Q&A on Electrostatic Discharge

Provided by the ESD Association

Q: We are not permitted to bring our jackets, plastic sheet protectors, or plastic foam cups into the work area. We're told it's an ESD problem, but we thought ESD damage was caused by a discharge from the human body. What gives?

A: There are a number of sources of ESD. A discharge from the human body is just one of them and is represented as the human body model (HBM).

Another source of ESD is described by the term field induction. Whenever any object becomes electrostatically charged, there is an electrostatic field associated with that charge. This field may extend as far as a foot from the object. You can feel this field when you hold your hand in front of your television screen. When an ESDS device is placed in that electrostatic field, a charge may be induced on the device.

If the device is then grounded momentarily while within the electrostatic field, a transfer of charge from the device can occur (i.e., a charged device model discharge). The device could become grounded by setting it on a work surface that is too conductive or by intentionally grounding it when testing the board.

Items such as sheet protectors, nylon jackets, and foam cups are high-charge generators. Their presence in the work area exposes products that are sensitive to static electricity to the electrostatic fields that may be present on these objects.

Another source of electrostatic fields in the workplace is the cathode-ray tube (CRT) screen on a computer monitor. Although the newer monitors have reduced the electrostatic fields emanating from them, older models can measure over 10,000 V. Checking the screen for fields should be done in both a power-on and a power-off condition. CRT screens may even induce a charge on a person if the individual is ungrounded.

The following are solutions to the problem: keep static-generating materials out of your work environment, use topical antistats on computer monitors, employ ionization techniques, and use shields over the monitor screens.

Bibliography

Chemilli, Robert; Unger, Burt; and Broussard, Peter. "ESD by Static Induction." EOS/ESD Symposium Proceedings, 1983.

Franey, John P; and Renninger, Robert G. "Field Induced ESD from CRTs: Its Cause and Cure." EOS/ESD Symposium Proceedings, 1994.

Additional information may be obtained by contacting the ESD Association, 7900 Turin Rd., Bldg. 3, Ste. 2, Rome, NY 13440; phone: 315/339-6937; fax: 315/339-6793; Internet: http://www.eosesd.org; e-mail: eosesd@aol.com.

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