Q: Can we make our own ESD Tools?
A: Perhaps. Since common hand tools can be at risk of creating an ESD event via CDM, what do we do about it? I was able to treat the screw driver in my last installation with our ESD anti-stat chemical Shock Stop. We also have Ultra Spray which lasts for a good deal longer than gimmicky products on the market, which are probably based on a fabric softener.
Our industrial products mentioned above will last longer, but need to be re-applied depending on usage.
I was brainstorming and came up with a more permanent solution to the ESD Screw Driver. Pretty effective, huh?
Until I can work out a deal with Western Forge in Colorado Springs (dreaming a bit here), ESD Finger Cots! Ground Zero ElectroStatics, Inc. has got them. We’ve also got GZ-Shock Stop and GZ Ultra Spray for industrial uses in the EPA.
Order a sample today.
What is the significance of the time to the charge generation in tribocharging?
Why is it that in tribocharging, there is a big charge produce in short period of time while small charge will be generated at long time? ( at the same force)
(I took the liberty here to respond to the question and go a bit further and look at CDM Testing as described in a recent issue of Conformity.)
A: First a little background about charge as it relates to ESD (ElectroStatic Discharge).
Triboelectric charge is merely the contact and separation of materials. “It involves the transfer of electrons between materials.” Which materials lose electrons and which gain them depends on the materials.
Static electricity can be measured in coulombs, and related to voltage potential via the equation: q=CV. q = charge in coulombs, C = Capacitance, V = Voltage
The industry typically uses electrostatic potential and thus uses voltage to look at this energy form. Voltage is merely charge potential with respect to a ground point or reference and measured in volts (v).
Insulators or materials with high resistance restricts or prevents flow of electrons across (surface) or through (volume) it’s material.
Conductors or materials with low resistance easily allows the flow of electrons across it (surface) or through (volume) it’s material.
Insulators and isolated conductors can tribocharge to high voltages and will remain for a long time… so long as energy is not transferred via induction (isolated conductors) by bringing other objects into it’s vicinity and grounding the other object, by grounding the isolated conductor, or by balanced ionization (isolated conductors or insulators).
When isolated conductors are grounded, they (becoming grounded conductors) will enable electrons to flow easily to ground and the charge upon it will become neutralized and reduced to near zero.
Insulators cannot be grounded. They can induce charge to isolated conductors and can cause electrical overstress/ESD events to isolated conductors at the time they are grounded via the charge field and do not need to contact the isolated conductors in order to do so.
Here’s another way to say that; “CDM (Charged Device Model) charging can produce two separate discharge events. Here’s how it works. If you ground a conductor (the conductive blade of a screwdriver for example) while it is in the presence of any item carrying an electrostatic field ( a charged piece of plastic or clothing), the conductor will acquire an electrostatic charge that may be sufficient to cause damage when discharged.”
Human Body Model, as is described in ANSI/ESD S20.20-2007… and the ESD control thereof, is concerned with limiting the voltage in the EPA for the protection of ESDS devices (ESD sensitive devices) to 100 volts and a discharge to within that level in less than 0.3 seconds for ESD Technical Elements (some quicker) at minimum.
I need to know what specifically are you interested in; the HBM, MM (Machine Model), or CDM (Charged Device Model)? Keep in mind, that “volt per volt, MM discharge is an order magnitude more powerful than HBM discharge because the resistance of human body has been removed from the equation.”
In the article in Conformity, “Demonstrating CDM Discharge Using Common Hand Tools” provided by the ESDA, they state; “The damage threat from hand tools is CDM charging of the hand tool, accompanied by MM discharge to the component or device.”
Source: Conformity : ESD Open Forum April 2009 pg 20.
The following pics depict the testing I did in my lab in accordance with what I’d learned from a recent Conformity article from the ESD Open Forum entitled Demonstrating CDM Discharge using Common Hand Tools. It involves charge, not by contact, but by induction;
Are ESD shoes and Conductive shoes the same thing?
The Static Conductive shoe will guarantee a combined resistance of personnel and footwear of less than 1.0E6 Ohms. I have a pair of Static Conductive shoes that when I’m standing on a static conductive flooring system (2.5E4 Ω to 1.0E6 Ω), my combined resistance from my body through the ESD footwear and through the ESD conductive flooring system to electrical ground or earth is less than 1.0E6 ohms per DoD 4145.26-M, C188.8.131.52.1: “The maximum resistance of a body, plus the resistance of conductive shoes, plus the resistance of the floor to the ground system shall not exceed 1,000,000 ohms total”… “The contractor can set the maximum resistance limits for the floor to the ground system and for the combined resistance of a person’s body plus the shoes, as long as the total resistance does not exceed 1,000,000 ohms.”
