An essential examination of FM Approvals' testing criteria for cleanroom materials and fume and smoke-exhaust systems used in cleanrooms

By Vinnie DeGiorgio & George Smith

In the mid-1990s, several semiconductor companies had fire losses totaling nearly $750 million. As a result, both the semiconductor industry and insurance companies realized the need to properly manage fire risk in this critical industry.

Because chips are susceptible to even the tiniest speck of dust, cleanrooms historically have been difficult yet critical areas to protect from fires. In the past, cleanrooms and wet benches often needed protection by sprinklers or more expensive special fire-protection systems like carbon dioxide, fine-water spray or halon.

But by the time a cleanroom fire propagated and triggered a sprinkler or special fire-protection system, damage could already have occurred. Contamination from a fire, no matter how small, could potentially put a chipmaker out of business for weeks, if not permanently.

As a result, commercial and industrial property insurer FM Global analyzed the causes of these losses-combustible tool construction and ducting, contamination, and water damage-and came away with a better understanding of the hazards found in a semiconductor facility.

In turn, FM Global developed engineering guidelines (FM Global Property Loss Prevention Data Sheet 7-7, Semiconductor Fabrication Facilities), established a dedicated group of semiconductor engineers within the organization to help clients understand the hazards, and had its product-testing business unit (FM Approvals) develop technology that determines which cleanroom materials and products can best reduce property loss in the event of fire.

FM4910 Flammability Test Protocol

FM Approvals' specification test standard, known as FM4910, is a test procedure that evaluates the fire hazards of engineered plastic materials designed for use in the manufacture of cleanroom products. Materials that meet the “Clean Room Materials Flammability Test Protocol” do not require, in and of themselves, fixed fire protection when used according to the appropriate FM Global data sheets.

FM4910-listed materials (see sidebar, page 24) may burn locally in the ignition area, but will not propagate a fire beyond that zone. Additionally, such materials produce little, if any, smoke or corrosive byproducts, thus minimizing nonthermal damage.

In its latest guidelines geared to reduce fire hazards within chip fabs, FM Approvals has issued Approval Standard 4911, “Wafer Carriers for Use in Cleanrooms,” which provides manufacturers and end users with parameters to earn certification from the nationally recognized testing laboratory. Here, a stocker-simulation apparatus was used to perform the new FM test.Click here to enlarge image

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It's important to note that the listed materials are not necessarily end-use products. (Note: FM Approvals “lists” materials that meet its specification test standards and “approves” products that meet its Approvals standards.) Some materials may be a single component in a factory-produced finished product used in cleanrooms. Other products may be FM Approved, if they meet the requirements of an FM Approval Standard like FM4922 (see below), in addition to the requirements of FM4910.

Semiconductor insurance companies and code officials specify that exhaust duct systems meet the FM4922 Approval Standard. Fab-Tech's Model Perma Shield Pipe (above) is one of the fume and/or smoke-exhaust duct systems that have been FM-Approved for use in cleanrooms as of May.Click here to enlarge image

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The following indexes are determined for the material tested, based on the results of a small-scale test. These include:

• Fire Propagation Index (FPI): This index is based on the fire propagation tests conducted and represents the rate at which the surface of the material is involved in a fire. Non-propagating materials have FPI values at or below 6.0 (m/s 1/2 ) / (kW/m)2/3.
• Smoke Damage Index (SDI): This index is defined as the product of the FPI index and the yield of smoke for a given material. SDI is an indicator of the smoke contamination of the environment expected during fire propagation. Materials expected to limit smoke contamination have an SDI of 0.4 [(m/s 1/2 ) (g/g)(kW/m)2/3 ] or less.

If the results of small-scale tests indicate borderline results with regard to FPI and SDI values, a medium-scale fire test known as the parallel panel test is then conducted.

FM4922 Approval Standard

Many semiconductor manufacturers, insurance companies and code officials also specify that exhaust duct systems should meet the requirements of the FM4922 Approval Standard for Fume and Smoke Exhaust Ducts Systems.

This standard outlines FM Approval requirements for ducts used in exhausting non-combustible chemical fumes and corrosive vapors or non-combustible chemical fumes, corrosive vapors and smoke in fire situations.

Click here to view the FM 4910 listed materials and FM4922 approved products for use in cleanrooms.

