SINGAPORE – Choosing the right kind of gloves for a particular cleanroom application is an important first step. But if they are not used properly their effectiveness is reduced.
An important aspect of glove use is cleaning. A number of techniques to reduce particle contamination were discussed by Roger Welker, principal scientist, R.W. Welker Associates, during a presentation at CleanRooms Asia '99 in Singapore in July. The event was cosponsored by PennWell, the parent organization of this newsmagazine, and Times Publishing Group.
A simple, yet highly effective, method of reducing particles on gloves is washing them in running DI water. This technique is most suitable for flat panel display, disk drive and semiconductor applications. There's also one added benefit: washing in DI water slightly improves glove performance from a static discharge standpoint.
While washing might seem an obvious way to clean gloves, it has only come to the forefront in the past year or so. In older cleanrooms, lack of utilties (eg, water supply and drainage) and shortage of space has meant the technique could not be adopted. And until recently there has been little data to prove the point. “When you think about it, its obvious glove washing will help. By how much? Well, you have to measure it to find out,” explains Welker.
In the mid-1980s glove washing experiments were conducted at IBM. During the early 1990s the technique was “discovered” in a leading Taiwan wafer fabrication facility, and thereafter spread to other facilities on the island. But in general, Welker says the practice is not widespread.
More recent data was presented based on tests with nitrile gloves, which have become the material of choice for the disk drive industry due to the widespread use of ESD sensitive MR heads.
The tests showed that washing of nitrile gloves is even more effective. Gloves as received into the cleanroom typically carried 1000 to 4000 submicron particles per square centimeter. After washing, the particle count dropped 25 to 100 times to about 40 per square centimeter.
For ionic contamination, the as-received levels of 1 to 4 micrograms per square centimeter were reduced 250 to 1,000 times to about 4 nanograms per square centimeter after washing.
After washing comes drying, which presents a problem. Rather than using a fan dryer that can take 1 minute to dry the gloves, Welker recommends using a typical cleanroon wiper with a bonded edge. Any further residual moisture can be removed by placing the hands under the air shower for a few seconds.
Although at $0.50 a piece wipers might seem an expensive way to dry gloves, Welker said the cost per use drops to less than 2 cents if the wipers are laundered and re-used more than 50 times.
Another glove cleaning technique discussed by Welker was the use of sticky mat in-situ cleaning, which offers several advantages. By touching the gloves on the mat periodically, users can reduce contact transferable contamination. Tests conducted by Welker at IBM in San Jose in the mid-1980s show this technique is 100 percent effective for larger sized particles.
In addition, Welker “accidently” discovered another use for sticky mats. Pin holes in gloves were identified as a problem several years ago, but operators were not able to tell when the holes formed. However, it was found that gloves with pinholes tear on a sticky mat. However, for those industries using more durable nitrile gloves, pin holes are no longer a problem.
Craig Addison