Cleanroom Ceilings, Floors and Walls

Since there is no Consumer Reports listing for the contamination control industry and if you`re building a cleanroom for the first time, chances are, you will need to do a lot of research on your own before choosing the right system. It`s up to you to do all the work. While some publications offer guidelines on material selection, there are no official suggested lists of materials to be used in cleanrooms. However, the ISO Working Group 4 “Cleanroom Design and Construction,” is currently addressing the issue.

Most consultants recommend consultation from a “cleanroom astute” architect and engineering (A&E) firm to do the research for you and pick the right materials for your space. The first thing you need to define, however, is what class of cleanroom you need. If you`re investigating materials for the first time, here are some tips.

The Top Five Qualities

If you`re undecided about whether to use aluminum honeycomb wall panels, epoxy-painted walls or 2-in. T-bar ceilings vs. the new “flush-grid” systems, consultants caution that your choice of material should not be made on any single criterion. There are some important qualities the materials you choose should have. Look for cleanability, cost, durability, flexibility, and maintainability. One good rule to follow during the selection process is: “Smooth surfaces with no rough edges,” recommends Ralph Kraft, owner of RKI Cleanroom Services (Rochester, NY), a consulting service for building and operational management of cleanrooms and cleanroom construction. With this in mind and knowing the class of room you need, you can begin to evaluate materials using the following yardsticks:

Cleanability. In the biopharmaceutical and biotechnology industry, cleanrooms usually need walls that can be easily cleaned and wiped down to prevent viral or bacterial contamination. “In the biotech market, cleanability is really a paramount concern,” says Architect Dana Watts of Symmes Maini and McKee Associates (SMMA; Cambridge, MA). Cleanroom materials need to be easily cleanable but rugged enough to dissuade bacterial buildup and stand up to strong cleaning solutions. The chemicals used for cleaning are a contributing factor in material selection. Says Technician Scott Kouri of Porta-Fab Corp., (Chesterfield, MO), “Depending on the concentration, chemicals may eat through certain materials, while other materials can withstand cleaning solutions. For example, a vinyl or fiberglass panel might not withstand a diluted HCL cleaning solution while a painted steel surface may.”

Cost. Microelectronics fab construction can cost up to $1 billion. The faster the fab is built, the sooner products can reach the market. Keeping a lid on construction costs is sometimes the greatest concern. “Cheaper and better” is the slogan for many semiconductor companies. However, knowing just how cheap you can go with material selection takes research and experience. “Money is going to be a very important factor,” says Fred Musano, Jr., vice president for Neslo Manufacturing (Wolcott, CT). “If your budget is thin, you`re not going to go with a Cadillac. The most important thing the customer is going to look at is dollars. Customers need to know what they`re going to spend.”

Cost and the “look” of the cleanroom can go hand-in-hand. A cleanroom can cost between $200 to $1,000 per ft2. Compared to $50/ft2 for space in an office environment, a cleanroom is almost 20 times the cost of a regular office. Says Gary Devloo, president of Lepco (Houston, TX): “The reason cleanrooms are so expensive is that all the air that`s moving through the room, the filtration, the ultrapurity piping, waste and exhaust systems–all those things drive up the cost of the facility. But you don`t necessarily see that when you walk into a room. When you walk into a cleanroom that costs $500 per ft2, you want it to look like it cost $500 per ft2. After spending all that money on mechanical systems, people will sometimes then put extra money into the walls and ceilings not for performance but just to make it look good.” Devloo adds that the cleanroom user who hires a turnkey designer/builder like Lepco will have a lot of say about how the cleanroom will look. “They`re trying to make the rooms look as hi-tech as possible,” says Devloo.

Durability. Obviously, cleanrooms must have durable materials to meet the requirements of the environment, the cleaning solutions, vibrations, the equipment, and the air filtration system. Lepco`s Devloo cites the chemicals used in the room as a very important criterion in material selection. “If the room is chemically intensive,” he says, “you`d put in a troweled-in epoxy resin flooring, like Stonhard for chemical resistance.”

Durability also depends on the class of cleanroom being built. For example, a sub-Class 1 room could not have a wood core wall system because the wood could cause particulate and contaminate the space. Semiconductor fabs tend to choose aluminum because of its durability. However, it`s not simply the material, but the construction of the entire wall, floor or ceiling system that must be durable for your application`s needs.

Flexibility. Some material vendors say flexibility will save money in the long-run. Flexibility means the system can be moved or changed without incurring a huge expense. However, it can also mean the ability to use cleanroom materials as the owner`s needs dictate.

Maintainability. A question that should be asked of any vendor is “are floors, walls and ceiling easy to maintain?” advises SMMA`s Watts. Cleanability and maintainability both should be priorities for material selection, he suggests.

