Resistance of ESD flooring

Hello,
We had non-ESD-voltage-generative flooring layed down in our electronics production facility. It is supposed to not generate high voltage ESD on people who walk on it...but it isnt the "ESD-dissipative" type of flooring, so in other words, it cant discharge somebody who walks into the room already-charged-up-with-ESD-voltage.
So anyway, we wish to measure the resistance of this flooring so as to define it in terms of its ESD prowess. Do you know how we can go about this?

Hi treez,

It's only a Google link about measure resistance of esd flooring but at first glance there may be a few short videos and online written resources that will point you in the right direction.

Thanks
https://www.youtube.com/watch?v=pqiydlMTF0s

..in the above video , he talks about the resistance of the flooring shoudl be between 25kohms and 35megohms...i presume he means resistance to earth ground. And i presume therefore that the floor tiles must all be on top of copper strip which is connected to earth ground?

Early in the video he says that the resistance is actually the resistance of the person, through the floor, to earth ground...so i am not sure how we measure that.

From what i gather, ESD flooring must have a copper strip connected to earth ground below it? If it doesnt, then its not ESD flooring?
Thats a lot of copper for our 5metre by 5 metre floor. -Gonna cost us a lot of money.

I would consider using the formula took from the following references for measuring sheet resistences (1, 2, 3) represented in a more general or simplified forms. Note that the 4.352 constant is present in all these equations, so all are based on the same geometrical assumptions; By the way, you should have to measure/estimate the thickness of the coating in order to obtain the expression of the resistivity in the classical form.

My experience of ESD flooring was at a large production plant (~50m x 30m) just north of Cardiff in South Wales. Investigations to find why static was causing problems were traced to several things but the main one was the copper mesh under the conductive flooring wasn't connected to anything! It went to a large connecting block which was supposed to be Earthed but the building contractor (it was a new purpose designed facility) had forgotten to connect it.

To be properly grounded, workbenches need Earthing and staff should wear conductive heel straps on their shoes, either ones with a tail that tucks inside the shoe or one that forms a second heel that fits around the back of the shoe.

Brian.

Thankyou, it’s a small world, I lived and worked actually in Cardiff (worked in Pentwyn, lived in Roath) but by then the production floor had closed and been sold off and got developed into flats. In fact, my dept also got closed down whilst I was there. Pretty horrendous day the announcement. 150 of us in a room, you could have heard a pin drop in there. Lovely place Cardiff though.

Anyway, thankyou, what you say is very interesting. It sounds to me that an “ESD floor”, in order to be classed as a true “ESD floor”, must actually have a hard wired connection to earth ground? This is our problem. We cannot find the earth connection either. All we see is what the “ESD installer” called the “ESD tiles”. These appear to be normal patterned lino tiles, (kind of like a bark-like wooden pattern) with what looks like a clear varnish finish to them. I suspect that if I prize one of these up off the floor, I will not see any copper sheeting or mesh underneath it. –This would kind of correspond to the fact that the “ESD floor installer” is no longer answering his phone to us. (I wonder if Del Trotter went into the ESD business.)
Years ago I worked in a PCB assemblers place in Redditch. I vaguely remember there was a “station” that we used to stand on and we would press a metal button on it, and a green light would flash if we were wearing our ESD shoes. I am wondering what circuit is being made here. Presumably its literally measuring the resistance from finger , through body, through shoe, through the footplate?

I’ve worked in a lot of places where PCB’s were being handled, but as far as I remember, in many cases the floor just appeared to be a “Normal” concrete factory floor. Maybe there was a copper lattice meshwork in the concrete…maybe not, I don’t know. I wonder how important it is to have a genuine ESD floor, with copper sheeting or mesh running completely underneath it? I wonder if a plain concrete floor makes a person generate a high ESD charge if one walks on it wearing so called “ESD shoes”. (or normal shoes with an ESD footstrap)?

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wow
Vermason surface resistance measure kit is 1000 pounds...
https://uk.rs-online.com/web/p/esd-test-meters/8772536/

The place I was working, as consultant, was a little further north, just the far side of Pontypridd from Cardiff direction.

The trick to ESD flooring is to realize you don't need to cover the complete are with copper conductors, one single wire every half metre or so in each direction is sufficient, the tiles do the rest for you. The 'drain' resistance can be quite high at any particular point but the underlying Earth is important. You have to consider that the floor sits between '0V' Earth underneath and just about every conceivable source of voltage from above. Making sure benches and machinery is well Earthed not only prevents them injecting voltage to the floor, it makes them 'drains' as well. Bear in mind that standard building construction, is to lay a reinforced concrete slab, cover it with a waterproof membrane ("damp course") then pour the final floor on top. The membrane makes an efficient insulating layer, even if it does have high capacitance from top to bottom.

