Is it appropriate to test an ESD garment on an operator?
This question was asked of Larry Levit during an “Ask the Experts” panel discussion at ESTECH, the annual technical meeting of the IEST (Institute of Environmental Science and Technology). Larry teaches a tutorial “ESD the Unseen Contaminant in the Cleanroom” for the IEST. Larry answered “Yes, but the results will be different when the ESD garment is worn versus when tested at the laundry for surface resistivity and triboelectric charging. Different environments will give different results.” q=CV where “q” is the charge on the garment; “V” is the voltage that is read on the static meter, and “C” is the capacitance of panel to ground. If the panel is near a person or grounded metal piece then “C” is a large numeric value. If there is nothing near the panel such as the garment hanging on a plastic hanger with three-foot spacing around the garment, then “C” is a very small number indeed. Therefore, “V” is a large or small numeric value depending upon the environment and the ESD garment will respond differently when worn on a person rather than on a hanger. The person is the capacitance. For that reason, it is not reasonable to set the same voltage limits on a garment on the rack because it does not represent the conditions in which the garment is used. Garments on the rack OVERESTIMATE the amount of charging that a garment will exhibit when worn, sometimes even by a factor of 10.
ESTABLISHMENT OF AN ESD CONTROL PROGRAM
There are several reasons to establish an Electrostatic Charge (ESC) Control program. Electrostatic charge causes damaging electrostatic discharges (ESD) and the attraction of contaminants or electrostatic attraction (ESA). In the disc drive, flat panel display, and semiconductor industries, static electricity can cause immediate or latent failure of the product through ESD or ESA. Therefore, it is imperative to control static electricity in cleanrooms and controlled environments.
Controlling static electricity and electrostatic discharge should be a multi-faceted approach. Each individual parameter supports the total ESD program. An ESC control program involves use of special personnel apparel, shoes, or grounding straps, ionizers, control flooring, work surfaces, and packaging. All surfaces that have some electrical conductivity should be grounded and insulative surfaces should be avoided. In some cases, temperature and humidity can be used to also decrease static charging. It is impossible for any one of the above components to individually control static electricity. Therefore the program should take into account:
- What is reasonable?
- What is enough?
- How to take the specifications and make them work.
- Common sense techniques based on the physics of what is happening.