ESD problem solved by using aluminium covered floor tiles?

Hi
Supposing an electronics assembly worker walks towards their workbench.
They first walk on a non-ESD safe carpet which gives them a high voltage bodily charge and
so makes them an ESD danger to the electronics components. However, around the bench, the floor
is covered with tiles covered with aluminium foil which is connected to earth via 2
MegOhms of resistance. Therefore, presumbaly when they walk over this aluminium flooring, the
high bodily charge they possess drains away?
That is, before they have even put on their wrist strap, the charge has drained away?
...Because their will be a conductive loop from their body, down their left leg, then along the
aluminium flooring, then up the right leg......as such, this loop will conduct away the bodily charge
and dissipate it?

You seem to be forgetting that some, possibly many, shoes are non-conductive. Especially those of vegan employees.

Something tells me that you are over-thinking the problem.

Static-safe carpet is the more usual precaution I think. Wrist strap (with regular testing) is sufficient for many workshops.

How will you assure H&S that your aluminium tiles never accidentally connect to hard earth and become a shock hazard?

Just my thoughts from out-of-date workshop experience - feel free to ignore

Wrist strap (with regular testing) is sufficient for many workshops.

Click to expand...

Thanks,
As you know, for static charge to dissipate, there has to be a circuit loop in which it can flow.
If a totally insulated person who is "charged up", then puts on a single wrist strap, where is the loop of current that will flow to let the charge "discharge"?

The wrist strap should be connected (via suitable resistor) to earth. That is where the controlled discharge happens before work starts.

Admittedly there is a possible risk of discharge before wrist-strap is worn, which is why, in a perfect world, all static sensitive equipment is kept either within it's chassis, or within static shielding bags until ESD precautions are in place.

I describe the perfect scenario, not necessarily a real-world scenario.

I suspect that LED luminaires and switch mode PSUs are less ESD sensitive than your average devices. Even PIC chips are pretty robust. Having said that, better to over-do ESD protection than under-do it

Thanks, the thing is, when wearing a single point wrist strap, where is the loop for the charge to flow in as part of the discharge process of the static charge?

Loops are for charge-neutral environments. You are
looking for charge -equalization- and that can happen
along any conductor. The return path is the universe.

Equi-Potential bonding is the technical term.

Equipment does not care what potential it sits at, so long as it never sees significantly different potential.

In the real world, "earth" is the simplest potential to choose as reference, even though nobody knows what absolute potential earth sits at relative to the cosmos. For us, we just define it as zero volts in ESD terms.

It could be that a worker has a wrist strap that discharges their body to earth potential (lets call earth zero volts for argument) but a board under repair/development could be sitting at thousands of volts above or below earth potential. When the worker touches the board, components may discharge slowly through wrist-strap resistor, hopefully without damage, but to be really sure, put the board on a static-dissipative mat to prevent static charge build up in the first place. I know from earlier posts that you are already doing this.

In my experience, anti-static equipment is pretty effective. The problem lies with convincing humans to even bother with wrist-straps and other precautions.

Somehow the equipment mostly seems to survive, though there will always be arguments over the importance and effectiveness of ESD precautions, with the added complication that components might be degraded by static damage, rather than failing completely and obviously

Loops are for charge-neutral environments. You are
looking for charge -equalization- and that can happen
along any conductor. The return path is the universe.

Click to expand...

Thanks,
So in the environment with the aluminium floor tiles that are connected to earth via 2 MegOhms, and the resistance through the soles of their shoes being 100kOhms, the person should not even need a wrist strap?

treez said:

Thanks,
So in the environment with the aluminium floor tiles that are connected to earth via 2 MegOhms, and the resistance through the soles of their shoes being 100kOhms, the person should not even need a wrist strap?

Click to expand...

As long as nobody puts an earthed chassis on the floor, and as long as nobody wears any kind of shoe other than a 100k Ohm shoe, everyone will be safe and the equipment will be ESD free.

.... or wear the wrist-straps just like everyone else in the industry does....

As long as nobody puts an earthed chassis on the floor,

Click to expand...

Thanks, presumably you are talking of electric shock hazard here?

Yes 100%

And you would be liable....

An earthed aluminium floor would be akin to a wet floor as far as electrocution hazard goes

You are already aware that the ESD mat on the bench has distributed resistance to prevent mains voltages reaching a nearby technician. A floor would have the same requirements, I would imagine.