This Static Conductive shoe is typically used for electrical safety requirements for facilities that deal with explosive environments such as ordinance, munitions, explosive powders, flammable liquids, etc. This is outside of the realm of ANSI/ESD S20.20-2007 and MIL-HDBK-263B.
If you’re goal is the protection of static sensitive devices, then Static dissipative shoes on a static conductive flooring system or a static dissipative flooring system will suffice so long as the combined resistance of personnel, footwear, and flooring to electrical or earth ground is less than 3.5E7 Ω as per ANSI/ESD STM97.1-2006. In that case, a good static dissipative shoe will be more than 1.0E6 or a meg ohm, but the resistance will probably be less than 35 Meg ohms. The best way to measure the footwear is to have personnel wear them for at least 10 minutes prior to going to the tester and checking for pass/fail low/fail high, as that’s the most practical way to test them. You can measure the resistance of the shoe from insole to outsole, but they aren’t used that way on the ESD flooring system. The ESD shoe relies on sweat from the personnel that wears them.
My combined resistance from my body, through my Static Conductive C4327 (men’s) or C437 (woman’s) shoes and through a static conductive floor to electrical/earth ground is about 7.0E5 Ω. My combined resistance from my body through my Static Dissipative C4341 shoes and through a static conductive floor to electrical/earth ground is about 1.6E6 Ω.
I hope this answers your questions. Please comment.
Thank you very much, Pat
Q: I have read the White Paper 1: A Case for Lowering Component Level HBM/MM ESD Specifications and Requirements and found the ESD Control Programs and Resulting Data (Chapter 1, Page 20-23) particularly interesting.
Assuming a production environment with ESD flooring, footwear (and clothing), by the time a person walks to a workstation and sits down, the voltage of this persons should not exceed 500V (or even 100V as seen in Figure 3). That would mean even a seated operator in this case would not need to wear wrist strap, that theory would be correct right? After sitting down and this person sits on a stool (feet off the floor) with resistance to floor < 1.0x10exp9ohms, any HBM risk would be further reduced wouldn’t it?
A: Hello ****. Nice try. Even if you have an ESD flooring system and even if you have ESD footwear and even if you have an ESD task chair with ESD casters or an ordinary task chair with an ESD chair cover (very effective as well), ESD smock on… you STILL have to wear the wrist strap when seated at an ESD workstation.
The only time, per ANSI/ESD S20.20-2007 page 4, 8.2 Personnel Grounding, that personnel in the EPA (ESD Protected Area) should be without a wrist strap is when doing standing or walking about operations, and then two conditions must be met;
· “When the total resistance of the system (from the person, through the footwear and flooring to the grounding / Equipotential bonding system) is less than 3.5E7 Ω…”
· “When the total resistance of the system (from the person, through the footwear and flooring to the grounding / Equipotential bonding system) is greater than 3.5E7 Ω and less than 1.0E9 Ω and the BVG is less than 100 v per 97.2…”
This is what is said about seated personnel:
“When personnel are seated at ESD protective workstations, they shall be connected to the grounding / Equipotential bonding system via a wrist strap system.”
Hope this helps. I guess you could say redundancy is good in the realm of ESD. It’s the weak link in the chain that will cause an ESD event. If someone lifts their ESD footwear from the ESD flooring system while seated, they can tribocharge to above 100 volts. It takes only 0.3 seconds of charge time to exceed 20.20 requirements. If personnel is seated and getting up to go to break, it seems best to stand up, remove the wrist strap from the wrist, carefully set it down and walk away from the ESD workstation. Worst case is to take the wrist strap off while still seated, set it down, put your hand on the ESD workstation and near ESDS devices, then stand up out of the task chair before leaving the work station. Under proper conditions and with good bench mats, clean ESD floors, ESD task chairs, etc. in place, no ESD event. The problem with ESD events is that we cannot see, hear, feel them.
The only alternative to not wearing a wrist strap while seated may be the used of a smock with a grounding coil cord attached to it. You can see the footnotes on the 20.20 document at the bottom of page 4 for further details.
