The do`s and don`ts of cleanroom swabs and wipes
By John Haystead
Swabs and wipes are essential for cleaning surfaces in a cleanroom. This is what they do, and it is their ability to do it effectively that gets them into the environment in the first place. Yet, for critical swab and wipe applications, the most important criteria is often not so much what they do, but what they don`t do.
In the semiconductor and disk drive industries, what they shouldn`t do is leave particulate, fibers or non-volatile residue. They also shouldn`t conduct electricity. In the pharmaceutical and medical device industries, they shouldn`t introduce living organisms or pyrogens. And for all users, what they shouldn`t do, is cost a lot.
To meet these increasingly demanding, often conflicting, requirements, cleanroom swab and wipe manufacturers are continually identifying and evaluating new swab and wipe fabrics and materials, as well as working to improve the efficiency, quality and cleanliness of their production processes. The questions from customers, however, remain the same: “Who`s doing the best, and is the best good enough?”
Tipping the scale
As observed by Dr. Mo Tazi, director of research and development for Coventry Clean Room Products (Kennesaw, GA), “ever-increasing miniaturization requirements throughout the electronics industry are at the same time leading to new requirements for cleaning.” And, Jack McBride, president of Contec Inc. (Spartanburg, SC), points out that, “while the products themselves will not change drastically, the scale against which they are measured will.”
It is this scale that remains elusive for both manufacturers and users. As pointed out by Andrew Marsh, vice president of sales and marketing for Berkshire Corp. (Great Barrington, MA), “cleanroom science is still very much a black art, and many product claims are difficult to substantiate when not everyone is on the same page when it comes to standards.”
While most manufacturers conduct extensive in-house and third-party testing of their products — and will make this data available — testing procedures are not standardized throughout the industry, nor are there established product performance standards for specific environments or applications. In fact, says Marsh, “When you`re measuring particulate microcontamination at 0.2 microns and ions at part-per-billion levels, there will be inherent variance in the contamination characteristics of individual items from the same vendor. Guaranteeing consistency within upper and lower control limits is increasingly critical.”
Likewise, while ISO 9000 programs can be an important indicator, users can`t assume that they constitute a defacto guarantee of high quality product. ISO documentation measures consistency of performance, not necessarily the highest performance. Quality levels can still vary from vendor to vendor, and in the end, suppliers rely largely on the sophistication and consistency of their own manufacturing, quality-control and testing practices — and their reputations — to win and maintain business.
What this boils down to is that the job of evaluating the cleanliness and effectiveness of new swab and wipe products ultimately falls on the user. It also means, however, that a large part of the process can be accomplished by evaluating the manufacturer.
Because the major swab and wipe suppliers do not themselves manufacture materials, they must rely largely on the development efforts of textile and foam suppliers. Given the relatively small size of the market, however, cleanroom applications do not get the lion`s share of research attention from these companies. Says Berkshire`s Marsh, “To date, there are really no fabrics that have been developed specifically for contamination-control applications.”
Instead, wipe manufacturers largely meet critical cleanroom requirements through value-added processing of conventional materials in controlled environments, principally, proprietary laundering processes. As Howard Siegerman, senior marketing manager for The Texwipe Co. (Upper Saddle River, NJ), describes, “Our real valued-added is in the laundering and packaging process. To provide the required level of cleanliness, you not only have to remove particulates but also emulsifiers, oils, ions and other possible contaminants.”
Water purity is a critical element of the laundering process, and several manufacturers place great emphasis on ensuring the cleanliness of their process water. Berkshire, for instance, uses highly-filtered water for its initial laundering processes, followed by 18-megohm semiconductor-grade DI water.
Similarly, Contec has transitioned from the use of contract laundry services to its own vertically integrated laundering and processing facility. Says McBride, “In particular, we invested heavily in our laundry`s water system to ensure low ion and extractable levels.” Contec uses Reverse Osmosis filtered water for its initial cycles and 18-megohm DI water for rinses. “High purity water not only controls the introduction of contamination and provides greater product consistency from lot to lot, but also generates greater synergistic effects and reactivity with surfactants,” says McBride.
