Educational Note 1

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INTRODUCTION

Static electricity is a natural phenomenon that occurs in all climates and levels of relative humidity year round. Most people can not feel a static spark or electrostatic discharge, ESD, unless the static voltage is greater than 2000 volts. An ESD event of less than 2000 volts can damage much of today’s microcircuit electronics. A wrist strap is a device to control the static electricity on people by connecting them to a ground potential, for the purpose of preventing ESD damage to electronic components. When people are charged with static electricity, they can throw static sparks toconductive objects such as door knobs and electronic circuits. These electrostatic discharges can damage sensitive semiconductor devices and other components used in modern electronics. This article covers practical information for the use, care and daily checking of wrist straps.

HISTORY

Most wrist straps in use today does not look much like a strap. They usually consist of a wrist cuff with an attached wire. In early times of ESD control wrist strap was made by cutting a strip of carbon filled polypropylene sheet plastic from a conductive work mat. One end of the strip was wrapped around a person wrist and secured with a snap fastener.

The other end of the strap had a battery clip that could be attached to ground. Modern wrist straps have come a long way since then. Soft and expandable fabric wrist bands with woven-in conductive fiber are as durable as comfortable to wear. Some wrist straps come as a metallic watch band or even a digital wrist watch.  Wrist strap description: Wrist straps have two major components, a cuff that fits on a person’s wrist to provide electrical contact to the wearer's skin and a ground cord that is usually a coiled electrical wire. The cuff is a flexible form fitting band that makes a reliable continuous connection to a person’s wrist. It has conductive surfaces on the inside, next to the skin that makes the connection to the person. The cuff also has a quick parting electro-mechanical connector that mates with a corresponding connector on the head of the ground cord. The ground cord is a coiled wire that connects the cuff to the ground. It usually consists of an insulated wire with a connector head that mates with the cuff on one and a termination device for connecting to ground. The connector most commonly used to mate the heads of the ground cord to the wrist cuff are metal snap fasteners used in garments and leather goods. Two sizes of snap fastener are used most often: the 3 mm (1/8 inch) button snaps and the 6 mm (1/4) floating ring snap. When the 3 mm button snap is used, the male snap is almost always on the wrist cuff and the female snap is on the cord head, while the 6 mm floating ring snap is used in both polarities among manufacturers, i.e. some have the male on the cuff and the female is the cord head; others reverse this usage. When qualifying or reordering wrist straps, it is a good practice to use a consistent snap size and orientation. This allows compatibility between old and new.

APPLICATION

It is a well-known fact among the ESD community that whenever static sensitive devices and assemblies are handled, personnel must always be grounded. The single most effective and most recommended method is by using a wrist strap. However, it is important to emphasize that while the wrist strap grounds the skin, it does not provide a means to eliminate static charges from clothing and footwear. Use To be effective, the wrist strap cuff must fit snugly and make full skin contact around the wrist. See Figure 4.

The wrist strap should not be worn uncomfortably tight or leave deep marks on the wrist. It must also be connected to ground. A continuous and secure connection will provide the proper dissipation of electrostatic charge stored on the body.

Testing of the resistance of the person to ground through the wrist strap, will assure the connection exists. While there are several types of Figure 2: Typical cloth cuff Figure 3: Typical metal cuff

Figure 1: Various types of wrist straps Figure 4: Wrist strap must fit well. Wrist straps, they all should be designed to provide personnel safety through a current limited resistor, protection of highly sensitive devices from static electricity due to charged personnel, and consistently maintains personnel at ground potential which should be lower than the sensitivity threshold of the components being handled. Selection While choosing a wrist strap, one have to consider the wearer’s comfort, wrist strap’s functionality, length of the ground cord whether it is retractable or straight, clean room compatibility (if applicable) and long term reliability. Human comfort plays a major part in the selection of a wrist strap design since it must be worn continuously and should not interfere with any job function. The industry has provided numerous designs which incorporates various techniques of getting a reliable skin contact. These range from fabric woven with conductive fibers, to metal bands as well as other special use designs. Adjustable fabric band such as the one shown on Figure 5 provides both comfort and functionality. Designs should all meet the goal of providing an appropriate ground. The ultimate choice will be up to the user since it must match the process where it is being used.

Selecting the right size and type "banana plug"on the wrist strap is also important. Often, users and suppliers pay so much attention to the functionality and comfort features of the cuff and overlook the other end's connection. There are two basic types of banana plug used by wrist strap manufacturers: single piece and roller type. As shown in Figure 6, the single piece plug has four fix spring leaves form the square retaining portion of

the plug. This square shape often restrict the plug’s rotational movement within the receptacle hole, thus provide little or no strain relief when the wearer move about. This restriction also allow the wrist strap to stay twisted and shorten its cord's life. The spring leaves are also much stiffer and can enlarge the receptacle hole or in some cases the spring leaves become permanently deformed and the plug comes loose.

