Fowler Associates Labs

First Published in EOS/ESD Technology Aug/Sept 1992

The Components of an Effective
Static-Control System: Part II

An effective program depends on how well the individual static-control components perform, on their won and as part of the system.

Hans-Peter Brandt
3M Laboratories (Europe) GmBh
Electro-Telecommunications Laboratory
Hamburg, Germany

This is the second of a two-part article describing some of the different components and equipment used to develop an effective static control program. Part I of the series, published in the April/May '92 issue of EOS/ESD Technology, covered wrist straps, flooring, footwear, and clothing.

Part II will cover work surfaces, air ionizers, and will examine some of the author's observations and conclusions.

A variety of devices are available to prevent static generation in electronics manufacturing environments. These items include work surfaces and air ionizers for the workplace. This article will discuss these static-control devices and their performance requirements. Also discussed are various factors that affect the performance of the individual system components and how these components interrelate in the overall static-control system.

Work Surfaces

Objects brought into the work area and placed on the work surface may have static charge. Work surfaces in static-safeguarded assembly stations must be able to dissipate any charge from such objects. If electronic components are carried in a tote box or packaging, these must of course also permit discharge.

Figure 1: Test apparatus for resistance to ground as per NFPA 99.

The resistance requirements for dissipative work surfaces are in effect the same as for floor surfaces, i.e., resistance to ground should be in the range of approximately 1-1000 Mohms. A test method used in evaluating resistance to ground of work surfaces is described in NFPA 99. This test method determines resistance to ground via an electrode of specified size and weight connected to a megohmmeter (see Figure 1).

Contact resistance. Another thing to consider with regard to charge drainage from an object on the work surface to ground is the contact resistance at the interface (inclusing any packaging). In contrast to the high contact pressure on worker's footwear, the contact pressure on components and other objects typically on the work surface is generally low, causing high contact resistance accordingly. In addition, many containers used for transfer of components incorporate lower bearing surfaces with reduced contact area. Such tote boxes will cause increased contact resistance, particularly on hard work surfaces. If residual charge remains on the packaging due to high contact resistance, static damage to handled components can ensue should a grounded worker holding a component come into contact with the packaging. This contact provides a path to ground for discahrge through the printed circuit board.

Capacitance. Capacitance effects of objects and packaging on the work surface can be significant; packaging containers have been found to vary in capacitance form 35-300pF depending on their size and shape.

A separately grounded conductive foam mat should be used as an intermediate storage zone at the workstation. The soft foam deforms under pressure, thus increasing the effective contact surface and allowing true discharge as opposed to temporary voltage suppression due to capacitance effects.

Figure 2: Static-controlled work surface with intermediate storage mat.

Laminates. Dissipative laminates should be used for the main work surface, especially where soldering operations are carried out or high physical stress and abrasion are expected (see Figure 2).

Hard laminates typically consist of a dissipative upper layer and a highly conductive lower layer. The upper layer must be sufficiently thin to permit vertical charge drainage to the conductive layer, while in effect insulating in the horizontal plane. Such tabletops allow handling of printed circuit boards or other components using power storage cells without causing cell discharge when conductive sections contact the work surface.

Tools. An additional detail which must be considered in a static-control system is any assembly or repair tools used at the workstation. These can provide an additional source of static. All tools should be conductive or dissipative in order not to inhibit static control.

Grounding of work surfaces must be carried out in accordance with local regulations which can vary from country to country. Care must be taken in the overall design of the work station that the current-limiting resistor is not bypassed in any way (i.e., by screw-mounting metal conduit to the tabletop). The screws would provide contact to the conductive layer of the tabletop and thus bypass the 1 Mohm resistor if the conduit is grounded.

All components of a static-control system should be separately grounded to the same potential, thus providing back-up protection in case a grounding cord should be damaged or otherwise defective. As noted earlier, resistance to ground measurements should be conducted at regular intervals for all static-control devices in the system to ensure overall integrity including the respective grounding lines.

Air Ionizers

In some instances, the charged objects brought into the work area are nonconductive (e.g. plastic housings for computers and office equipment). In such cases, charge neutralization can only be accomplished by using ionized air.

Figure 3: Voltage decay vs. time in ionized air stream.

An air ionizer should neutralize charge quickly and completely. Neutralizarion can be monitored by a test apparatus consisting of a charged electrode placed in the ionized air stream connected to a high impedance voltmeter with a plotting device giving a plot of voltage vs. time (see Figure 3).

Two characteristics of the curve are typically used in evaluating neutralization performance: offset voltage and static decay time.

Offset voltage. Offset voltage (U_o) is the equilibrium voltage resulting on objects exposed to the ionized air stream over a sufficiently long period of time to normally eliminate any net charge due to electrostatic buildup.

Figure 4: Ionizer test apparatus.

Static decay time. The static decay time (t_sd) is the time period required to reduce the electrostatic voltage to a defined lower voltage level. A test method for evaluation of air ionizer performance has been proposed in EOS/ESD Association Standard No.3. In this procedure, a 6x6 in.(152x152mm) metal plate with a total capacitance of 20 pF is charged to a defined potential. The apparatus is then placed at a defined distance from the air ionizer. A high impedance voltmeter is used to monitor voltage decay on the plate as a function of time. (see Figure 4).

Aside from the intended neutralization effect, air ionizers can cause certain side effects which may result in problems in work environments. These effects include air turbulence, ozone generation and electromagntic interference. These factors should be taken into consideration when considering air ionizers for use in a static-control system.

Conclusions

A static-control system in an electronics manufacturing environment should ideally consist of a combination of several devices that reliably prevent static-charge accumulation. Many factors must be taken into account in evaluating such components for use in an overall system. Each individual component system must be well understood with regard to the static-control problem at hand and compatibility with other components of the system. Some static-control devices give rise to other limitations not pertaining to static control. These limitations must be considered during system design.

An important aspect of any component system is the ability to continuously monitor functioning to indicate any malfunction for immediate resolution and prevent long periods of defective production. Understanding the individual components used in static-control systems is essential in electronics workplace design to ensure selection of the correct combination of required components.

References

1. ANSI/NFPA 99, Health Care Facilities, 1984.
2. Herhahn/Winkler, Elektroinstallation nach VDE 0100 13.Auflage 1984.
3. EOS/ESD Draft Standard No. 2/3, 1987.
4. Stephen A. Halperin, Guidelines for Static Control Management, Proceedings Eurostat Conference, 1990.
5. Donald M. Yenni, Basic Electrical Considerations in the Design of a Static-Safe Work Environment, presented at Nepcon/West Conference, Anaheim, CA, 1979.
6. Tech Response, 3M, issued July 25, 1983.