Increasing Product Quality by Particle Deposition Control

Solutions

A product or process surface can be contaminated by depositing particle or microbe carrying particles directly or indirectly by contact surfaces. The likelihood of surface contamination depends on the local particle deposition rate and the time of exposure to the environment.

A cleanroom or clean controlled environment is used to prevent unwanted surface contamination by particles. This is achieved by an air handling system that supplies and circulates clean air. Particles that cannot be removed by airflow must be removed by cleaning. Through operational procedures for the entry and work in a clean environment the introduction of particles can be limited. This will lead to a particular particle deposition rate and product contamination risk.

A particle deposition monitor determines the effectiveness of the established control solution. The data shows how much a product surface is at risk. The time of particle deposition event and the particle size distribution provide information on how to improve the contamination control measures. It predicts surface cleanliness of various surfaces in a controlled environment and the required cleaning frequency. Alternative a surface cleanliness monitor can be used.

Since human contamination is the major contribution particle deposition monitoring is most useful in clean environment where personnel is present. In those situations the contamination by particles larger than 5 micrometres is important. The impact increases with particle size.

Various industries can improve their operations using particle deposition data:

Industry	Critical particle size in µm	Vulnerable area in cm²	Time of exposure in hours
Electronics	≥ 10-100	0.01 to 1	0.5 – 5
Electromechanical	≥ 25-100	0.1 to 100	0.5 – 5
Automotive	≥ 50-500	10 to 1 000	1 – 20
Aerospace	≥ 25-250	10 to 10 000	5 – 50
Medical	≥ 20-100	1 to 100	0.5 – 2
Semicon equipment	≥ 5-100	100 to 10 000	0.5 – 5

Product contamination in a clean controlled environment

Surface cleanliness Monitoring
Particle deposition causes an increase in particle concentration on all surfaces in a cleanroom.
Particle deposition rate predicts increase of surface concentration, Unclean surfaces contaminate product in contact.
Unclean surfaces contaminate product in contact.
Activity incombination with unclean surfaces increases the particle deposition rate.
Human activity will increase particle deposition rate and by that the surface cleanliness.
Surface cleanliness on surfaces can be measured by black light inspection and particle measurement.

NO DIRECT CORRELATION BETWEEN THE CLEANROOM CLASS AND SURFACE CLEANINESS
In a cleanroom, the air cleanliness is classified for particles from ≥ 0,1 to ≥ 5 µm. These particles stay airborne and can be removed by airflow (generated by cleanroom installation). The removal efficiency of particles larger than 5 µm decreases with increasing particle size. Particles ≥ 25 µm tend to deposit depending on their size, mass, shape and momentum. Particles get momentum by air currents and turbulence from people and moving objects. Above 30 µm field data prove that there is no direct correlation between cleanroom class and the way the cleanroom is used becomes more important.
Deposited particle can only be removed by cleaning.

APMON FIELD RESULTS: 24/7, VALUABLE DATA
In many applications cleanroom users and managers are surprised by the measured particle deposition rate levels. Often these are much higher than expected. Often particles ≥ 100 µm determine the particle deposition rate level.
The real-time data show the moments of deposition. Often this happens when people and/or goods are entering the cleanroom. But also, when work is prepared or when people come together in the cleanroom.
The particle size distribution helps to decide where and what type of improvements should be made: cleaning (cleanroom, work benches, tools and equipment), people (number, garments, changing, working methods) and cleanroom installation.

MONITORING THE QUALITY OF CLEANING
By cleaning a surface the concentration of all particles is reduced. In a cleanroom the majority of the particles arises from human contamination. Most of these are 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.

Importance of particle deposition rate in contamination control

A cleanroom is a controlled environment with an installation that removes airborne particles by air flow and does not introduce particles by this air flow. Not all particles > 5 µm are removed by air flow because of deposition on all surfaces. The number of particles that are not removed because of deposition increases with particle size (at 15 air changes: 50% for particles > 10 µm, 80% for particles > 25 µm and >90 % for particles > 40 µm).

The resulting air cleanliness for all particle sizes during operation (when people are working in the cleanroom) can be monitored by measuring the concentration of airborne particles up to ≥ 5 µm. Larger particles are more difficult to measure by airborne sampling. By measuring the particle deposition of particles > 15 µm and the resulting surface cleanliness at critical locations.

Application of particle deposition rate monitoring

Particle deposition rate is the determining parameter of product contamination by macroparticles. To prevent product contamination in a controlled manner a limit for the particle deposition rate during operations can be set. Control is executed by the combination of a cleanroom installation and the operations control program. The larger the critical particles size the more important the operations control programme.

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