We adhere to and meet or exceed requirements put forth in ANSI/ESD S20.20-2007 or IEC 61340-5-1, which assumes a target HBM of 100 volts and less.
Q: How do we test ESD conductive or dissipative gloves?
A: The glove industry offers gloves for the protection of ESD sensitive items by using materials that will provide specific measurable “intrinsic electrical resistance of gloves and finger cots” as per ANSI/ESD SP15.1-2005.
Some materials are being used which reduce the amount of charge generation “and/or have static dissipative properties to reduce charge accumulation”, such as Nitrile or vinyl. I would image cotton could be effective based on the layer of sweat on our skin. But if you require ESD gloves in the Static Conductive range, those would need to be specifically made for that purpose. I’m currently working on nailing down an exact value of what these gloves should read and how that affects the ESD testing of it and the closest I could find comes from a test fixture from Prostat called the CAFÉ, or Constant Area & Force Electrode. They recommend using 1.5 to 10 volts when the measurement of glove in combination with personnel through a wrist strap assembly without the 1 meg Ω resistor is less than 1 meg ohm. They use 10 volts between 1.0E6 Ω and 1.0E7 Ω. Then they use 100 volts for above that. This is fairly easy to do using a sophisticated megger like the 801 in manual mode, otherwise the mere testing of the glove per 15.1 could be a challenge.
Here’s what confuses about ANSI/ESD S20.20-2007 and -1999 …
What’s the range of the glove and finger cots? Only in 20.20-2007 Tables 1, 2, and 3 final column does it give us “Required Limits” to measure up against. So then what? Go to manufacturing specs. Some list a value, some don’t. Be careful how they’re categorized; anti-static (describes that it’s low charging but doesn’t really quantify a resistance range unless you’re talking about packaging), static dissipative (1.0E6 Ω to 1.0E9 Ω ??), and static conductive (less than 1.0E6 Ω but greater than what?? 1.0E4 Ω rings a bell, but I’d hope it’s not less than that.).
Ok, so for our Static Conductive or black finger cots, they measure between 1.0E6 Ω and 1.0E8 Ω per ASTM D257 and meet the static decay specs per MIL-STD-81705B from 5000 to less than 100 volts in less than 0.01 seconds.
So here’s the upshot; My improvisation in measuring ESD gloves and finger cots involves using the PFA-861-H Handle (see attached), a DUT (esd glove), and a wrist strap without the 1 meg ohm resistor for measurements known to be below about 1.0E7 Ω , I hook that up to my meg ohmmeter and see what I get (see attached photos).
This ESD TR20.20-Handbook has a wealth, a plethora of information about ESD gloves and finger cots, such as referring to yet other standards such as ANSI/ESD STM11.11 Surface Resistance Measurement of Static Dissipative Planar Material , and let’s not forget ANSI/ESD STM11.12 Volume Resistance Measurement of Static Dissipative Planar Materials, oh, and of course ANSI/ESD STM11.13 Two-Point Resistance Measurement of Static Dissipative and Insulative (what the??) Material, then it goes on to tell us to use the CAFÉ method, which is specifically designed for resistance measurements at the thumb and fingertips, which can yield much lower results than those obtained by the above test BECAUSE THEY INVOLVE A REAL LIVE PERSON, THE WAY THEY ARE ACTUALLY USED IN PRACTICE! Oh, and they say you can only measure once due to a “person’s skin emissions”. Fair enough. Time to reorder?
So… If this info helps anybody, let me know and send over a comment.
Q: Does anybody know the reason behind the upper limit resistance (3,5×10E7Ohms)of a grounding system (personnel+conductive shoes+conductive flooring)? Why not 1×10E8Ohms?
We have tried many waxes and all of them either give an overall reading for the system that is barely, when it is, within the limits above (IEC 61340-5-1 Table 1 – Note 2.
A: That reading is for ANSI/ESD STM97.1-2006 Floor Materials and Footwear- Resistance Measurement in Combination with a person.
So make sure you’re measuring a clean spot on the floor, someone wearing good clean heel grounders, sole grounders, or static dissipative shoes with one probe from a megger in the palm of their hand to earth or machine ground and the voltage on the meter set for 100 volts, as the resistance is greater than 1.0E6 ohms. Now if they fail this test and are less than 1.0E9 ohms, then they pass if they generate less than 100 volts as per ANSI/ESD STM97.2-2006 Floor Materials and Footwear- Voltage Measurement in Combination with a person.
