Choosing the correct product for a wet chemistry application in a cleanroom has become complicated. One must balance the chemical resistance, physical properties of the materials, and fire resistant characteristics, to satisfy the insurance underwriters and provide a safe work environment. All this must be accomplished without over-engineering, since this will add unnecessary cost to a project. In today’s budget conscious environment, you should not pay for fluoropolymer when a CPVC product will do the same job.

The choices for FM approved materials have grown to a mind boggling number. The number of choices has apparently clouded the process of choosing the correct materials for each wet application. In this article we will explore simplifying the material selection process for any wet clean room application by looking closely at the controlling parameters.

If one has recently browsed the FM Global website, you would have noticed that the list of FM 4910 approved sheet and resin products has grown to over two pages. The vast majority of these products are PVC and CPVC products-however, there are also listings for Polypropylene, PVDF, Halar® ECTFE, and anti static formulations. What do all of these products bring to the table? They help to meet the insurance and fire safety guidelines for clean rooms, yet pose a question. How do you get the right performing product for the job at hand?

Most clean room applications that involve wet processing, use a range of chemicals. In any given area, a product may come into direct contact with the chemical or it might be located in the “splash zone” where the contact would be intermittent. In addition to the individual chemistry, the process temperatures, ambient temperature (for splash zones), and the purity of the process needs to be considered. Products that will be used in constant contact with the process bath may be good candidates for using a high purity product like PVDF or Halar. In contrast, splash zone areas may require structural strength or secondary containment where a PP, PVC, or CPVC product should be considered. Also to be taken into account is how critical the process is to the final product, and if the rinse water is recycled or sent to a waste treatment system.

How To Select

To simplify the selection procedure, start by looking at the project to determine what parts require high purity. High purity processes1 are areas that demand a material that has a low extraction level, low out gassing values, and is stable in a variety of chemistries. The SEMATECH organization conducted a series of tests some time ago on materials used in wet benches to determine the “purity” of a sheet of material by defining an out gassing parameter, leaching parameter, and extraction parameter. These tests were carried out in a variety of chemical environments common to the semiconductor industry.

Looking at the materials that were tested, PVDF and Halar ECTFE were two that offered chemical stability without compromising the integrity of the solution. These two polymers offered process purity because they are pure polymer products with no fillers, modifiers, or colorants added. In addition, these products meet the FM 4910 guidelines without formula modifications. PVDF and Halar are on the higher end of the pricing spectrum, therefore they should only be used in areas where the specifications match the material performance.

Most of the other areas of a wet bench, portions of chemical distribution equipment, or a wafer cleaning station would be classified as structural, secondary containment, splash zone or “non-process critical.” For these areas, consider using some of the other products that are available - Flametec™ Polypropylene, PVC-C, PVC or CPVC. The common thread among these products is that they are compounds made up of several mixed ingredients. Each product is compounded in such a way as to enhance the fire characteristics, smoke development, or physical properties. Flametec CP7-D Polypropylene is compounded to meet the FM4910 fire and smoke classifications, where PVC and CPVC compounds are modified to reduce smoke.

In any case, by adding a filler of any kind to a polymer base material, the filler can be extracted in certain environments and pose a contamination threat in high purity areas. Therefore, it is best to use these products for the benefits they have–structural strength for wet bench cabinets and shells, auxiliary equipment, and work surfaces, etc. In contrast to PVDF and Halar the “non pure” products are more economical to use and help to control the cost of wet processing equipment.

Conclusion

With the voluminous choices that are available for materials, keep in mind the environment in which the product will be used and what parts require high purity. Select the material that fits the requirements and be aware of costs, as a less expensive material may meet your needs.

1 “High Purity” is defined by the end user and the process in a particular clean room or fab. The required process purity will be different just as the requirement for classifications of clean room are different.