BY Phil Winters, Chairman, IEST Working Group 007: Testing ULPA Filters
The Institute of Environmental Sciences and Technologies (IEST) has long been a leader in the area of cleanroom filter definitions and performance testing. IEST maintains recommended practices (RP) for the following filter-related topics:
IEST-RP-CC001.3: HEPA and ULPA Filters
IEST-RP-CC002.2: Unidirectional Flow Clean-Air Devices
IEST-RP-CC006.3: Testing Cleanrooms
IEST-RP-CC007.1: Testing ULPA Filters
IEST-RD-CC011.2: A Glossary of Terms and Definitions Relating to Contamination Control
IEST-RP-CC021.1: Testing HEPA and ULPA Filter Media
IEST-RP-CC034.1: HEPA and ULPA Filter Leak Tests
In addition, IEST Working Groups (WG) are developing recommended practices for areas such as “Design Considerations for Airborne Molecular Contamination Filtration Systems in Cleanrooms” and “Testing Fan Filter Units.”
Several of the abovementioned recommended practices have recently been revised and republished so that advances in air filter construction and testing can be included. As part of that overall upgrade to IEST’s documents, WG-CC007 is putting the finishing touches on a revision of their recommended practice to be released as IEST-RP-CC007.2. The latest draft of this effort contains new information on a wide range of topics; however, the core content of the RP remains unchanged.
When is an ULPA filter not an ULPA filter?
As markets continue to globalize, the language that we use to define our industry is changing. For instance, it can be said that a Class 5 cleanroom (per ISO 14644-1) is almost the same as a Class 100 cleanroom (per Federal Standard 209E). The two definitions are not exactly the same, but the two classifications define a performance level that is quite similar. There are also two dominant documents that define the test methods for classifying ULPA filters. These documents are IEST-RP-CC007.1 (originally published in 1992) and the European test standard entitled EN 1822 (consisting of five parts, published from 1998 to 2000). EN 1822 defines efficiency classifications that are similar to, but not the same as, the classifications defined in the IEST recommended practices. In addition, there are substantive differences in the test methods defined in these two documents. IEST-RP-CC007.2 will include an appendix that describes some of those differences and provides the user with some guidelines to use when comparing filters and test data between these two documents.
Along similar lines, our industry has begun to see the test methods defined in IEST-RP-CC007.1 applied to filters in the HEPA range of efficiencies (efficiencies from 99.97 percent to 99.999 percent). The results from such tests should be perfectly valid, but there was no accommodation for such a test within the IEST procedures. As a result, the Working Group added text to the latest version that acknowledges the validity of the methodology for filters outside of the efficiency and particle size range.
With all these changes and more, the Working Group has tried to determine how the data from different test methods and different test equipment will affect the results obtained. Starting earlier this year, the WG undertook a round-robin testing program to evaluate results obtained at various laboratories, using different test equipment and different test methods. This work is being coordinated by Air Techniques International (ATI) in their test facility in Oak Ridge, Tennessee. Filters were obtained from four different manufacturers who tested their filters prior to sending them to Oak Ridge. The filters were then tested by ATI upon receipt, using various test methods. Preliminary results from this test program were presented at the IEST’s ESTECH 2005 Conference on May 5, 2005.
The role of media advancements
Traditionally, ULPA filters have been made using wet-laid fiberglass paper media. This media has well-known properties and has been suitable for cleanroom operation for decades. However, there are newer media options that offer different characteristics and are beginning to be offered for use in cleanrooms. WG-CC007 wanted to inform users of the new RP about the advantages and disadvantages these media choices might offer.
The use of membrane media has had a major impact on semiconductor applications. Traditional fiberglass media uses a borosilicate glass that can outgas elemental boron into the cleanroom. Since boron is a dopant, this can significantly reduce yields in the semiconductor facilities. Wet-laid fiberglass media manufacturers have developed low-boron media, which can greatly reduce this effect; however, they do not eliminate it completely. One of the benefits of membrane media is that it does not contain any borosilicate glass, which eliminates the boron outgassing problem completely. There are, however, issues associated with testing membrane media filters that need to be addressed before the user can be comfortable that they understand all the issues involved. First, the most penetrating particle size (MPPS) for today’s membrane media is significantly less than 0.1 μm, when compared to a value of 0.1 to 0.25 μm for typical wet-laid fiberglass ULPA media. As a result, testing this variety of filters at the 0.1 to 0.2 μm range specified in IEST-RP-CC007.1 may underpredict the penetration of contaminant particles. Second, a membrane media is comprised of a thin layer of fibrous structure. Preliminary testing of membrane filters indicates that they can become loaded with particles quite quickly, causing premature pressure drop increases and potentially shortened filter life.
The second type of media that is beginning to show up in ULPA filters is a synthetic fibrous material that uses an electrostatic charge on the fibers to enhance filtration performance. This type of media has been in use for general HVAC filters for years, but some data is showing a gradual decrease in filter efficiency over time. This is due to charge dissipation (or masking), which is particularly evident when the filters are exposed to very small particles (less than 0.1 μm). For many applications that require ULPA filters, these very small particles comprise a major portion of the contamination that the ULPA filters are designed to capture. As a result, this charge dissipation could reduce the performance of the ULPA filters in an area of critical interest. The revised RP indicates that these issues should be carefully considered before any electrostatic media is used in critical applications.
IEST recommended practices are the result of consensus among experts in the field, and are used the world over by contamination-control professionals. To serve the industries represented by IEST, recommended practices are reviewed on a regular basis and, when necessary, updated to reflect current thinking, technological advances and best practices. For more information about joining an IEST Working Group, contact [email protected]. III
Philip Winters is the director of product development for Filtration Group, Inc. Mr. Winters holds a Bachelor’s and a Master’s degree in Engineering and is a licensed Professional Engineer (PE) within the state of Illinois. He is actively involved in Technical Committees within ASHRAE and IEST and is currently the Chair of IEST Working Group 007, “Testing ULPA Filters.” He is also IEST’s Education Vice-President and member of the IEST Executive Board.
ABOUT IEST
IEST is an international technical society of engineers, scientists and educators that serves its members and the industries they represent through education and the development of recommended practices and standards. IEST is the Secretariat for ISO Technical Committee 209, Cleanrooms and Associated Controlled Environments, charged with writing a family of international cleanroom standards. For more information, visit the IEST website at www.iest.org.