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First Published in EOS/ESD Technology April/May 1992 The
Components of an Effective An
effective program depends on how well the (Click Here for Part 2 of this Article) Hans-Peter
Brandt Note: This is the first of a two-part article on the different components and equipment used to develop an effective static-control program. Part 1 will cover wrist straps, flooring, footwear, and clothing. Part 2 of the article, to be published in the next issue of EOS/ESD Technology Magazine, will continue with work surfaces, air ionizers, and a look at some of the author's observations and conclusions. A variety of devices are available to prevent static generation in electronics manufacturing environments. These items range from wrist straps, footwear, and work apparel for personnel, to stationary devices such as special floor materials, work surfaces, and air ionizers for the workplace. Described here are the different types of static-control devices that can be used in the design of an electronics workplace 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. System Performance Requirements A static-safeguarded work area must be free of static charge and must dissipate any static on objects brought into the area. How the charge is removed and the makeup of the static-control system will depend on whether the charged objects are conductive or nonconductive. ESD safeguarding and precautions must be continuous over the entire production sequence since static accumulation, for even short periods of time, can cause defective production. Automatic monitoring can provide maximum reliability and ensure continuous protection. Prior to implementing a static-control system, it is necessary to determine the maximum permissible voltage levels that the most sensitive components or assemblies handled in the particular workplace can accept. Wrist Straps A wrist strap is designed to continuously ground a worker to prevent harmful static voltage levels resulting from simple physical movements. The voltage from static-charge accumulation measured on a human body, or any other object, is equal to the charge divided by the capacitance.
U=
voltage measured on the person Capacitance. The effective capacitance of a worker is a function of his proximity to other objects in the work environment such as the floor, workbenches, or equipment. These distances can charge as a result of physical movements, thus causing voltage fluctuations. Resistance. The total resistance through the wrist strap and worker to ground is primarily dependent on the contact resistance between the wrist strap and the adjacent skin surface. A number of factors influence contact resistance including the texture and dryness of the worker's skin, presence of hair, and most importantly, the contact pressure (i.e. the fit of the wrist band around the wrist).
Testing. Functional testing of a wrist strap must include the effects of all of these factors. The most suitable means of doing this is by measuring the overall resistance from the worker's hand to ground (See Figure 1).
The overall resistance to ground should be in the range of approximately 1-10 Mohms to provide static protection and ensure operator safety. Experiments have shown that a resistance to ground below 10 Mohms is required to provide adequate charge drainage and preclude voltage build-up in excess of the damage threshold of most sensitive electronic devices (See Figure 2).
The lower resistance limit of 1 Mohm is required for operator safety (i.e. when in contact with live electrical circuitry). This is typically provided by a 1 Mohm current limiting resistor in the ground cord. If the worker contacts a 240V AC line, the resistance of 1 Mohm will result in a current of 0.2 milliamperes. The current level is at or below the threshold of sensation in humans (See Figure 3).
Monitoring. Since wrist-strap performance can vary with skin dryness and foreign material accumulation on the wrist band or adjacent skin surface, resistance to ground should be checked at least daily. In many cases, continuous monitoring is preferred over periodic checking to prevent the possibility of defective production between control intervals. This type of monitoring system uses a comparator to continuously compare the resistance to ground through the wrist strap with the reference ground. An acoustic or visual alarm gives a signal to the operator in the event of high or low resistance in the wrist-strap grounding circuit (See Figure 4). Flooring, Footwear, and Related Components Objects brought into electronics production environments can carry static charge with them. Floor materials that can rapidly dissipate static accumulation is an important static safeguard. In addition, all components contacting the floor (such as transport carts, chairs, and footwear) must be electrically compatible with the floor surface. To minimize the risk of static damage, discharge should ideally take place as quickly as possible. The time required for discharge is a function of the capacitance of the discharging object and the resistance of the path to ground. The instantaneous voltage on the discharging body is described by the equation:
U0
= initial voltage This function is illustrated in the plot of Figure 5.
The discharge time to 1% of the initial voltage should generally not exceed one second to prevent static damage. For a worker with a typical capacitance of 200 pF, the maximum permissible resistance to ground is given by the following equation:
t1
= 1 sec
Total resistance. The total resistance to ground is the sum of the resistance of the worker, footwear, the contact resistance between footwear and floor, the resistance over the floor to the ground connection and the current limiting resistor in the ground cord (See Figure 6). The static-dissipative characteristics of shoes can be changed significantly by factors such as wear, foreign material accumulation, and perspiration. The resistance of a worker through his footwear should be checked daily.
Standards. The CECC 00 015 standard has specified a maximum permissible resistance of worker through his footwear of 35 Mohms (See Figure 7 for test setup.) Generally accepted standards or specifications of minimum permissible resistance for dissipative footwear are not presently available. Experience has shown that a minimum resistance of 1 Mohm in the floor grounding line will ensure personnel safety. The resistance should not exceed approximately 1000 Mohms in order to provide rapid charge dissipation. The resistance to ground measurements is described in a variety of international standards including NFPA 99, ASTM F 150 and DIN 51953. These methods are also employed for resistance to ground measurement of work surfaces (See Part 2 of this article for a detailed look at work surfaces). Clothing Even if the measured voltage on a person is close to zero, some induced charge can exist on his body due to static charge on his clothing. The selection of correct work apparel is therefore a very important factor and requires a reproducible test procedure for reliable evaluation. A suitable test method for evaluation of charge-dissipation performance on clothing is given in the draft version of EOS/ESD Standard No. 2. Garment testing. The garment is charged by a high-voltage source and the voltage buildup is monitored by a high impedance voltmeter. After obtaining a defined potential, the garment is disconnected from the voltage source and grounded. The resultant voltage vs. time plot is used to evaluate static-drain performance, which is particularly critical for garments with one or more seams across the conductive path. The clothing must ensure galvanic contact to the wearer's skin; for safety reasons, the resistance of such work apparel should be at least 1 Mohm.
Objects in the work area. Resistance to groundmeasurements through conductive flooring cannot always be performed in conjunction with other components of interest at the workstation such as moving carts, sliding objects or a worker in a conductive chair. Evaluation of static discharge performance in such cases is best performed by monitoring the voltage level of the respective conductive body with a high-impedance voltmeter. A monitoring apparatus of this type is shown schematically in Figure 8.
A typical example of such a case would be a worker approaching the workstation who is not wearing static-dissipative apparel. The high contact pressure between show and floor normally ensures good electrical contact and thus low static voltage levels. If the worker sits down on a chair and raises both feet, the path to ground changes from footwear-floor to clothing-chair-floor. This rapidly changes his capacitance, causing a transient voltage peak of significant magnitude due to the increased resistance to ground. A typical voltage pulse plot monitored for an individual is shown in Figure 9.
Part 2 of this article, covering work surfaces and air ionizers, is scheduled for publication in the June/July 1992 issue of EOS/ESD Technology Magazine. References 1.
ANSI/NFPA 99, Health Care Facilities, 1984. Click here for The Components of an Effective Static-Control System Part 2 |
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