The Materials

Ceilings. Depending on the application, the ceiling can be the most important part of the floor-wall-ceiling trio. The cleanroom ceiling contains the HEPA or ULPA filter units which make the room “clean,” as well as lighting and sprinkler heads. Also, ceiling systems may be required to support separate minienvironments or material handling equipment. SMMA`s Watts says, “If somebody had a really limited budget but a little extra money in the kitty, you might find them going to a more expensive ceiling system rather than a more expensive wall system, especially if it`s a laminar flow room.”

The most popular systems in use today range between the newer “flush grids” or conventional T-bar grid systems. Both types of ceilings are based on a support grid suspended from the building`s structural system. The grid is composed of extruded aluminum shapes or an inverted “T” cross section to support the ceiling components, which can be installed from the top. Ceiling components include the HEPA/ULPA filter units, lighting units, or may also include blank panels made out of the same materials as the walls. The advantage of this type of system is that ceiling components can be placed anywhere without serious structural alterations. The disadvantage is the potential for particle leakage emanating from the space above the suspended components.

Other considerations in choosing a ceiling component in combination with the filtration systems are light fixtures, the class of cleanroom, and whether or not your cleanroom will need a seismic bracing system. According to Pat Beatrice, cleanroom products sales manager for Farr Co. (El Segundo, CA), if your cleanroom is located in an earthquake prone area, seismic considerations are a “must.” “A 1.5-inch T-bar is fine, but it`s not considered a seismic system. A 2-inch T-bar or gel system is.” The 2-inch system is hung from turnbuckles and threaded rods. “And depending on the location, you may have to go to a higher seismic level. That means hanging every filter and light fixture and having a sound structural system.”

Floors. Choosing materials for a cleanroom floor is very dependent on the application. This is the one area that probably takes the most abuse in the form of heavy traffic from pedestrians and wheeled carts, as well as cleaning solvents and chemical spills.1 (Also, see “Evaluating Flooring for Cleanrooms,” CleanRooms, October 1995, p. 29). If the cleanroom is in a microelectronics facility, conductive or static dissipation is required via the flooring or by ionizers and by other methods.

One of the lowest cost floor coverings is vinyl tiles or sheets laid over a concrete structural floor. Brittle floor covering can cause particulates, therefore, the floor covering should be pliable but not too soft that it will tear. Also, outgassing must be considered. In his book, author Michael Kozicki says, “When you begin to see floor wear, it means that the floor is actually in an advanced stage of breaking up. The wear you see is actually due to degradation of the surface finish.”2 Once this happens, particulate generation increases. Not all cleanrooms require vinyl floor coverings. Many require perforated, solid or grated panels and/or raised access flooring. Perforated flooring may be required to maintain laminar air flow.

Walls. One of the most economical ways to construct a wall system is to use standard construction materials and then cover or coat them with a low-contamination layer. Gypsum wall board can be bonded with a thin vinyl layer, and in some cases, vinyl can be used instead of paint, which outgasses. Also, epoxy painted wall board is frequently used in Class 100,000 and above cleanrooms. Vinyl, too, has a tendency to outgas, but high quality vinyl mixed with another substance can mitigate the problem. For rooms at Class 100,000 and above, a more expensive wall system would be a wood core with a hard plastic laminate. However, wood cores can often create particulates. Stronger and safer than a wood-core wall, is an aluminum honeycombed panel system.

The more expensive wall systems are made of aluminum wall panels, which are usually produced in sheets. While aluminum forms an oxide to protect itself from the air, it is susceptible to harmful acidic vapors that can cause corrosion. To counter corrosion, the aluminum is sometimes treated with chemically stable hard coatings. Anodization–a process that deliberately thickens the oxide coating on the surface is then used to make the aluminum inert. One advantage of this process is that it chemically bonds the oxide and metal. Enamel can also be used to coat aluminum. A less expensive alternative to aluminum is sheet steel, however, steel also must be coated because it corrodes easily and cannot be anodized. If the coating becomes chipped, it could flake off. Another alternative is high-quality stainless steel that does not require a coating. But stainless steel too, generates particles.

The use of glass in cleanrooms can be of value to allow cleanroom workers visibility outside the room and add aesthetic appeal. Glass must be tempered or laminated and coated to block ultraviolet light. Says SMMA`s Watts, “Frequently, clean spaces have walls between process areas…We`ve put glass in these areas to open up one room to another and make the space feel larger–or just to achieve visual communication in order to make spaces feel less claustrophobic.”

Stick-build vs. Modular

One of the most important considerations in material selection is whether to stick-build or use prefabricated, modular construction? Do you build from the ground up or do you have major cleanroom systems constructed off-site and brought in when ready to be clipped together?