ESD and Static Control Questions
Answered by the experts at Ground Zero Electrostatics
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Q. What is the importance of having the personnel within an EPA to wear ESD footwear on an ESD Flooring system?
A. Thanks for the inquiry. It was good speaking with you earlier. You had stated that you had concerns about the grounding of your flooring system and my first question to you was about the use of ESD footwear. I believe that you had stated that not everyone was using esd footwear.
You pose a situation here that needs to be addressed as a top priority at all levels of ESD Awareness for every client that we come into contact with. I don’t mean to over-simplify this but I am seeing a common trend in the ESD industry; your ESD system is only as strong as the weakest link in the chain. Another way to say this is; If you buy a Plasma TV, it needs to come with a power cord and you need to plug that in.
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Q: How does ESD Gloves, Aprons and Bunny Suit help for static discharge? WIll it not be mandatory to wear the wrist band, footwear and heel strap along with it?
A: People are a great generator of static electricity, among other things. The ESD gloves, aprons, etc. enable you to work and interface with sensitive components and equipment while protecting them from an ESD event as well as other things like contamination from human skin, street clothes, dirt, dust, etc. It will be mandatory for personnel ground to wear a wrist strap when in seated operations in a manufacturing environment or in the field. For personnel ground in an environment where you are standing or walking about, you do NOT have to wear a wrist strap if you have sufficient ESD footwear in conjunction with an ESD flooring system. The above requirements are per ANSI/ESD S20.20-1999 Table 1.
Q: Is it necessary to use the ESD garments? Is it not possible to ground the charges developed through a person’s clothing through the human body which is grounded through a wrist strap, footwear or heel strap while working at an EPA?
A: This depends who’s in charge. If the ESD manager wants to require ESD personnel clothing and redundancy of ESD compliance for improved ESD control, they are the one to determine that. More and more I’m seeing the use of ESD smocks outside of the cleanroom environment. The company and their clients are reaping the benefits of increased quality control and reduced “out of box failures”. Cloth is proven to be a better conductor of ESD charges than some types of clothing, but not everybody wears cotton. Some people have dry skin and the ESD garments are a good idea for added protection and a requirement in some environments.
I’d like to add that when seated in an EPA, you must have your wrist strap on regardless of ESD Garments, ESD footwear, ESD flooring system, etc. That’s a requirement of ANSI/ESD S20.20-2007 page 4 section 8.2 Personnel Grounding; “When personnel are seated at ESD protective workstations, they shall be connected to the grounding/equipotential bonding system via a wrist strap system.”
Further, per Table 2 note 2, “For situations where an ESD garment is used as part of the wrist strap grounding path, the total system resistance including the person, garment and grounding cord shall be less than 3.5E7 ohms.”
Q: Is it necessary to use an ESD Chair when the humany body is already grounded through a Wrist Strap, Footwear or Heel Strap when working in an EPA?
A: I think it’s a great idea. I can be sitting at a workstation with an ESD flooring system, have a wrist strap on and when I get out of my chair, not generate more than 50 volts. I may not generate more than 5 volts. But what if I take my wrist strap off and jump out of my chair? I can easily generate 100’s of volts. Oh, by the way, jumping should not be allowed in an EPA. If you have great ESD shoes or sole grounders and you keep at least one foot firmly planted on the ground at all times, then maybe your ESD chair is redundant, but still an added piece to the chain in your EPA system.
Q: What does ASTM F2413-05 compliant mean?
A: I’ve got the document from the American Society for Testing and Material Standards (ASTM) in front of me now.
They sent me a copyrighted document on June 2nd of 2005.
It’s significance and use section is pretty encompassing;
- This specification contains requirements to evaluate the performance of footwear for the following:
- Impact resistance for the toe area of footwear.
- Compression resistance for the toe area of footwear.
- Metatarsal protection that reduces the chance of injury to the metatarsal bones at the top of the foot.
- Conductive properties which reduce hazards that may result from static electricity buildup, and reduce the possibility of ignition of explosives and volatile chemicals.
- Electric shock resistance.
- Static Dissipative (SD) properties to reduce hazards due to excessively low footwear resistance that may exist where SD footwear is required.
- Puncture resistance of footwear bottoms.
- Chain saw cut resistance, and
- Dielectric insulation.
There’s a section in there that describes Performance Requirements and Workmanship, Hazard Assessment, Labeling and Identification, Marking and Compliance Requirements, and Keywords.
Q: We have an ESD tile floor in our testing lab. During a recent AS9100 audit we were asked why we don’t use wrist straps. Since our ISO9100 & 2 certified calibration provider only uses them when they have to certify the repair of a system and we don’t do that do we need more than the flooring? (booties, straps, etc.)
