1. What is an EPA or Electrostatic Protective Area ?
Prevention of ESD bases on Electrostatic Protective Area (EPA). EPA can be a small working station or a large manufacturing area. The main principle of an EPA is that there are no highly charging materials in the vicinity of ESD sensitive electronics, all conductive materials are grounded, workers are grounded, and charge build-up on ESD sensitive electronics is prevented. International standards are used to define typical EPA and can be found for example from International Electrotechnical Commission (IEC) or American National Standards Institute (ANSI).
ESD prevention within an EPA may include using appropriate ESD-safe packing material, the use of conductive filaments on garments worn by assembly workers, conducting wrist straps and foot-straps to prevent high voltages from accumulating on workers’ bodies, anti-static mats or conductive flooring materials to conduct harmful electric charges away from the work area, and humidity control. Humid conditions prevent electrostatic charge generation because the thin layer of moisture that accumulates on most surfaces serves to dissipate electric charges.
2. I usually wear one foot grounder, but have been told to wear two, one on each foot. Why?
The foot grounder only works properly when worn correctly and in conjunction with a conductive or dissipative floor. The act of walking is a perfect example of triboelectric generation. If your properly worn foot grounder is in contact with an ESD floor, then all charges on your body (skin) are drained to ground so the net difference (voltage) between you and ground is zero. But, as soon as that foot with the foot grounder is lifted into the air, your body becomes charged again, either through induction from your clothing or triboelectric generation from the contact and separation of your foot (feet). If you were to wear two foot grounders, one on each foot, then you further minimize the chance you have of holding a net charge much greater than a few volts (typically 2,000 – 5,000 Volts) because you are increasing your time in a grounded state. We recommend wearing a foot grounder on each foot or a pair of ESD shoes when you go mobile.
3. If a mat and the wrist strap are connected together, do they need to be connected to earth (ground) potential?
No, but the equipment being worked on should be both unplugged and electrically connected to the ESD mat. As long as all conductive parts, (body’s skin, ESD mat, equipment chassis, parts being handled, etc.), are at the same potential, there will be no opportunity for an ESD event (discharge). Earth ground happens to be a very convenient reference voltage to connect to. If you ensure that all conductive parts are tied to earth ground, then you meet the same requirement that they are at the same potential. It is when you have a difference in the potential (voltage or net charge) between two conductors that, (either touch together or come in near proximity), will lead to a discharge (ESD event).
4. Are wrist straps necessary if all other ESD precautions are taken (i.e., two ground [foot] straps, jacket, dissipative flooring, grounded mats
, etc.)? If so, why?
Wrist straps are not necessary if an operator is wearing two foot grounders on a conductive grounded floor and doesn’t lift both heels/toes at the same time, like some people do when sitting down. If an operator is also wearing a esd jacket, but is not electrically connecting the jacket to either their body’s skin or ground, then the jacket is providing only partial protection. Charges on the jacket may have no where to go or discharge to if the jacket is not grounded. A popular way to ground the jacket is with a coil cord either attached to a snap on the waist area of the jacket or via a wrist strap snapped to the inside cuff of an ESD jacket. Our wrist strap WB2500 can be used with most industry standard jackets with this feature.
5. We are setting up work stations for handling static sensitive devices. What are some guidelines we should follow to be sure we are adequately protecting our components?
An ESD protective workstation refers to the work area of a single individual that is constructed and equipped with materials and equipment to limit damage to ESD sensitive items. It may be a stand-alone station in a stockroom, warehouse, or assembly area, or in a field location such as a computer bay in commercial aircraft. A workstation also may be located in a controlled area such as a clean room. The basic concept of the ESD protective workstation is to keep all materials and personnel at the same electrical potential. Electrostatic discharge occurs when two objects at different potentials come into contact with or in the proximity of each other. If the potentials are equal, no discharge occurs. The key ESD control elements comprising most workstations are a static dissipative work surface, a means of grounding personnel (usually a wrist strap), a common grounding connection, and appropriate signage and labeling.
6. Some of our ESD benches have the wrist strap going directly to ground and others to a work bench ESD mat which (on the opposite corner) then goes through a common point ground cord with a resistor to ground. Which is right?
Neither are necessarily correct because of the dependency on the relationship of the wrist strap that is directly grounded and the potential of the workbench mat. The concept of grounding at an ESD safe bench is to bring to the same potential all conductors, thus minimizing the possibility of a potential difference between two conductors or its resultant discharge. Power ground or earth ground happens to be a convenient potential to connect into because it is consistent within the controlled environment. If both the wrist strap and the ESD mat are connected to the same point at ground, then they have the same potential and are properly setup. The wrist strap that connects first to the ESD mat, second through the mat, third from the opposite corner of the mat and fourth to ground has the addition of the serial impedance of the mat (resistance and capacitance) to ground to contend with. At times the operator may be at a different potential than grounded bench equipment or ESD sensitive devices on the mat.
7. How well does relative humidity of the surrounding atmosphere control the ESD?
The relative humidity (RH) directly affects the ability of a surface to store surface charges. The higher the RH, the less time an item will hold a charge. The method of surface charge reduction [due to increased RH] can be attributed to recombination and or conduction. As the RH increases, so does the natural conductivity of the air, but even at 100% RH the increase in the natural discharge rate cannot be substituted for proper ESD control practices.
8. We have a conductive floor in our plant that gives readings less than 1.0 x 105 when I test it. Is this a safety concern?
