Making Informed Choices in Wet Bench Fire Safety
As the costs and consequences of failure grow, so does the list of fire-safe, approved materials.
In the precision manufacturing and process industries, the term “wet bench” generally refers to cleanroom process equipment that contains, dispenses, rinses, or in some manner processes or utilizes corrosive chemicals. The sheet plastics traditionally used in the construction of wet benches, polypropylene or polyvinyl chloride (PVC), provided good construction properties and corrosion resistance along with relative ease of fabrication and welding. These early materials, however, did not offer a high level of fire or flame resistance. The introduction of fire retardant polypropylene in the mid-1990s improved the flame resistance of wet bench materials.
Until recently, the industry relied on UL94-V0 and V5a and ASTM E-84 standards to evaluate the performance of plastics when exposed to flame. UL94 measures the flammability of plastics used as components in appliances and other devices. E-84, also known as the Steiner Tunnel Test, measures the tendency of building materials to spread flame and produce smoke. None of these standards addressed specific concerns about plastics used in cleanrooms.
Risk underwriters and insurers, facing financial losses from major fires in wafer fabrication plants, demanded a reduction in fire risk from their insureds. Wet bench plastics were identified as culprits in accelerating flame spread and worsening damage to work in process and equipment from excessive smoke. In 1997 Factory Mutual Research Corporation (FMRC) released a new standard to measure the performance of plastics in these areas. This standard, titled Cleanroom Material Flammability Test Protocol 4910, references a fire propagation index (FPI) to measure a plastic’s fire propagation potential and cites a smoke damage index (SDI). Together, these indices are used to evaluate a plastic’s suitability for cleanroom use.
Using the FM4910 standard, plastic sheet manufacturers began to develop and modify sheet formulations to meet it. Submitted products were tested by FMRC. Successful candidates were given official listings as compliant with FM4910. In 2000 Underwriters Laboratories (UL) released |L2360, Test Methods for Determining the Combustibility Characteristics of Plastics Used in Semi-Conductor Tool Construction. This protocol also measures the fire propagation and smoke generation qualities of wet bench plastics. In addition, it provides a class rating of 1, 2, or 3, depending on the material’s FPI and SDI levels. Manufacturers have submitted materials to UL for testing and several sheets and resins have been UL2360 listed.
Spreading the News
With more options for wet bench construction, material selection has become problematic. For those buyers/specifiers whose insurers favored a protocol (FM or UL), their choices are limited to the listed fire-safe materials. In other cases, the end user has mandated fire-safe plastic use. In some municipalities the authority having jurisdiction (AHJ) has stipulated fire-safe plastic use or compliance with a protocol or industry standard. Yet there are also those situations in which no clear-cut directives are in place for the buyer/specifier that would limit their choices. Wet benches made from polypropylene and FRPP are still being built and installed in retrofitted and newly constructed fabs.
Dissemination of information about fire-safe plastics has been a slow process. Industry associations such as SEMI, Semiconductor Industry Association (SIA), Semiconductor Safety Association, National Fire Protection Association and others have reviewed the FM4910 and UL2360 protocols. In some cases they have endorsed them or included them in their own standards.
Adoption of FM4910 and UL2360 into building codes is slowly under way. Fire and building approval professionals learn about new materials through official documentation, their membership in relevant organizations, their industry contacts, the reading of published data and sales materials, and through their personal research.
The promotion by FM and UL of their own standards has become a primary source of information. The publishing and announcement of their protocols are important methods for tooling buyers, specifiers, builders, AHJs and interested third parties.
Choosing the Right Material
At best this objective approach provides the designer/builder with merely a list of approved materials. Still to be resolved is how to determine which listed materials are best suited for the wet bench in question. Three significant factors must be considered to help narrow down the choices:
* Functionality. The types of process chemistries which will come in contact with the wet bench must be examined to ensure that the appropriate fire-safe plastics are used. In some cases, tooling models offered by manufacturers may be used with a variety of chemistries. Thorough research will ensure that the material selected is compatible and rugged enough for all intended applications. The average workload of the installed bench may also affect the decision of which sheet material to choose. The conditions in which the bench will be located are a key factor. A hostile manufacturing environment may require a more durable, corrosion-resistant cabinet than if the only process chemistries contact were in the wetted areas. A trend toward hybrid tools has developed among equipment designers. In such units the shell is constructed from a fire-safe but less expensive cabinet-grade material.
* Budget. Opinions vary about the relative cost of wet bench plastics. In the overall cost of a tool the plastic sheet content may represent only 15-30% of the total expense. This is not to say that the comparative increase from one fire-safe material to another is not relevant or important, however. When multiple benches are constructed, significant savings may be realized when materials are selected according to performance requirements. The hybrid bench built from cabinet-grade materials (outer shell) and process chemical-grade materials (wetted areas) can be both cost-effective and well suited to the cleanroom environment.
* Builder Selection. The importance of which tool builder is selected cannot be minimized. Experience, technical expertise, reputation, and quality are important considerations. Judgment based on hands-on knowledge of fire-safe materials is invaluable. A savvy fabricator can save money, time and labor and build in tool longevity by helping select the best plastics for the job.
There are many ways specifiers, designers, builders, AHJs, or third parties with approval responsibilities, can find up-to-date fire-safe plastics information in trade journals, at trade shows, and on vendor websites. If one were to ask wet bench fabricators, specifiers, buyers, and especially plastic sheet manufacturers, which information source has the greatest impact on fire-safe product selection, it would be the FM and UL websites. Proof that a resin, compound or sheet is indeed fire-safe must be documented by a listing on the respective websites. Until the product is posted on the FM or UL site, even with the written test and listing results in the manufacturer’s hands, the industry is reluctant to accept it.
The FM Global website, www.fmglobal.com, encompasses more than 1000 pages with information about its standards, insurance services, and its resource center. While its main function is to help its clients prevent and control property loss, it also provides information for customers, including manufacturers of listed products. A downloadable listing of FM4910 materials (40, as of this writing) is included the Sidebar.
The UL website, www.ul.com, unlike the FM Global site, does not offer a unified posting of all UL2360-listed products, but instead offers search capability by company name and product number.
It is clear that the web has become a viable, vital research, sales, and teaching tool. An internet presence is now a necessity; since 1995, when many companies were either starting their websites or merely thinking about the idea, the number of sites has exploded exponentially. The ways we use the web now to make decisions about fire-safe plastics, for example, have changed how we do business and on which experts and data sources users rely.
For Further Reading
FM4910: Cleanroom Materials Flammability Test Protocol, September 1997, Factory Mutual Research, 1151 Boston Providence Turnpike, Norwood MA.
NFPA318, Standard for Protection of Cleanrooms, 2000 Editions, National Fire Protection Association, 1 Batterymarch Park, Quincy MA.
“Process Compatibility Parameters for Wet Bench Plastic Materials,” International Sematech, 2706 Montopolis Drive, Austin, TX.
SEMI S2-0200, Environmental, Health & Safety Guideline for Semiconductor Manufacturing Equipment, Semiconductor Equipment and Materials International, 805 E. Middlefield Road, Mountain View, CA.
UL2360: Test Methods for Determining the Combustibility of Plastics Used in Semi-Conductor Tool Construction, Underwriters Laboratories, Inc., 333 Pfingsten Road, Northbrook IL.
“Wet Bench Fire Safety: Update,” A2C2, February 2000, pp 17-19.
“Wet Bench Fire Safety: One Issue Fizzles, New Ones Ignite,” A2C2 February 2001, pp 19-26.