Joachim Ludwig, Colandis CEO, explains what it takes to determine the compatibility of machinery, supplies and tools for use in a cleanroom given air cleanliness required by ISO 14644
Photo for illustration purpose only
Investigations regarding the suitability of machinery, supplies and tools for use in the cleanroom tend to be a secondary issue in most companies. Non-experts would simply rely on the information provided by manufacturers. But, a detailed and individual analysis of the requirements for a clean production environment is highly recommended to avoid contamination risks and secure a continuous operation.
Cleanroom suitability has become a definite quality parameter with machine and component suppliers. The concept describes the suitability of a machine, operating utility, material, or tool for use in a room where air cleanliness and other parameters are controlled in accordance with ISO 14644.
As a quality property, it can be found in products for which their suitability must be documented. The quality seal confirms that a product complies with specific requirements regarding the emissions of a particulate or gaseous nature. However, all users should be aware that this quality seal is connected to the conditions prevailing during the cleanroom suitability test. In this regard, a question arises about the extent a mechanical product should conform to an air cleanliness class. This characteristic is not in direct relation to the product and yet very often misinterpreted in the documentation.
The German directive VDI 2083-9.1 entitled “Compatibility with required cleanliness and surface cleanliness”, explains the differences between cleanroom suitability and cleanliness suitability. A simplified definition is: cleanroom suitability describes the effect of a machine or component on the air cleanliness class of the room.
While both parameters (cleanroom and cleanliness suitability) are important, greater emphasis should be placed on cleanliness suitability.
Cleanroom suitability is used frequently without further differentiation, but its singularity plays an important role if the parameters determined are important for the facilities or production departments. It also plays a role in production, especially when issues of cross-contamination are important. Indeed, cleanroom suitability is a part of cleanliness suitability, hence both areas should always be evaluated in context without neglecting their differentiation.
Product specifications, data sheets, or brochures would state “cleanroom compatible ISO Class 4 according to ISO 14644-1” for the identification of machines or components to be used in cleanrooms. What does this imply? It indicates:
Point 1 describes classic cleanroom suitability: nowhere on the machine are more particles emitted into the surrounding cleanroom than allowed by the classification of the values resulting from the air volumetric measurements into the classification of air cleanliness of ISO Class 4. However, some questions remain open. For instance, how many of these machines can be used in the respective cleanroom?
Point 2 describes the cleanliness suitability regarding the product environment. Because, in the end, it is the product that counts and not the cleanroom; the cleanroom is only a means to an end. If you manage to keep the immediate product environment clean, then the cleanroom plays only a secondary role. The semiconductor industry, for example, has solved the aspect of cleanliness suitability in a practical way using acceptance tests.
Regarding point 3, it must be said that no machine or component can have an air cleanliness class. But no matter how this is communicated, the question of how resilient these values are is raised. Only the measurement protocol can answer this.
The measurement protocol forms the basis of each certification and it should provide information regarding the measurement conditions. These are necessary to make a comparison with your circumstances. It is not exclusively about environmental parameters (i.e. temperature, relative air humidity) but about the process itself for which cleanliness suitability tests have been carried out.
Cleanroom technology differentiates between three levels of qualification, which are based on the following conditions:
As built: construction of the cleanroom is complete. The room is empty during the measurement.
At rest: the cleanroom is equipped with all machines and facilities, the cleanroom is operational as well as machines and facilities (idle). There is no staff inside.
In operation: the cleanroom with all its machinery and facilities is operational, the staff is present and production is ongoing.
The client and the measurement service provider agree on how these conditions are applied to the cleanliness suitability tests. In most cases, the "at rest" condition is applied. That makes it clear that there can be a gap between the measurement and the actual use.
In practice, we have seen that while the classification correctly specified in the datasheet was ISO Class 1, the results of other measuring points of the tested component were withheld since they were much worse. In other cases, a re-evaluation of the values has shown that the initial parameters were taken, inadvertently, at the initialisation speed of a transfer component, thus the results at real-time process speed were considerably worse. These circumstances cause the end customer to use a different product.
So, what is the statement “Suitable according to ISO Class 4” actually mean? It simply states that, under certain conditions, in as many measuring points as possible, a particle concentration was estimated according to ISO 14644-1 to correspond to air cleanliness of ISO Class 4.
It is not the machine that corresponds to the air cleanliness class, but the measured particle concentration.
The distinction tells the user of the machine and/or component that it has been identified as a suitable product for the application. This suitability should be confirmed in a comparison of one’s own operating conditions with the protocol that needs to be added to the certificate.
There are still other important questions that remain open:
Does the VDI directive 2083-9.1 consider these types of measurements with the necessary diligence? The holistic method measures the particle emission per unit of time. The disadvantage is that the results cannot be compared to any classification as shown in ISO 14644-1.
It is not the machine that corresponds to the air cleanliness class, but the measured particle concentration
The holistic method should always be considered in connection with the local measurement method where estimations are taken at defined points. The analysis set up means that the object is measured in a testing room with clean air flow through, then particle concentration is determined at an air outlet, as well as the volume flow at the air outlet.
Multiplying both values results in the particle flow. The particle concentration (P/m3) multiplied by the volume flow (m3/s) results in the particle flow (P/s):
P/s = P/m3 x m3/s
The following example explains why the holistic method makes sense: five measuring points were agreed at a duct by two different manufacturers. The duct of brand A showed the following results at the measurement points using the local measurement method (0 – 0 – 200 – 0 ‒ 0).
According to VDI directive 2083-9.1, this means that the particles emission of this duct equals a classification to which the highest measured value, in this case 200, must be assigned.
The result for the duct of brand B is (100 – 100 – 100 – 100 – 100). According to the directive, in this case, a classification of 100 must be assigned. Although the duct of brand B emits more particles, it has a lower classification. This contradiction can be resolved using the holistic method.
A blanket statement on cleanliness is often desirable, but it does not always give the right information about the most important component in the manufacturing process: the product.
Statements on cleanliness suitability should always be made about the product and the process. Many conditions can influence the result of such measurements, including long-term performance, the stability of the manufacturing conditions and the overall environment. These issues should be factored to make a decision about the cleanliness suitability.
N.B. This article is featured in the March 2019 issue of Cleanroom Technology.