Although most swab manufacturers wash their fabric material before assembling the products, Coventry`s Director of New Product Development, David Lance, points out that swabs may still have residue or particles generated by the manufacturing process. To address this, some companies post-process their swabs using different cleaning solutions. For example, Coventry uses an “Aqua-Prime” aqueous cleaning process on its polyester head swabs to reduce particulates and ion contamination and a “C-Prime” chemical process to lower NVR on its foam-head materials.
Texwipe`s Director of Marketing, Rob Linke, says automation is another critical factor in swab manufacture, providing both better cleanliness and economy. “Swab sizes are getting smaller each year, and this is difficult to accommodate if you don`t have a highly-automated production line in a controlled environment.”
Testing and documentation
Product testing is a major value-added service that should be provided by all cleanroom swab and wipe manufacturers, providing users with an important tool for product evaluation and comparison. Leading manufacturers either conduct in-house testing or use third-party testing services. As pointed out by Deborah Leonard, vice president of sales and marketing for Micronova Manufacturing Inc. (Torrance, CA), in addition to testing cleanliness levels, “a wide range of performance characteristics is tested, such as tensile strength, absorbency, temperature range, and chemical compatibility.”
Unfortunately, however, there are currently inadequate industry standards for swab and wipe testing procedures and no standards for acceptance criteria.
Contec`s Director of R&D, Dave Nobile, is also Chairman of the Institute of Environmental Sciences working group currently developing standard test methods for cleanroom swabs. According to Nobile, the new standard is expected to be published later this year, and will include particulation and residue testing. While the test standards will certainly provide valuable guidance for users and suppliers alike, Nobile acknowledges that product cleanliness criteria (acceptable levels of particulate or residue) will still not be available, leaving product evaluation largely to the user`s own functional evaluation process.
Regardless of the value-added processing added by the supplier, understanding the basic characteristics of raw materials is critical to selecting the right product for the job. Parameters include particulates, absorbency, solvent tolerance, abrasion resistance, ESD, ion content, non-volatile residue and economy. Though not written in stone, and variable with new technologies and processes, these are generally accepted guidelines for material/application suitability.
Where high-capacity (absorbency) wipers are required, and where microcontamination control is not as stringent, latex and polyurethane foams are often used. While not recommended for micro-critical environments, these materials are useful for applying cleaning solutions to large surface areas and are used extensively in the pharmaceutical industry.
Other materials can also be used at these non-critical levels. Lym-Tech Scientific (Chicopee, MA), for example, has a polycellulose wipe (60 percent polyester, 40 percent cellulose) targeted for cleanroom construction applications. Contec`s TaxFre wipe is a non-woven wiper treated with an acrylic-based adhesive that works like a tack cloth without transferring adhesive to surfaces. Says McBride, “While certainly not appropriate for critical areas, it can help lower overall solvent utilization and gross contaminant levels outside the fab.”
In semiconductor applications, requirements can be generally divided into three broad categories. Controlled areas (Class 1,000 to 10,000 and above), Subcritical (Class 100 to Class 1,000), and Critical (Class 100 to Class 1 and below). The semiconductor industry is also particularly concerned about ion content. Metal ions such as sodium and potassium are killers for semiconductor wafers, and chloride ions can be a corrosion problem for metal gas and chemical delivery lines. According to Texwipe`s Siegerman, “The stringency of cleaning protocols varies significantly between semiconductor facilities, but in general, cleanliness requirements ramp up quickly as you move into the operational areas.” Most experts agree that cellulose (cotton/wood/hemp) is to be avoided at all critical levels of semiconductor manufacture. Although cotton has good heat-resistance properties compared to polyester or blended fabrics, and is also good for mild-abrasive applications, there is a high penalty to pay in particle and fiber contamination levels. Nylon, on the other hand, remains in fairly wide use because of its low particle and fiber characteristics. Nylon, however, suffers from higher non-volatile residue (NVR) values compared to polyester. Many foams and non-woven fabrics are suitable for use in controlled areas. As opposed to woven fabrics whose fibers have a regular geometric pattern, non-woven, “spun-lace” fabrics are made through a hydro-entangling process using fine jets of water to tangle the fibers together in a random orientation. Although there is no additional laundering done on these products, the hydro-entangling process provides a level of cleaning action. According to Siegerman, “hydro-entangled wipes will never be as clean as a laundered polyester product, and woven fabrics usually provide greater tear strength, but they are less costly than woven wipes and some fabrics are surprisingly strong, even when wet.” DuPont Nonwoven`s (Wilmington, DE) “Sontara CritiClean
