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.
�
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.”
�
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

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

Static Dissipative shoe C4341

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?

ESD Dissipative / Cleanroom Paper

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?
Continue reading…

Q:  We have a number of carts, racks, and chairs in the clean room. When we do our weekly, monthly and semi-annual checks for ESD and grounding, do we do a statistical sample or do we check all of each item. Is there a standard that explains this?

A:  You ask a very valid question and it just so happens that I received a similar question on this not long ago.

I wish there was one and only one ESD Bible that’s all inclusive and complete; from design to audit. The ESD Association has the ANSI/ESD S20.20-2007 document which gives us guidelines in establishing an ESD control program. But as far as a standard explaining when to audit each and every ESD technical element, I have yet to find a good source. The tables contained within 20.20 will give you “required limits” for various technical elements, but not a frequency of when to audit or test these systems. Between JEDEC Standard 625-A Table 2 of page 9 and other sources, I’ve put together these recommendations, but it’s up to you to implement them and tailor them to your unique processes;
Continue reading…

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.