A: The answer to your question is, YES. In accordance to EOS/ESD standards, an ESD floor in conjunction with Heel Grounders, ESD Booties, ESD Foot Wear does NOT take the place of using Personnel grounded Wrist Straps at the workstation. So the basic ESD program would include the technician wearing a Wrist Strap.
Protective personnel grounding products such as; heel straps, booties and shoes working in conjunction with an ESD floor are designed for processes within your ESD program where the technician has to be mobile within an ESDPA, (ESD Protected Area) where the electrostatic sensitive device is not protected in an ESD protective container or shielded in some manner, e.g. ESD Bag, Bin, Tote, Box, Tray, Container, etc.
If your process has technical personnel setting or standing at a workstation they DO require grounding via a Wrist Strap. So one doesn’t necessarily take the place of the other, and both may be required depending on your personnel and their mobility throughout the plant.
If your technical staff removes their wrist strap to transport an ESD sensitive device or component outside the ESD protected area, the device also needs to be shielded as well as the technician being properly outfitted with ESD shoes, booties or heel straps. Within the ESDPA, they do require the additional shielding protection of the device, but it is always a good idea when practical.
I hope this helps address your question, please let us know if we can be of further assistance with any ESD questions or ESD protective product requirements you and your team may be sourcing.
Q: How important is humidity in the control of electrostatic problems? What is the approximate decrease in electrostatic control if the humidity is reduced from 50% to 35% RH?
A: I have linked a white paper below for your review that puts this topic into perspective.
This data seems most relevant in this case, because it points out how well humidity from 30% and up reduced tribo-charging, but not good enough for the thin-film applications. Notice that voltages of up to 1.5 kV are still generated with ease in an 80% environment! Dropping from 50% to 35% might be beneficial to prevent corrosion, but still require an EPA (ESD PROTECTED AREA) with a total ESD protection system.
Download White Paper (PDF Reader Required)
Q: I have one question/problem that I would like to ask regarding an experiment to test a bench top air ionizer. From the attached documents, there are 2 graphs of decay time versus effective distance, one for decay time on negative charges and the other one for decay time on positive charges. As you can see, the closer the bench top air ionizer (effective distance), the lesser the decay time will be. My question is, why is it during the distance of 20 to 24 inch the graph line become a straight line (saturated) and not growing linearly like the other points?
A: I see you’re using the Bench Top style air ionizer. The “PC” means that it is “targeted” coverage as opposed to “extended” coverage. The posted operating range appears to be from 1′ x 5′ or 12″ to 60″.
Your graphs range in distance from 4″ to 32″. I’d consider using a test method recommended by EOS/ESD S3.1 if you haven’t already.
Was your humidity really at 80%?
This ionizer also has a heater function. I’m not familiar with that particular feature, whether or not it is a factor here.
Ionizers have two properties that ANSI EOS/ESD S3.1-2000 defines through the use of a CPM; discharge time and offset voltage.
Ionizers increase the electrical conductivity of air, which is especially useful in environments that use insulators which cannot be removed from the EPA (such as PC boards). Grounding an insulator doesn’t remove it’s electrostatic charge. They also reduce the effect of the earth’s field, which increases with the altitude above the work surface of the DUT(device under test).
Now things get complicated.
Discharge time:
Ionizers decrease the charge on the CPM exponentially with the time constant RC.
R = resistance of air (Keep in mind that air resistance increases with the distance that ions must travel).
C = capacitance of the plate. ( Keep in mind that smaller objects have lower capacitance and the time to discharge them maybe shorter than the discharge time).
Offset Voltage:
This deals with induced potential on objects. It has been found that the mobility of negative and positive ions are different. Thus, there is a small electric field generated which is zero at the work bench surface (if dissipative ESD mats are being used) and largest as you get closer to the ionizer. We use an isolated system, which reduce this effect by a simple law of nature- charge cannot be created or destroyed in an isolated system. With the more sensitive devices, such as an MR head on a disk-drive, I’d be very careful here as no system is perfect. Will electrical potential damage a device, or will the rate of current discharge do the damage?
Unfortunately, there’s other things involved with your chart, such as the mere dimensions of your plates and the distance from them. Really close to the plates, the effects of decay time are linear and are affected by a plane source. Further out to some point, you may be dealing with a line source which drops off as the function of L/2 where L = length from the plate, and ultimately, when you get 7 times that length or 7L, you are dealing with an inverse square or L / 4 equation- or point source- out there you are parallel to the source or detector. An example of this equation is to calculate the surface area of a sphere with a diameter of 2 units as opposed to one with a diameter of 4 units. Suffice to say, nature is too complicated to be linear.
I’ve only scratched the surface on your question here and I’d like to give it more thought. I’d run the experiment myself here, but I currently have some equipment in for calibration. For now, let’s move in the direction of testing per EOS/ESD S3.1.
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