Reusable wipe products are appealing to companies trying to minimize their waste and disposal costs. Like garments, users can either launder the wipes themselves or send them out to a cleanroom laundry service. Coventry offers a reusable polyester/nylon wipe called the “Chamois,” which, according to Tazi, is suitable for Class 1 cleanrooms. Another reusable wipe material is polyvinyl alcohol (PVA). According to Micronova`s Leonard, users are moving away from the use of Isopropyl Alcohol (IPA) cleaning products in Class 1 and Class 10 environments, opening the way for reusable wipes made of non-woven PVA. Leonard points out that since IPA is used as an evaporating agent, “if you remove it from the equation, you need something else to guarantee a dry surface, i.e. high absorbency.” Since PVA is substantially more absorbent than polyester, Leonard says its higher cost can be balanced against the number of wipes required to do the job. Still, Leonard acknowledges that where large amounts of alcohol (30 percent) or acetone are being used, PVA would not work well. According to Leonard, the company`s PVA-based “Nova” sponges and wipes are suitable for Class 10 environments.Berkshire`s Marsh says the use of IPA remains a question of what is the best solvent to get the job done. “While IPA isn`t necessarily the best solvent to remove some materials, the industry will continue to use it until something better comes along.”
Presaturated wipers are becoming increasingly popular in the electronics industry because of convenience, safety and environmental issues. Material considerations are also important for pre-saturated swabs and wipes. According to DuPont`s de Jong, “There are significant differences in requirements between pre-saturated wipe materials and dry wipe materials. For example, with wet wipes, since you don`t have to worry as much about the rate of absorption, polyester materials are useful, while dry wipes are more oriented toward absorbency and so tend toward cellulose and rayon-type products. In addition, optimum materials vary according to the kind of solution being used. Aqueous solutions, for example, will differ from other solvents in terms of substrate absorbency. Also, since the longer the material sits in a chemical solvent, the greater will be the risk of extraction, it`s also important to use materials that will not leach over time.”
The disk drive industry is currently the largest industrial user of cleanroom swabs, with most cleanrooms operating at Class 100 with some areas to Class 10. Swabs are typically used at the head-gimbal-assembly (HGA) level, and since industry practice is to use one swab per cleaning pass, a very high volume of swabs is needed. Texwipe`s Linke sees the disk drive industry rapidly catching up to semiconductor levels in terms of contamination-control requirements and sophistication. While Contec`s Nobile agrees, he also observes that cotton swabs are still being used to a large extent. Tim Templet, sales manager for Hardwood Products Commercial Products Division (Gilford, ME) explains. “Somewhere in their process, the use of cotton is justified, such as picking up a fiber from a disk drive. Where a polyester swab might cost them $60 to $80/1,000 vs. 10,000 cotton swabs, users instead buy high-quality, wooden-handled cotton swabs, put them in a double polybag and take them into the cleanroom.” Even so, Templet acknowledges that cotton swabs aren`t appropriate for completely-contamination-free applications, “which is why people are moving to polyester in those instances. For disk-drive manufacturers,” Templet says, “the important swab characteristics are handle rigidity and low-lint. Today, it`s relatively easy to produce a lint-free cotton swab, but the wooden handle remains an issue.” Though wooden handles provide rigidity, they also introduce contamination which must be addressed through non-contaminating coatings that need to also be impervious to solvents such as acetone. Ultimately, says Contec`s Nobile, “The challenge is to come up with materials that can effectively clean surfaces of residues and particles without abrasion.” While polyester swabs are being used increasingly for these applications, foam swabs are also an option, although not as clean as polyester in terms of particles, ions or non-volatile residue. Chemical-resistant swabs can be made for use with