Sorry so long for the response time.
Q2: Many thanks for you help.
What you are actually saying, if I understand it correctly, is that “if the combined resistance of an operator wearing whatever shoes over a a conductive flooring is greater than 1 x 3,5E7Ohms he will generate more than 100 Volts” and
currently in many electronic plants static generation above 100 Volts is not tolerated.
A2: No, that’s not what I’m saying. I’m saying, as per ANSI/ESD S20.20-2007, that if you fail the < 3.5E7 ohms test, you may pass the less than 100 volts test and still be compliant to 20.20
Look on table 2 of page 4 of 20.20 and you’ll see what I mean.
Let me know if that helps.
ADD: I guess what needs to be understood with 20.20-2007 is that the < 100 volts and the < 1.0E9 Ohms still stands as well. But if you’re testing per 97.1 and you get >3.5E7 ohms, then you can still pass 20.20-2007 if you have < 1.0E9 ohms per 97.1 AND < 100 volts per 97.2.
If you go to the table 2 chart on page 4 of 20.20-2007, it makes more sense.
Q: Do the heel strap cords need to be placed inside the sock i.e. between skin and sock or can the cord be outside the sock i.e. between sock and shoe? Is this specified in any ESD documentation? If so, which standard?
A: I like this question because I used to always put the strap between my foot (skin) and the sock, aka stuff it into my sock and under my heel. I later came to find that this wasn’t necessary.
I don’t believe it’s mentioned in any ESDA standards, perhaps in ESD SP9.2-2003 Footwear-Foot Grounders Resistive. But Foot Grounders are checked on a combo tester hopefully before you enter the EPA and if you pass and get a green light, you’re good to go. I’m seeing instructions to say the strap is to go “inside the shoe or sock”, so…
MIL-HDBK-263B Section 40.1.1 Personnel ground strap on page 100 says, “Personnel handling ESDS items should wear a skin-contact wrist, leg or ankle ground strap.” So this would imply skin contact, but I found it’s not necessary. If personnel wears cotton socks, the sweat and salt in our body make ESD shoes work, so thusly, the strap can go outside the sock and on the shoe’s conductive insole. Be sure to lay the strap across the heel and not just across the arch if you do it this way.
So just put the strap between your sock and shoe insole and use your combo tester or test yourself from palm to earth ground and see that you’re reading less than 3.5E7 Ω per ANSI/ESD STM97.1-2006 Floor Materials and Footwear. If you feel more comfortable putting it between the sock and skin, that’s fine too.
I find it to be more comfortable, convenient, and hygenetically sound to place it between the sock and insole as well. If there’s any mention of this to this detail elsewhere, it’s unbeknownst to me.
Q: Is it ok to wear ESD shoes or heel, toe, or sole grounders outside?
A: No. MIL-HDBK-263B Appendix 1 page 101 40.1.2 states, “Conductive shoes, shoe covers, or heel grounders should be used to discharge personnel on conductive floors. These items should only be worn in the ESD protected areas and should be kept clean so that contaminants do not inhibit their conductive interface with the floor.”
So, to protect your investment, for good house keeping and maintenance, longevity of the ESD personal equipment, just don them before going into the EPA and take them off when you leave.
Do this and keep them clean (vacuum inside of shoes weekly and maybe clean outsoles and grounders with soapy water once a week) and you’ll get more wear out of them and they’ll be more effective.
Q: In our organization, there is a lot of paperwork that accompanies the product. Is this harmful to the product due to static generation? I have measured all the documents surface resistivity and find it to be dissipative in nature. Our production environment humidity is controlled from 40%-60%. I also tried to tribocharge the paper but there is no static voltage generated. The funny thing is when I rub my plastic comb and put it near bits of small paper, the bits get attracted to the comb. So is paper really harmful to the semiconductor products that we manufacture for our customer?
A: Hello. It’s good for you to observe the possible generators of static in an EPA (Electrostatic Discharge Protected Area) and to remove all non-essential insulators and to ground conductors or soft ground them, as the case may be, and to use neutralization on isolated conductors and essential insulators (ionization). You may notice low static charge potential or voltage on that paper, but what happens when it tribocharges with other materials in the EPA? Read More