Operators were seen to use tweezers to ply open these spring leaves to make the plug stay in the receptacle. In the roller type of banana plugs, see Figure 7, the retainer spring portion has six to eight small leaves that can rotate freely around the plug's shaft. Smaller spring leaves provide resiliency while the large number of leaves provides good retention to the receptacle. This type of plug can also accept a wider range of receptacle sizes compared to the one piece type. The final attribute in selecting a wrist strap is the factor of reliability that the device will exhibit. Keep records indicating the initial service date and the date that the individual wrist straps are retired due to wear or test failure. This data will be valuable in determining the reliability of the product. Ground Connection As mentioned earlier, the wrist strap must be grounded to drain charges from the wearer’s body. Where the wrist strap is plugged in is as important as the type wrist strap used. Since most, if not all modern wrist straps have a built-in safety resistor, the strap must make direct connection to an approved grounded receptacle. It should not be connected in series with other static dissipative systems such as table mat or table top, etc. In other words, the resistance between the wrist strap’s plug and a ground point should be as close to zero as possible. High impedance path between the wrist strap and ground may not help dissipate charges from the user's body fast enough. There are some commercial wrist strap grounding receptacle available to simplify the grounding task and assist in recognizing correct grounding point for wrist straps. There are also devices called Continuous wrist strap monitors which come with grounded receptacles. The ground lead of these devices should be connected directly to an approved ground, such as earth ground of equipment ground. In some most cases, a hole with correct diameter can be drilled into the equipment metallic chassis or metal frame of the work bench, provided the parts are correctly grounded.

CONTINUOUS WRIST STRAP MONITORS

The Continuous Wrist Strap Monitor (CWSM) is a device that provide a continuous path to ground for wrist strap and monitor to make sure that the operator is always connected to ground. There are two popular way to achieve this result: a) measuring the resistance between two points on the operator's wrist via skin resistance, or b) measuring the operator's resistance and capacitance (RC) combination impedance to ground.

Figure 5: Adjustable wrist strap.

Figure 6: One piece plug

Figure 7: Rollertype plug.

Figure 8: Commercial Wrist Strap Ground outlet

Figure 9: Wrist Strap Connection with Mat.

The resistance measurement method seems to be simple enough. All it takes is two metal buttons on the wrist strap. Skin resistance is usually between 1000 ohms to 5 megaohms, depending on the skin moisture. This method, however, requires two electrical wires, one each connects the metal button on the operator's wrist. Dual wire wrist straps are available, but are normally more expensive than the conventional single wire straps.

Impedance test method for the single cord wrist strap is much trickier and at time less reliable than the dual type resistance monitor system. To monitor the operator's impedance, an AC wave is sent out through the operator's wrist strap. Depending on the impedance at the end of the wrist, the monitor circuit is able to determine if the wrist strap makes contact with some object with a definitive resistance and capacitance.

Some system can only determine if the wrist strap make contact with a person or a thing. For practical intend and purpose, this impedance system works well to ensure that the operator is wearing the wrist strap correctly. It can not tell if the operator is cheating the system by hooking the wrist strap to a large metal cart or holding the straps with the metal button in contact with her skin.

WRIST STRAP TESTING:

The purpose of testing the wrist strap is to confirm that the system of the human wrist, the cuff, the cord and ground connection are reliable. This test can be accomplished through the use of an ohmmeter or other specially designed tester made to test the system. Numerous testers have been designed by the industry to accomplish this task. If an ohmmeter is used, it is important to understand that the resistance of the human is considered in the total resistance of the system and that the value of this resistance will vary from person to person.

Wrist straps do not last forever, and changes in weather and changes in people can affect the resistance of the personnel ground. Wrist straps should be tested at least twice daily. Frequency of testing however, must be driven by the amount of risk exposure that could be derived in the period between tests.

Because wrist straps do not have an infinite life, it is important to develop test frequency that will guarantee integrity of the system. Typical test programs will recommend that wrist straps that are used daily should be tested daily. However, if the products that are being produced are of such value that knowledge of a continuous, reliable ground is needed, than continuous monitoring should be considered and may be required.

