given by RD.A.T. The change of surface cleanliness ∆Cs by particle deposition rate is given by ∆Cs = R.T.
A new standard ISO 14644-17 on Particle Deposition Rate is expected in 2019.