For a duct to qualify as FM4922 Approved, it must demonstrate its ability to limit fire spread on the inner and outer surface, retain its structural integrity, and continue to exhaust smoke in the event of fire.

Not all FM4922 Approved ducts are FM Approved, however, for use in a cleanroom. Those FM4922 ducts that are approved for use in a cleanroom also have had their exterior surfaces successfully tested according to FM4910, and meet its criteria for fire propagation, smoke damage and corrosion damage.

While the FM4910 criteria involves one test procedure, the FM4922 criteria involves a series of tests that include a horizontal-duct fire test, a test for smoke removal ability, and a horizontal/vertical-duct combination fire test. For ducts not intended for smoke removal in a fire situation (i.e., fume exhaust only), the exhausting capability of the duct is not evaluated.

Horizontal-Duct Fire Test

In the Horizontal-Duct Fire Test, the duct is exposed to a 1 square-foot heptane pan fire with a heat output of 10,000 Btu/min (176 kW) for 15 minutes. During this period, the induced draft velocity of 600 ft./min draws the heptane flame into the inlet end of the duct.

To pass this test, all of the following conditions must be met for the entire 15-minute test:

1. Flaming must not spread on the interior of the duct from the fire-exposed end to the 23-foot point.
2. The interior duct temperature recorded at the 23-foot point must not exceed 1,000 degrees F.
3. The duct and all field-applied joints must maintain their structural integrity without developing any through openings.
4. There must be no flaming on the exterior of the duct due to autoignition or temperature transmission from the interior of the duct. If flaming appears on the exterior surface as a result of the external fire exposure, the flaming must be intermittent in nature, and must not propagate beyond the first field joint located 4 feet from the fire exposure.
5. No portion of the duct or joint material may fall, drip or melt off the test sample.
6. No smoke or other particulates shall be emitted from the inlet end or from the exterior surface of the duct.

Smoke Removal Ability

The test for Smoke Removal Ability is performed immediately after the completion of the Horizontal-Duct Fire Test and on the same ductwork.

In the test for Smoke Removal Ability, the fire exposure source is removed and the fan speed is increased to induce an air velocity of 2,000 ft./min; that air velocity is maintained for 10 minutes. Performance for Smoke Removal Ability is considered satisfactory if all of the following conditions are met:

1. There must be no reduction in the cross-sectional area of the duct;
2. There must be no interruption or reduction of air movement through the duct;
3. No smoke may be emitted from the inlet end or from the exterior surface of the duct;
4. The duct must conform to all performance criteria specified in the Horizontal-Duct Fire Test.

Horizontal/Vertical-Duct Combination

Finally, the duct must pass the criteria of the Horizontal/Vertical-Duct Combination Fire Test, which is performed on a new section of ductwork. This test is similar to the Horizontal-Duct Fire Test, except that there is a 90-degree downward elbow in the ductwork after the horizontal test section, which then connects to a vertical section of ductwork. The heptane pan fire is placed at the bottom of the vertical section of ductwork.

The FM4922 standard is not intended to determine the suitability for all end-use conditions of a product. Conditions under which a duct assembly is used vary widely.

For example, ducts may be subjected to environments not represented by standard corrosion tests. This standard, therefore, does not evaluate the corrosion resistance in every environment where duct materials are used for handling corrosive fumes.

It is the manufacturer's responsibility to determine the duct system's suitability for specific corrosive environments. The manufacturer should be consulted for recommended corrosive fume applications. Further, it is required that the manufacturer comply with all requirements of the Accepted Industry Practice for Industrial Duct Construction, published by the Sheet Metal and Air Conditioning Contractors National Association, Inc. (SMACNA;www.smacna.org).

For general limitations and restrictions of FM4910-listed materials and FM4922-Approved products, see the individual product listing in the Approval Guide, a publication of FM Approvals. To order the Approval Guide and Approval Standards or FM Global data sheet 7-7, call FM Global customer services at (781) 255-6681, or visit www.fmglobal.com/store

Vinnie DeGiorgio is semiconductor industry leader, FM Global. He can be reached at [email protected]; George Smith, PE, is manager of FM Approvals' materials group. He is a member of ASTM Committee EO5 on Fire Tests, EO6 on Buildings and DO8 for roofing and Waterproofing, and is chairman of ASTM DO8.23 on Laboratory Accreditation of Roof Testing Laboratories. He can be reached at: [email protected]