Testing of the wrist strap must be include the resistance of the cord, the limiting resistor, the snap connector, and the resistance of the interface of the cuff and the wrist. The range of resistance most often used for proper ESD grounding is at least one megohm and less than 10 megohms. (+/- 20%)

Data taken from the test program set will allow the user to make the choice of how often the wrist strap should be checked and which wrist straps have the most useful service life and dependability.

Wrist Strap Test procedure:

While wearing the wrist strap, connect the loose end of the cord to the tester terminal (or ground if they are the same) and depress the test button or metal test surface with a finger or hand. If resistance is over 10 megohms, test the cord alone for continuity. If the resistance of the cord alone is approximately one megohm, check the fit of the band around the wrist and adjust it for a snug fit. Snap the cord back on the cuff and retest. If the resistance is still over 10 megohms, substitute a new band. If that does not work, the problem might be dry skin. Dry skin conditions can be resolved by applying moisturizing lotion on the wrist and repeating the resistance test again. The moisturizing lotion should be one that is designed specifically for the electronic industry. It should not contain oils, fats or grease that could contaminate contact surfaces or affect solderability.

Performance/Reliability:

As test programs are implemented data will surface relative to wrist strap system failure and remedy them with corrective action. Four modes of wrist strap failure are typically experienced. Contact to human skin failure. This mode of failure occurs when the wearer does not put on the wrist strap properly, i.e. cuff conductor is too loose, or wearer's skin is too dry. Often wrist strap wearer can use hand lotion to moisturize her wrist to achieve good contact. Care must be taken not to allow excessive build up of lotion on the wrist strap itself. Once dried, the lotion residue can form an insulative layer around the cuff and render it useless.

Cord fracture failure

This is one of the most common failure mode on a wrist strap. Since the operator usually move about her work station, the cord is continuously stretched and strained. Fracture in the conductor is difficult to detect unless the cord is tested while stretched out. This is where the monthly resistance check of wrist strap can weed out bad or intermittent cords.

Cuff conductor fracture failure

Rigid cuff conductor can crack because of stress and strain during putting on and off. This failure mode does not cause intermittent failure of operator's ground contact. When failure happens, the cuff usually break off soon afterward and must be replace.

Soft and expandable fabric cuff can become stretched and would not fit well around wearer's wrist. On adjustable types, the conductive elements of the cuff would become stretched out so far that their start to lose contact with the wrist.

Loss of ground contact failure

Accurate measurements will allow users to quickly identify sources of problems associated with various elements with the wrist strap

WRIST STRAP SAFETY:

Modern wrist straps have a current limiting resistor molded into the ground cord head on the end that connects to the wrist cuff. The resistor most commonly used is a one megohm, 1/4 watt with a working voltage rating of 250 volts. Resistors limits the flow of electric current as defined by Ohm's Law which states the current is equal to the voltage divided by the resistance.

In practical application, the maximum amount of current that can flow through a wrist strap ground cord if it were placed across a 250 Volt source is 250 micro amps or 0.25 milliamps. This amount of current is safely below the 0.7 milliamps that Underwriters Laboratories uses as the threshold of danger to people.

CONCLUSION

• Wrist straps provide the most effective means for maintaining personnel at ground potential. People who are at ground potential cannot zap ESD sensitive product by touching it. This is true for all conditions of floor, footwear and clothing.

• Wrist strap grounds are a primary method for grounding personnel when handling unprotected product.

• Wrist straps have a current limiting resistance, usually a one million ohm resistor, molded into the ground cord near the point where the cord attaches to the cuff. The resistor usually has a working voltage rating of 250V.

• Wrist straps should not be worn by personnel working near open and exposed metal parts where they could come into contact with hazardous high voltage over 250 volts AC or DC.

• Wrist strap grounds do not control the electrostatic field on clothing or other insulating personal items.

• Wrist straps are sometimes supplied with a one megohm resistor molded into both ends of the ground cord. this is done when both ends of the cord have the same snap connector.

• Wrist strap ground cords have a quick parting connector to the cuff so personnel will not be tied to the work station.

• Wrist strap cuffs should be worn comfortably snug around the wrist making full skin contact.

• Wrist strap ground cords should be connected to an ESD common point ground or hard ground. Do not connect to a resistive ground exceeding 2 megohms. Do not connect to a resistive ground exceeding 2 megohms. Do not connect to a snap on a dissipative mat unless it is also the ESD common point ground. Do not clip a wrist strap to the edge of a dissipative mat.

• Disposable wrist straps made from linear resistance material are designed for single use applications such as customer installation of an option module. They should not be used in any application of an option module. They should not be used in any application where hazardous voltage is present. They should not be used on equipment that has not been removed from the power source.

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