... But I am no expert... so I could be wrong. You would not catch me taking the chance though

Look at it this way: You make an all metal conductive floor and charge it up to 100KV, on it you place a workbench and on the workbench you place your static sensitive devices. Will they be damaged by touching them?

(answer - in case you haven't worked it out is no, because they are at the same potential as their surroundings and the person touching them)

It is potential difference between objects that causes the static problem, the action of the potentials equalizing as they connect together is what causes damage. Regardless of absolute potential, even if you could quantify it, if the parts are all at or close to zero relative to each other there is no problem.

The discharge, unless you have a serious electrical fault, is only that of the charge difference between touching objects. That basically boils down to voltage difference and the ability of the bodies to hold charge, in other words capacitance (q=cv). Body capacitance is usually quite small and in most cases there will be leakage paths that keep it discharged. If you take reasonable steps to maintain a discharge path, ie. a wristband or shoe strap you will not have any problems. In the UK where humidity is normally quite high, even the atmosphere keeps charge levels low.

As Hexreader states, you really are expending your efforts on chasing ghosts, the time proven solutions to a trivial problem are all you need.

Brian.

As Hexreader states, you really are expending your efforts on chasing ghosts, the time proven solutions to a trivial problem are all you need

Click to expand...

Yes i know what you mean, but when you are busy, and constantly going to and from the bench multiple times each day, its easy to once forget to put the wrist strap on when touching something...and zapp...damaged.
With a conductive floor youre always stood on it and thus connected to it.......it would be connected to earth via 2 megohms so no electric shock hazard.
I dont think anyone would put an earthed chassis on the aluminium floor, none of our production process's allow that.

An earthed aluminium floor would be akin to a wet floor as far as electrocution hazard goes

Click to expand...

Thanks....the alu coated floor is earthed via 2 megohms.

Another point is that when you are constantly taking on/off/on the wrist strap, they eventually rip............this is because they have to be tight to work properly, and thus they need stretching out to put on the wrist, and eventually they break or become non-elastic and wont tighten to the wrist.

- - - Updated - - -

Look at it this way: You make an all metal conductive floor and charge it up to 100KV, on it you place a workbench and on the workbench you place your static sensitive devices. Will they be damaged by touching them?

(answer - in case you haven't worked it out is no, because they are at the same potential as their surroundings and the person touching them)

Click to expand...

thanks, so you are assuming that the legs of the workbench are conductive and the components are not on high resistance (though still "ESD safe") materials?

Also, we've been criticised by our contractors for wearing wrist straps when testing PCBs connected to raw mains..but dont see how we can get round this one......we need to be ESD safe...we dont want to damage what we are testing. Do you know how to mitigate danger here?

thanks, so you are assuming that the legs of the workbench are conductive and the components are not on high resistance (though still "ESD safe") materials?

Click to expand...

Your workbench is supposed to be grounded via 1M already.

https://www.rapidonline.com/bjz-c-204-001h-esd-conductive-heel-strap-one-size-blue-51-8534
(But don't forget to take your feet with you :lol: )

You can buy testers to make sure they work but in truth all they are is a metal plate to stand on and a handle to touch with a resistance tester between them.

Brian.

I see two problematic points.
1. You are reporting parts of the floor being covered with aluminium foil but no overall ESD flooring.
2. Apparently no mandatory ESD shoes.

Consider that components and modules are handled at work benches but also transported through the room. Wrist straps are useless in the latter case.

Consider that components and modules are handled at work benches but also transported through the room.

Click to expand...

Thansk yes, we dont have overall ESD flooring, we just put PCBs into the pink plastic ESD bags when we transport them from place to place.

Reply #15 on this thread might give pause for thought:

https://www.eevblog.com/forum/chat/does-you-soldering-iron-have-grounded-tip/

Seems like overkill to me, but it shows mat connection (with no real detail). I have no comment to make regarding validity.

Hi,

Thanks, the thing is, when wearing a single point wrist strap, where is the loop for the charge to flow in as part of the discharge process of the static charge?

Click to expand...

You may see it like this:
* The person is one electrode of a capacitor.
* the other electrode is the floor, the walls, any other - even low conductive - material.... with any (low conductive) connection to earth ground.
* the dielectric between the two electrodes is the air, the shoes, the isolating floor...

--> then the wrist band between earth ground and the person closes the loop.

Realistic or not ... it may help to understand the voltage potential, current flow and loop problem.

Klaus