The integrity of many products built in a cleanroom is compromised by electrostatic discharge (ESD). Semiconductors, disc drive, and flat panel manufacturers are particularly at risk when static electricity attracts contaminates. An effective ESD control program is multi-faceted and continually improving. There are many elements in an ESD program; here we will focus on the basics of training and garments.
The major generators of static electricity are people and machinery in the cleanroom or controlled environment. Cleanroom workers and engineers inadvertently promote static electricity because they do not know what causes ESD or why it is bad. According to Jan Eudy in her “Testing ESD Garments” article in Controlled Environments, engineers are often the worst offenders when it comes to ESD.
Therefore, no ESD program will be successful without separate training courses for every level of personnel affecting ESD. This includes engineers, operators, and managers. Each group needs to know specific ESD protocol for their jobs and why it exists. When people are educated on an issue, they both care more about it and are more compliant.
Remember that if training veteran employees, old habits are heard to break. People carry out actions without even thinking about them, so it is wise to have people look out for one another. For example, a worker may be sitting in a chair with his feet tucked back to rest on the chair rather than on the grounded floor.
Cleanroom garments are vital elements of an ESD program. There are 3 ways that garments can control static:
- Connect to a groundable point
- Establish an electrical path to the groundable point
- Establish electrical resistance from one point to another within the garment.
The last kind of garment is the most commonly used. It is important to know that the seam construction, fabric, and ESD yarn in an ESD garment for a cleanroom is different than those in an ESD garment for a controlled environment. Pockets and emblems are not recommended on ESD garments.
Other apparel plays an important role in ESD protection. Eudy’s article states, “Specially designed ESD shoe covers, heel straps, or shoes are used to impart some ESD control when a facility has a dissipative grounded floor. Grounded static dissipative garments, when worn correctly, are designed to minimize the charges that may or may not be present on an operator’s undergarments. However, it is recommended that other measures of ESD control are also used.”
Wrist straps are also available. When properly worn and connected to the ground, a wrist strap keeps its wearer near charge neutral.
For all ESD apparel, daily testing is recommended.