Use of plastic materials in cleanrooms and semiconductor fab plants presents a significant fire exposure and risk of property loss. This has been known to the fire protection, property insurance and semiconductor industries for some time now. In many cases the best way to mitigate this risk is to limit the use of plastics (used in construction of wet benches, wafer stockers, polishers, spin dryers and other semiconductor tools) in these areas. However this is not always practical since some of the chemicals used in processing semiconductor chips are not compatible with other materials. The semiconductor industry is therefore supporting the development of new plastics.
Until the new UL 2360 was developed and issued in May 2000, the only test for evaluating the combustibility of plastics in cleanrooms was the Factory Mutual Research Corporation (FMRC) Test Standard 4910, FMRC Clean Room Materials – Flammability Test Protocol1. This test measures fire characteristics in a specially modified calorimeter. Fire propagation, smoke damage, and corrosion damage indices are then calculated from the measurements. It has been found that materials with a fire propagation index of less than 6 in this test are not self-propagating in the referred Parallel Panel test. As the search for better plastics escalated, numerous plastics manufacturers expressed the need to test their own products during development. They were not able to run the FM 4910 test because the nonstandard test apparatus is not readily available and the test results were not reproducible. In addition, the test is complex and expensive.
At the request of the semiconductor industry, UL and IRI teamed up to develop a simple, reproducible test using a standard apparatus. To do this, Parallel Panel and ASTM E1354 Cone Calorimeter tests were performed on eight representative plastics. The test results from these tests were correlated (see the UL report2 for more details) and used to determine the Thermal Response Parameter (TRP), Fire Propagation Index (FPI), and Smoke Damage Index (SDI) as specified in the FM 4910 test.
Two combustibility classification schemes were developed, one a prescriptive scheme and one performance-based. The prescriptive classification scheme is shown in the table below.
| UL 2360 Prescriptive Classification Scheme | ||||
Test | Property | Acceptance Criteria | ||
Class 1: Non-propagating | Class 2: Limited Propagating | Class 3: Slow Propagating | ||
ASTM E1354 | FPI | 6 or less | Parallel Panel Required | Parallel Panel Required |
SDI | 0.4 or less | 0.4 or less | <1 | |
Parallel Panel | Propagation (ft.) | 4 or less | 8 or less | 8 or less at 10 min. |
Pooling of melted material | No | No | No | |
The performance-based classification scheme lists the properties of the tested plastics for use in fire hazard assessments. The properties are determined at a radiant flux of 50 kW/m2.
Properties reported include the following:
Ignition Properties
– Critical Flux
– Thermal Response Parameter
Combustibility Properties
– Mass Loss
– Heat Release – Smoke Release
– Effective Heat of Combustion
– Specific Extinction Area
To conclude the project UL then developed the standard UL 2360 – Standard Test Method for Determining the Combustibility Characteristics of Plastics Used in Semiconductor Tool Construction (issued on May 10, 2000).
Plastic construction materials are just one of the exposures to the semiconductor industry. See NFPA 3183 and FM Global Data Sheet 7-74 for information about other fire protection concerns of this industry.
CREDITS
Excerpts from the article “UL 2360 – A New Test for Wet Bench Plastics” by Jane I. Lataille, P.E. in Fire Protection Engineering, Issue No. 12 were included in this article.
REFERENCES
1. FM 4910, FMRC Clean Room Materials – Flammability Test Protocol, Factory Mutual Research Corporation, Norwood, MA.
2. Report on the Research Investigation of the Combustibility of Plastics Used for Semiconductor Tools, Underwriters Laboratories Inc., Northbrook, IL.
3. NFPA 318, Standard for the Protection of Cleanrooms, National Fire Protection Association, Quincy, MA, 1998.
4. FM Global Property Loss Prevention Data Sheet 7-7, Semiconductor Fabrication Facilities, FM Global, Johnston, RI.