ESD Mitigation - Floor mats or conductive floor?

[Icy Mountain]

I need some expert ESD mitigation assistance. We have recently discovered that our production facility’s concrete floors were sealed with a conductive epoxy when our building was constructed. The floor taken 15 years of wear. A measurement to earth ground (or from any two points) from anywhere on the floor yields a 1x10^8 ohms (100,000,000 ohms or 100 MegaOhms).

We use a variety of benchtop mats that measure from 10^7 to 10^8 (10,000,000 to 100,000,000) ohms to earth ground. Combined with these is a wrist strap system that is 1 Meg from wrist to mat.

We use a variety of floor mats that measure from 10^5 to 10^6 (100,000 to 1,000,000) ohms to earth ground. Combined with these is a heel strap system that is 1 Meg from ankle to floor mat.

We have several areas where the floor has conductive paint whose measuremnet to earth ground (or from any two points) from anywhere on the floor yields a 1x10^5 to 5x10^5 ohms (100,000 to 500,000 ohms).


Supposedly, the use of the floormat/heel strap allows a person to work safely on a product on the bench mat without the wrist strap. All of the mat and floor measurements have been taken using the same ESD surface test equipment.

Summary:

Bench mat system 11Meg to 101Meg from wrist to earth
Bench mat system 10Meg to 100Meg from any product on the mat to earth.
Floor mat system 1.1Meg to 2Meg from ankle to earth.
Painted floor 1.1Meg to 1.5Meg from ankle to earth.
Epoxy floor ~101Meg from ankle to earth.

Question:
Do we really need conductive floor mats?
Is it a correct assumption that product on the mat can be safely manipulated while wearing only heel straps on the floor mat? Painted floor? Epoxy floor?
Is it a correct assumption that product at earth ground can be safely manipulated while wearing only heel straps on a floor mat? Painted floor? Epoxy floor?

[Al Rosen]

You can find the answers to your questions in ANSI/ESD s20.20-1999, available free of charge at www.esda.org.

[Icy Mountain]

Al. I knew somebody here could point me in the right direction.

[Marc]

Any help with replies here would be nice, too.

[Al Rosen]

Originally Posted by Marc

Any help with replies here would be nice, too.

In a nut shell:
Where unprotected ESDS devices are handled, a grounded static protective worksurface with a resistance to ground of less than 10^9 Ω must be used.

Grounded flooring or floor mats are only required when personnel or mobile ESD protective workstations utilize floor grounding methods.

Each person handling or within twelve (12) inches of unprotected ESDS devices must be grounded using either:
a) Wrist straps that:
1) Provides a continuous electrical path from the user directly to ESD
ground.
2) Have an integral resistance at the wrist band end of the grounding wire that will limit current to less than 0.5 mA through that specific path to ground at the highest power supply voltage that may be encountered.
3) Be worn by operators handling unprotected ESDS devices when seated.

b) ESD protective footwear (heel straps, toe straps or shoes) that:
1) Provide a continuous electrical path from the user directly to the ESD protective flooring or floor mat.
2) Be worn on both feet.
3) Limit current to less than 0.5 mA through that specific path to ground at the highest power supply voltage that may be encountered.
4) NOT be relied upon for grounding of seated personnel.

[David Hartman]

Yes, relying upon the epoxy and/or painted floor is adequate. The foot straps are adequate as long as the strapped foot (feet) remain on the floor. If an operator sets down they should either be setting on a grounded (via chains, straps, etc.) chair, or implement a wrist strap.

These are the conditions we are currently working under - although we do implement the use of ESD floor mats where an operator is going to be on their feet much of the time in a specific area (for operator comfort, not additional ESD control).

[Icy Mountain]

Thanks for the replies in plain English. What I was really concerned about was a situation were the resistance to ground was 100 times larger from the product to ground than the person to ground. It doesn't look like it matters as long as:
a) "Ground" is less than 1 ohm from "Earth Ground"
b) Person to "Grounding System" is < 35 x 10^6 ohms
c) "Grounding System" to ground is < 1 x 10^9 ohms

We use the painted floor for low resistance to ground in our automated electronics assembly area since product must be handled without Faraday cage protection to get it from the line into the protection.

The floor mat usage developed when people wanted to stand and work on the final assembly lines without being tethered at the wrist. At that point, many years ago, we were not measuring or testing our mats, floors, or straps. Here people are removing PCBs from protection and placing them in final product that is resting on a protected surface.

Now that we regularly test mats, floors, straps, etc. for compliance, the conductive epoxy sealer was discovered on the concrete. The only way to know is to test because this was not documented in the building specifications nor was anyone aware that such a sealer had been applied. It looks like we've just freed our work force from the wrist strap without spending $200 a pop for 4'x8' floor mats or conductive paint at ~$1 per square foot (over about 15k square feet).

We have wrist straps and heel straps (plural, both feet) that are tested daily on everyone in the production area. They also wear semi-conductive smocks and protective eyewear. Every bench has an anti-static mat connected to ground with a wrist strap port. Every cart has a metal drag strap and is made for electronics handling. Every PCB is in some sort of Faraday cage, either in an anti-static bag or in an anti-static tote with lid, unless it is being transfered from production line to storage or storage to production line. Unplugging everyone that is standing (just not necessarily on a mat) is going to make some folks very happy.

[ralphsulser]

I used to work for a company in Marietta, OH that manufactured electrostatic dissipative mats sold to U.S. Government for Navy Nuclear subs.
Each roll was 100% tested and the Gov't. inspector came in from Columbus, O to check each shipment and documents of test results. Company name is RJF International (formerly a B.F.Goodrich plant bought by a private owner)
Good company.