Walking on concrete itself doesn't create a high voltage, it typically comes from friction from flexing and 'scuffing' of synthetic soles. If a conductive strap joins the person tot he floor, bridging the shoes, it solves the problem. That's why all anti-static shoes clips and strap-on heels have a tail that tucks inside the shoe and provides a resistive contact to their occupier.

As far as I know, all the commercial machines for testing personnel for working anti-static protection are just Ohm meters with some kind of good/bad comparator to alert the user. I don't think they actually check for residual charge. They all have a metal footplate for the person to stand on and something for them to touch by hand, I think all they do is check for conductivity between them.

I do know of one building where military aerospace parts are made that has a solid copper floor - I bet that was expensive but then money isn't in short supply for defense contracts!

Brian.

I do know of one building where military aerospace parts are made that has a solid copper floor

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Thanks, I cant find it now, but I had a discussion on edabord some months back where we considered the acceptable resistance of ESD mats etc. The conclusion from some edaboarders was that if the resistance of ESD mat/flooring etc is too low, then it can actually exacerbate ESD damage, because the low resistance means a high surge current spike from an ESD discharge. I’m not sure if this is true? The concensus seemed to be that the resistance to earth of an ESD mat or floor should be around 1 to 2 Megohms…….if any more or less than this then ESD protection is not optimised?
Another point is nature of the kit needed to measure the resistance of an ESD mat or flooring tile to earth ground…….it appears that normal cheap DMM cannot be used for this. This is said to be because the pointed probes don’t garner enough surface contact with the mat or floor tile. As such, large area “surface contact probes” are required as follows…
http://documents.vermason.co.uk/pdf/TB-7545.pdf
…these cost around £1000.
I think we could easily make one of these “surface contact probes“ using a sheet of aluminium with a dollop of clay on it to weigh it down onto whichever surface, and a 4mm plug attached to it so that we can connect it to a cheap $4 DMM?

The bigger danger of low resistance is to the user if they handle equipment with high voltage on it.
It boils down to what exactly do you want to test. There are specifications, as mentioned in the link you gave but they are to quantify the resistance, not how safe it makes it. You can use any sensible resistance without increasing or reducing the risk to operator or equipment, it isn't critical. Obviously, if you are working with mains powered lighting systems you want to use a resistance that limits the current through an operator touching the circuit so they are safe from electrocution. Equally, you don't want a resistance so high that it can't discharge 'normal' ambient levels of static charge. In general >1M and <20M should be adequate in a normal production environment as part of a good general protection regime. Remember that the idea is to prevent build up of a voltage rather than instantaneously discharge one.

Yes, you can make your own contact probes. They wouldn't be certified to meet standards unless you submitted them to a test lab but they should work perfectly well. The drawback to using a DVM (note a voltage is applied in the Vermason tester) is it may take a while to settle it's reading because of their high input impedance but it will eventually give a stable reading. Remember that you want to do two tests, one from probe to probe and another from probe to an Earth point.

Another point to note is how the floor is cleaned. A polished floor may have an insulating layer over it. There are static dissipative floor polishes on the market that are designed for assembly plants. I do not know of a supplier off hand, I've been working from home for several years and lost contact with staff in factories who I could ask.

Also check the type of floor tile. There are some with a marbled pattern where the conductive additive is only present in some of the colour. You might find different results by moving the probes only slightly over different part of the pattern but consider that a human footprint is probably big enough to ensure some part is in contact at all times.

Brian.

Thanks yes, our "ESD floor tiles" look like they are covered in a clear varnish layer, which surely must be highly insulating.....so i actually doubt they are ESD tiles at all.

Yes, you can make your own contact probes. They wouldn't be certified to meet standards unless you submitted them to a test lab but they should work perfectly well.

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Thanks, i wonder if we can just do a cheap esd tile resistance measurement method by putting loads of copper tape over the esd floor tiles, and then connecting to these two separate patches of copper tape with the normal , pointed DMM probes?

It will work as long as your copper tape doesn't have an adhesive backing and you can keep it in intimate contact with the tiles. If it has a sticky backing it will probably insulate it from the tiles.

You could, using any waterproof tape, make two small dams (a bunded area) on the floor and pour saline solution into them, it only needs to be a few mm deep. Provided your DMM probes are made of the same material so you don't make a battery, it should give you a good contact to the floor surface. Reverse the probes (at the DMM end) and check again to be sure. The resistance of the saline will be insignificantly small compared to the tile resistance.

Brian.

http://www.youtube.com/watch?v=pqiydlMTF0s&t=13s
..this video says that a esd floor can have a resistance of anywhere between 25kohms and 35megohms. That’s us blown away, because DMM’s that can measure up to 35megohms cost several hundred pounds, and no way will that be affordable.
I presume they mean the resistance bewtween say probes that are say 30cm apart or so....most videos show floor probes being about that far apart.