I know that the re
adings shouldn’t go above 1.0 x 109, but is there a lower limit? What documentation will back this up? We have been using a dissipative wax on the floor and the readings are consistently between 1 x 105 and 1 x 109; however, we would like to discontinue this practice to save money.
Lower limits of floor to ground resistance are a matter of company or local electrical codes. Some areas allow as low as 1x10E4 ohms to ground and some areas require no lower than 1 x 10E6 ohms to ground. A common value is at 1 x 10E5 ohms to ground. If the resistance to ground values vary in an area, then it is wise to create a “map” that shows the “hot spots” where it might be wise to treat them with something to raise the resistance to within the acceptable range based on the maximum current exposure people should experience (using Ohms Law – E=IR).
9. What is the difference between antistatic, conductive and static dissipative?
The terms conductive and static dissipative typically refer to resistance or resistivity ranges used in the evaluation of ESD control materials and products. By definition, a conductive material has a surface resistivity of less than 1 x 10E5 ohms per square or a volume resistivity less than 1 x 10E4 ohm-cm. A static dissipative material has a surface resistivity of 1 x 10E5 to 1 x 10E12 ohms per square or a volume resistivity of 1 x 10E4 to 1 x 10E11 ohm-cm. These definitions appear in the ESD Association Glossary as well as in various other static control standards documents. For some materials, surface resistance rather than surface resistivity is often used to define these terms. In this case, a simple conversion factor is applied, dividing the resistivity ranges by 10. Thus conductive becomes less than 1 x 10E4 ohms and static dissipative becomes 1 x 10E4 to 1 x 10E11 ohms, provided that the appropriate electrodes with the correct geometric conversions are used. ANSI/ESD S11.11 provides additional information on this issue. The term antistatic, however, does not refer to resistance or resistivity. By definition, the term refers to a material that resists tribocharging. At one time, the term referenced a resistance value, but it was severely misused and today no longer represents any resistance range.
10. Are there any problems using cell phones near ESD devices or assemblies?
Some investigators in the disk drive area have found that under certain test conditions that GMR heads can be altered or damaged if near an activated cell phone. The testing was contrived a bit in that a large coaxial antenna was attached across the magnetic stripe of the GMR head and the cell phone operated within inches of the antenna. Naturally, this sort of condition should not occur in actual practice but it does show that the sensitivity of the particular item was quite high (easy to damage). Of course a charged piece of plastic waved in front of the antenna would have resulted in the same problem or worse. The signal strength of a cell phone should not cause problems to anything of a practical nature. Have a look at papers presented in the EOS/ESD Symposia from 1998-2000 regarding testing of sensitive items like GMR heads for disk drives.
11. Do I need to take any ESD precautions when I service or repair products in the field?
If you’re servicing ESD sensitive products in the field, you need to take the same ESD control precautions that you would take in the manufacturing environment. This includes using wrist straps, protective work surfaces, and even placing products in protective packaging. These items should be part of your ESD field service kit. The challenge that you face in field service is that you are working in someone else’s environment, one over which you have little control. You may face low relative humidity, or an office with static generating carpet, or a lack of work space. Your customer may not know whether the products you work on are ESD sensitive or not. You need to be adaptable to the environments in which you are working. For example, in addition to your wrist strap, carry extra alligator clips, jacks, and extension cords. You should assume that all the products you work on are ESD sensitive and handle them accordingly. Take the extra time to clear a work area large enough to place your protective work surface. Follow good ESD protection procedures at all times. Your responsibility is to help solve your customers’ problems, not contribute to them.
12. How can I choose the best ESD ionizer, and how should I verify them?
A charge plate monitor can be used to verify that an ionizer is in proper working order. A charge pate works like this: An electrical potential is applied to a metal plate of fixed size and capacitance. A non-contacting voltmeter reads the applied potential. A timer measures the time it takes an ionizer to neutralize the plate potential usually from 10
00 volts to less than 100 volts. Other ranges can be established depending on the application. The test methods most often used are available from the ESD Association at www.esda.org.
Another important test for ionizers is to determine the balance of the ionization process. It is optimal if an ionizer produces and delivers a near equal amount of positive and negative air ions. If an ionizer is off-balance, it can actually charge a surface. The balance is measured using the same equipment described above, only in a slightly different mode. Most ionizers today are fairly stable in performance. They should be verified for performance probably quarterly or twice a year depending on your program requirements. For more information on testing ionizers and application of ionizers, contact the ESD Association as mentioned above. There is a Standard ANSI/ESD S20.20 and other information that could assist you for nominal dollars.
Transforming Technologies offers ESD Consulting Services through LBL Scientific. LBL Scientific can guide you to a more profitable factory by elliminating the sources of ESD damage, mitigating microcontamination issues and identifying EMI from ESD that is effecting your robotics and automated test systems.
Some static generative items (i.e., monitors, key boards or special items) must be used in a static controlled area, how do you provide static protection under these circumstances?
Ideally, we should try to keep these static generating materials to a minimum in any ESD-protective environment. However, this is not always possible. If you have to use them, there are several steps you can take to reduce their possible impact on your ESD-sensitive products. First, keep the static generators as far away from your ESDS products as possible, preferably several feet. Second, be sure to keep ESDS products from entering any field that may be produced by the static generator. Third, if the static generator is a conductor, ground it if possible to remove any charge that may be generated on it. Fourth, treat the surface of the static generator with a topical antistat to reduce the charge generated on the material. Fifth, ionize the material to remove any generated charges. Sixth, if the charge generator is employee clothing, use static protective garments to help suppress any fields from the clothing. You may need to implement several of these steps to provide adequate static protection.
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