School of Electronic Engineering Science,
University College of North Wales,
Dean Street, Bangor, Gwynedd LL57 1UT
It is now widely accepted that Electrostatic Discharge (ESD) events are a significant cause of device failure and that instituting static control measures is not only desirable but essential. The exact cost of ESD induced failures to the Electronics Industry is difficult to calculate since many of the costs cannot be quantified, e. g. loss of customer confidence as a result of early product failures in the field. However, it has been shown¹ that while the cost of static control measures can be high, nevertheless if correctly applied, the return on investment does justify the implementation of such measures.
MOS devices are generally regarded as the most prone to ESD damage but, in fact, all devices and technologies are susceptible, differing only in the degree of sensitivity. Furthermore, it is important to remember that ESD damage can occur at any stage from device production through system assembly, testing and packaging to final use in the field.
2. THE ORIGIN OF THE PROBLEM
ESD problems have arisen in the last decade because of two major developments.
(b) As complexity increases, it is necessary to fabricate integrated circuits from smaller and smaller device elements in order to achieve higher operating speeds and improved production
The intrinsic electrical properties of man-made fibre and plastic materials are such as to render them very good insulators, their bulk resistivities exceeding 1014Ωm. When brought into contact with other insulating, or even conducting materials they will become electrically charged by a process known as triboelectrification. Such charging cannot be prevented since it is a natural consequence of electron (or possibly ion) transfer between two contacting surfaces2, a process which brings the surfaces into thermodynamic equilibrium. As a result of the charge transfer, one surface acquires a positive charge and the other a negative charge. The degree of charging depends on (i) the intimacy of the contact, (ii) whether any rubbing occurs during contact and (iii) the manner in which the surfaces are separated. Generally, the highest charge levels are generated when surfaces are rubbed rapidly together under high contact pressure and then separated quickly from each other so as to minimise the opportunity for the transferred charges to recombine. Once generated, static charge can remain on the surface of good insulators for minutes, hours and even days unless steps are taken to neutralise it. Electrically isolated metal surfaces can also retain static charge thereby posing an ESD threat.
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by Rick Cardinale, Bird Electronic
Bird Electronic, founded in 1942 by J. Raymond Bird, soon became a leader in radio frequency instrumentation. Today, Bird also has moved into digital instrumentation test equipment.
With the development of digital instrumentation came the increased need for controls to prevent ESD events. Improving ESD protection has been an ongoing process since the late 1980s. In 1997, the company determined that an automated PCB production line would be installed and that the entire manufacturing area should be protected against ESD.
This decision led to an evaluation of ESD protective flooring. In 1998, 20,000 square feet of conductive floor tile were installed in the main production area. To help brighten the area, white tile was selected. The floor resistance measured less than 1.0 × 10^6 ohms.
A bright, high-gloss appearance was part of the selection criterion for the floor. While the electrical properties were unchanging, by 1999, the floor was starting to dull. It was being maintained like a regular tile floor. No waxes or finishes were used; however, the tile manufacturer did recommend using buffing pads.
After consulting with the tile manufacturer and the installer, maintenance was increased to sweeping clean and damp mopping two times per week and buffing once per month. Monthly floor maintenance was $1,700 per month, a $20,400 annual expenditure.
In late 1999, the maintenance schedule was modified to add more buffing since this was the only way to keep the floor shiny. The floor now was swept and damp mopped weekly and buffed twice per month. The floor was clean and shiny, but the cost went up 41% to $2,400 per month, a $28,800 annual expenditure.
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