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Though Fluke 114 does up to 40megohms for £114, so with some home made surface probes, (using bounded salt water pools like Betwixt kindly described) that looks like our cheapeast way to test our esd floor.
Fluke 114
**broken link removed**

woops, the 35megohm figure is the resistance of the floor and the person together, so therefore i wouldsay that the floor is going to be less than 1 megohm, which means we can check our esd floor tiles with a cheap £5 DMM and some home made surface probes.
I am just going to check what is the resistance of a person wearing ESD static shoes.

i must admit i wonder how "ESD shoes" work, because you get them, with the yellow lightning icon on them, but the person's socks will insulate them from the shoe.....so unless they have some kind of tab to be put against the persons skin, inside the sock, then surely they are not going to work?

If your feet as like mine, socks become conductive after they have been worn for a few minutes! By the end of the day, I have to change the yellow lightning symbol to a bio-hazard one. :-D

ESD shoe straps are typically equivalent to wristband resistors at about 1M. They use carbon loaded plastic to make them conductive so to some extent the pressure on them (persons weight and stance) changes the resistance.

Try standing bare foot on two metal plates and measure the human body resistance between them, I think you will be surprised how low it is.

As I mentioned before, the idea of these devices isn't to be able to discharge an existing voltage, they are to prevent it building up in the first place so a slow drain through high resistance is quite in order.

Brian.

the idea of these devices isn't to be able to discharge an existing voltage, they are to prevent it building up in the first place so a slow drain through high resistance is quite in order.

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Thanks, though say a PCB worker goes to the toilet, and then comes back into the ESD sensitive area, then on their way back from the toilet they may have picked up a charge and so it would be best if they could discharge this when coming back into the ESD sensitive area?

Or does that just mean you have to have a highly conductive , earth connected "doormat" over which the person has to walk as they come back into the ESD sensitive area?

ESD shoe straps are typically equivalent to wristband resistors at about 1M.

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Thanks, though i am surprised they are not zero ohms, since the 1 Meg would be provided by the resistance from the floor tiles to the earthing point?

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Aah...the Tenma DMM, (£7.50) measures up to 200 megohms, so it will be fine for our esd floor measurement via pools of salt water to act as surface probes and dipped into with the tenma dmm's pointed probes.

Tenma 2-7770A DMM
https://uk.farnell.com/tenma/72-7770a/dmm-handheld-manual-3-5digit/dp/2499508?MER=mktBestSellerB

...I just can't work out why people are spending £1500 for ESD resistance measurmeent kits?

they may have picked up a charge and so it would be best if they could discharge this when coming back into the ESD sensitive area?

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The amount of charge they accumulate should be fairly small, it is mostly capacitance to ground which will only be a few hundred pF at most and assuming they don't remove their shoe strap it will disappear within a few seconds.

Brian.

thanks, so when they walk back on to the esd floor, the charge will disappear i take it.

Hi,

Sounds like a sort of inverted dodgems car for footwear, instead of powering from above, it drains from below or something.

thanks yes good analogy....but in fact, we are thinking of having one of those "stranded door thingys" whereby we have metallic bead danglers coming down over the doorway, and these are earthed, and people have to touch them as they go through them, and then they are discharged as they walk into the esd protected area.

the "thingys" which i refer to are like those things people put over a doorway in the summer to stop the wasps coming in, but allow the door to still be open....you just kind of separate a gap through the danglers as you go through it......i am sure you know what i mean.....we would have these danglers earthed.....cost a couple of quid instead of a couple of grand which is what the esd companies charge for their merchandise.

The amount of charge they accumulate should be fairly small, it is mostly capacitance to ground which will only be a few hundred pF at most and assuming they don't remove their shoe strap it will disappear within a few seconds.

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Thankyou, sorry to keep asking, please dont answer if youd rather not. At what point do you believe they will dischareg the charge they have built up after coming back to the esd area from say the toilet etc?......when they put on the wriststrap which is attached to earth on their bench?....for us, that is too late, we want people to be discharged as they come into the esd room.... either via the flooring or something else.

That is, if a charged up person comes into the esd protected room, then we want them to be discharged before they get to their work bench. So we need a good, highly conductive esd floor

Hi,

for us, that is too late, we want people to be discharged as they come into the esd room.... either via the flooring or something else.

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Use an earthed (with series resistor) metal door or at least a metal door handle....

.. or make the people to press a metal earthed (with resistor) push button when they enter the room.
The button could just show a OFF-delayed LED, maybe for 5 seconds, so that every other in the room can verify that he pressed the button.. and discharged himself /herself.

Klaus