ESD and Static Control Questions

Q:

Are ESD shoes and Conductive shoes the same thing?

A: There are two types of ESD shoes, Static Dissipative and Static Conductive.
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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, C6.4.7.5.1: “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.”
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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.
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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.
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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.
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I hope this answers your questions.  Please comment.

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Thank you very much, Pat

Static Conductive shoe C4327 Resistance per ANSI/ESD STM97.1-2006

Static Dissipative shoe C4341

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 < 1.0×10exp9ohms, 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.
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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…”
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This is what is said about seated personnel:
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“When personnel are seated at ESD protective workstations, they shall be connected to the grounding / Equipotential bonding system via a wrist strap system.”
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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.�
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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.
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 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: 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
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Look on table 2 of page 4 of 20.20 and you’ll see what I mean.
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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: Into our manufacturing area we have a dissipative floor installed, some areas has lost the dissipative properties, we are using ESD wax to correct those areas.

We are not having good results, we measure point to point and point to ground and these areas still measure insulative, my question is:
The ESD wax only works in floors with dissipative properties, or could it be applied on areas where the floor has lost the dissipation properties?”

A: Thanks for contacting us.  This is one of those questions that I like to pounce on, because it brings up many current misconceptions in the ESD flooring industry.  I’m going to answer your question about using chemicals to perhaps patch up certain areas in your flooring system, but I’m going to reach beyond that question and give a comparison of the ESD flooring system vs an otherwise non-ESD flooring system with an ESD sealer applied to it.  We supply both options here, by the way.  I hope you find what you need here and that this response helps guide you in your application.

Continue reading…

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.
Continue reading…

Q: What is the difference between Low static 3.5kv carpeting and static dissipative carpeting? When used on walls is 3.5kv carpeting ok in electronic equipment rooms?

A: When people refer to 3.5 kV carpeting I believe they are referring to the threshold voltage that people can feel as a nuisance static shock. We deal primarily with manufacture, test, assembly, and application environments where the end-user is protecting expensive electronic components, explosives, assemblies, etc. and the threshold for their needs is down to 100 volts and less.

Q: Some ESD material suppliers claim that Copper Mesh Grid is not required in case of two layer flooring. Is it correct and if so, why only in case of three layer?

A: This question may be off topic. It sounds like you are talking about ESD flooring, not bench or floor matting. If that’s the case, some manufacturers of esd flooring make a conductive backed tile or sheet good. This backing may be so conductive and along with a conductive esd adhesive, they claim that you don’t have to lay down an expensive copper grid. That’s fine. But if I’m installing the floor, I’ll use copper (or aluminum if requested) tape and run a standard grid the length of the room (along the x-axis) and cross it up (y-axis) so as to ground the floor at least once every 2500 square feet or a couple of times per room minimum. I’ve found that this helps prevent hot spots from tile to tile or gives more consistent RTT (Resistance Top-to-Top or Point to Point). One roll of copper tape would enable me to ground a room that was 60’ x 40’ without a problem. If I have more tape to use, I’ll use it. A liberal amount of copper tape and ESD adhesive is provided free of charge with the purchase and installation of an ESD floor from Ground Zero Electrostatics, Inc.

Copper mesh grid just doesn’t apply to 2 layer or 3 layer mats. They use ground cords. One per every 10 feet, I believe.

Q: What is the difference betwen two layer and three layer ESD PVC Mat, and which one will be advisable for flooring?

A: We have a variety of bench and flooring mats and runners. The two and three layer mats would generally refer to our bench or table mats. Our Duro-Stat line is actually a homogenous vinyl ESD matting with great mechanical and electrical properties. But most BM’s are of the two-layer or three-layer type. The top layer would give the mats its resistance to chemicals, resistance to solder, flux, and the ability to keep it clean. The backing would typically promote an anti-skid and durable surface. The three-layer mats are going to have a conductive scrim center layer and work well with most wrist strap constant monitors.

For floor mats and runners, we’ve got our UltraCon Floor Mat which is made of highly conductive rubber. Your flooring mats and runners are going to be typically homogenous and don’t have layers per say. Our Tough One! line is made of embossed homogenous solid vinyl. The exception being our Anti-Fatigue II line, this static dissipative mat is a vinyl mat with a foamed vinyl backing. Our No-Slip II is made of a corrugated slip-resistant vinyl.

Q: What is the difference between anti-static and static dissipative floors? Which floor will be better to use if I’m going to have an electronic assembly line?

A: Anti-static is not the proper term to use for ESD flooring.

Anti-Static refers to the ability to suppress charge generation or the prevention of static build up. Anti-static materials will not safely attract or decay a static charge before it randomly discharges. Anti-static material is usually indicated by an electrical resistance range, measured in ohms, of a minimum of 1E10, (10 giga ohms), to a maximum of 1E12, (1 trillion ohms).

ESD flooring systems are referred to as static conductive (more conductive) or static dissipative (not as conductive, but will dissipate charges in an orderly fashion).

Static dissipative floors: 1E06 Ω or 1 Meg Ω to 1E09 Ω. I would consider SD material to be the minimum requirement per ANSI/ESD S7.1-2005 via ANSI/ESD S20.20-2007. If you choose this flooring system, the RTG readings may be at the low end of the scale and be in the E06 to E07 range or it may be at the high end and be in the E08 range. You want to keep you flooring system clean and always below a gig ohm.

Static conductive floors: 2.5E04 Ω to 1E06 Ω. These floors are the superior choice for an assembly environment and offer the lowest charge generation and quickest charge dissipation. These floors require proper cleaning and maintenance, but will likely exceed the requirements for 20.20 throughout its lifetime.

I’d recommend a static conductive flooring system for your application. You are dealing with ESD sensitive components, raw boards, and/or sub-assemblies that have a low threshold voltage tolerance. With a static conductive flooring system and proper ESD footwear, you will have an optimal ESDS area.

Q: We always face a problem of selling your anti-static/dissipative vinyl floors to our clients. What is the solution for client satisfaction if they want to make sure the vinyl floor is anti-static or not and if the vinyl floor works as a dissipate of current.

A: The ESD vinyl flooring that we sell, service, and install is typically manufactured from a permanently conductive, non-humidity dependant blend of conductive additives interspersed with a pure virgin solid vinyl.

Our tile is specified as static conductive (2.5E04 Ω to 1E06 Ω) or static dissipative (1E06 Ω to 1E09 Ω). The term antistatic typically refers to a range of resistance outside of industry accepted standards for ESD flooring. There are some commercial and residential applications where antistatic flooring is acceptable but would be outside of the range of Static Conductive or Static Dissipative. Antistatic often refers to products used in packaging or where materials that resist tribocharge but aren’t necessarily conductive enough to bleed off charges to ground in a timely fashion.

Attachment of a typical GZ flooring system to an existing substrate is typically accomplished via GZ-C2000-4R releasable liquid conductive fiber-loaded adhesive. There are a variety of adhesives for various types of application. Grounding of an ESD flooring system is comprised of appropriately placed copper grounding tape running beneath flooring overlayment adhesive and attached to primary electrical building grounds located throughout the facility, as well as installation of GZ- ground plates. The primary electrical building grounds serve as the Common Point Ground or are bonded to the CPG for the ESD flooring (ESD technical element) and comply with ANSI/ESD S6.1-2005 as per ANSI/ESD S20.20-1999.