In electronics manufacture, and particularly disk drives, the ESD properties of both swabs and wipes are becoming increasingly important. According to Coventry`s Tazi, “The triboelectric effect is much greater today than it was 5 to 10 years ago, and conventional swabs may no longer be usable in 2 to 3 years for these applications.” Tazi observes that most companies are transitioning to magnetoresistance (MR) technology for their swabs using either conductive or static-dissipative handles made from specific polymer materials. Since swabs are generally used wet, the ESD characteristics of the tip material are not as critical as the handle material. While dealing with the ESD issues, however, manufacturers must also be concerned with ensuring that their handle materials are compatible with the solvents commonly used in the cleanroom such as acetone, HFCs and alcohol. Some handle materials will rapidly deteriorate in these liquids. Texwipe`s Linke says they prefer to use static-dissipative vs. conductive handle materials for their ESD-safe swabs. “Over the next six months, drive makers will be pushing to control charges to as low as 5V, and conductive handles can easily transfer that level of charge,” says Linke. In fact, to reach its goal, Linke says the industry will have to take a system-level view of the entire manufacturing process ensuring that everything in the workstation area is both grounded and made of static-dissipative materials. Since wipes are generally used wet, until recently, they have not had major issues in terms of static-dissipation. Now, however, increasingly stringent ESD concerns are calling attention to these products as well. According to Texwipe`s Siegerman, static-dissipative wipers are increasingly important to the disk drive industry as well as for back-end semiconductor processing after devices have been fully formed and wired. Texwipe`s “ESD-wipe” uses either a nylon or polyester substrate material and a proprietary treatment process. A
Pharmaceuticals and medical devices
In the pharmaceutical and medical device industries, live microbials (5 to 10 microns) are the principal concern, as opposed to microparticles. Therefore, the use of sterile swab and wipe products either via gamma radiation or autoclaving is usually mandated for aseptic areas. Heavy metals and pyrogens must also be dealt with, and as Contec`s McBride points out, the pharmaceutical and medical device industries are also usually dealing with water-based solvents that don`t absorb well in standard polyester materials. Hardwood Products` Templet says foam swabs are used heavily in the medical diagnostic industry, “which is becoming increasingly sophisticated in terms of its contamination requirements.” Hardwood Products makes swabs using a non-particulating polyurethane foam. Of the two types of foam used, reticulated and non-reticulated, reticulated foam is more absorbent since it is blown open after being extruded to create cavities. According to Lym-Tech Vice President Peter Hogan, presaturated wipes are also becoming increasingly popular in both the pharmaceutical and medical-device industries. The company has a new “Lym-Sat” presaturated polypropylene wipe product with 70 percent IPA and 30 percent DI water in a resealable pouch. Contec also offers a number of presaturated wipe disinfectant products including phenols, quaternary ammonium compounds, and alcohol-based wipes. Veltek Associates Inc.`s (Exton, PA) “Alcoh-wipe” is an individually-packaged, pre-saturated, sealed-edge polyester wipe. Art Vellutato, Veltek vice president of sales and marketing, advises that wipers used in aseptic environments should only be used once, and warns that bulk-packaged wipes can cause problems as well. “There`s no such thing as a sterile room, and you can actually grow bacillus spores using isopropyl alcohol,” says Vellutato. Vellutato also sees particles of increasing concern to the pharmaceutical industry. “Particle counts can be as important as actual organisms, since they nee
Back to the source
Despite the sophistication of their post-processing, treatment, packaging, and testing techniques, swab and wipe manufacturers frequently find themselves frustrated with the capabilities of the raw materials they have to start with. For example, says Lym-Tech`s Hogan, “Knitted fabrics are expensive and we`re always looking for an economical non-woven product that can be used in a Class 10 cleanroom. Though we`ve tested product from a number of mills, the results are usually disappointing.” This situation may be changing somewhat, however. According to Contec`s McBride, “As the cleanroom market has grown, we`ve begun to see more interest from the fabric manufacturers and more emphasis on product development and process control for cleanroom applications.” In particular, DuPont Nonwovens is frequently mentioned as a possible source of advanced cleanroom fabrics. According to de Jong, this is because “we already view the cleanrooms market as significant in terms of the application of leading-edge technology, and it will be even more so as it broadens into other critical manufacturing areas.”
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When producing their wares, cleanroom swab and wipe manufacturers must juggle the needs of the disk drive, pharmaceutical, and semiconductor industries.
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Berkshire`s Microseal edge is smooth as opposed to the edge of a standard non-sealed polyester.
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Micronova`s PVA-based Nova wipes are suitable for Class 10 environments.
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Texwipe`s ScourWipe is among the latest developments in cleanroom wipers.
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Berkshire is among those in industry racing to develop smaller, more tightly contoured swabs.
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Coventry swabs are cleaned by an aqueous cleaning process to reduce particulates.