Why was i electrocuted?

[c_mitra]

Sorry I was not clear enough.

i just cant see how its going to get induced up to 1000's of volts

Suppose your body is charged (and you are wearing insulating shoes) and you touch a part of the circuit.

Because of the low impedance connection to the earth, a large current flows via the circuit components (to earth). This current induces potential differences between parts and that is the primary cause of all damages.

If there is no connection to the earth, the circuit board (say unpowered) is raised to a high potential; that itself is not damaging. The whole board will be floating.

If there is a high impedance connection to earth, the high voltage will dissipate slowly and the current will not induce large potential differences. (it is the potential differences that need to be tackled)

You feel the shock only when the charge on your body is discharged (or vice versa) but you do not feel anything while you carry it along...

(I hope I am little more clearer now)

 

[treez]

Thanks,
May i speak about ESD damage to the circuit board here..
Supposing i have an ESD mat with an unpowered PCB on it, and i am worried about ESD damage to that PCB..
With the "conductive mat", if i am charged to 1000s of volts and touch the "conductive mat", then the potential discharges through the 1Meg resistor in the press-stud earth plug...... or if i am using a "dissipative ESD mat", then the 1000's of volts again discharges through the press-stud earth plug......but just "saw" greater resistance on its way to earth......i dont see how the "conductive mat" presents a greater ESD hazard than the "dissipative mat" to the circuit board thats resting on the mat.

I do realise that posts #12 (kindly sent by C_Mitra) and #18 (kindly sent by Ads-EE) and post #33 (kindly sent by Klaus) have answered this in kind.....but our "conductive mat" is not like a sheet of metal, it is maybe several kOhms per centimetre...so its not like a short circuit.

..the thing is, when I am wearing the ESD wrist strap, which is press-stud connected to the “conductive ESD mat”, then my body would not be able to float up to 1000’s of volts anyway, because I would be connected to earth via the Wrist-strap—Conductive ESD mat---press-stud earth plug.

I would have thought that the “conductive mat” would better ensure that my body did not get induced up to high static voltage, because its got a lower resistance than a typical “dissipative ESD mat”?

Post #33 states that "dissipative ESD mats" have a conductive inner layer anyway, so as to conduct the charge to earth expediently....so it appears that this high conductivity property is advantageous?

Post #33 kindly tells of the danger of what could happen if a PCB say was carried to the “conductive mat” and then placed on it…but the thing is that we only ever carry PCBs in that pink plastic ESD stuff, and so the PCB would not see the ESD discharge of our body through the mat……and once we are wearing that ESD wrist strap which is press-studded to the mat, then there surely is absolutely no ESD risk…..in fact, once the wrist strap is ON, there is surely less likely-hood that our bodies would get up to 1000s of volts with the “conductive ESD mat”, than with the "dissipative ESD mat"?

 

[c_mitra]

With the "conductive mat", if i am charged to 1000s of volts and touch the "conductive mat", then the potential discharges through the 1Meg resistor in the press-stud earth plug...... or if i am using a "dissipative ESD mat", then the 1000's of volts again discharges through the press-stud earth plug.

In the first case you will not get a shock (and yes, you will feel a shock in the second case). In reality, it is not 1000V but 10-20 kV (that we are concerned)- you can see sparks flying in a dark room.

If you touch the PCB after you have discharged the charge on your body, there is no problem of ESD. In addition, I believe that individual components are far more susceptible to ESD (compared to assembled PCBs). Assembled PCBs have many more electric paths (relatively low impedance) to ground and it is rather unlikely that an individual component will see suddenly a large dV/dt because of high discharge current for a couple of microsec. Perhaps others can enlighten better on this but an assembled PCB is far more robust in my opinion.

 

[betwixt]

Like others., I have to reiterate, it isn't the actual voltage that matters, it's the potential difference that causes the problem. If you want to take ESD precautions to the extreme, buy a voltage field probe and be very worried about what it tells you. They are hand held voltmeters with very high impedance wire or flat plate connections, they tell you the voltage between your hand and the probe.

Just walking around the room will reveal many KV can build up between your body and the floor, mostly from friction between shoe soles and the floor material. Normally we are completely unaware of the charges and they discharge through direct contact or tiny arcs without us noticing anything at all. One of the interesting things you might spot is the voltage on door handles, they are usually well isolated from discharge paths and can be left at high voltage by the previous person using them.

Consider this scenario which I experienced while investigating a poor production yield:
It is a construction factory turning out thousands of PCBs for a well known multi-national brand, every conceivable ESD prevention measure has been taken but the component failure rate is higher than predicted. The problem is traced to a hand assembly station where connectors are manually soldered to an otherwise all SMD board. Prior to this stage, ATE equipment has fully tested the components and functionality of the board so it is deemed to be working, when actually powering it up, it doesn't work. The problem turns out to be the operator doing the assembly work, they are grounded through a standard wrist strap and they are working on a metal bench which is earthed which has on it's top an ESD dissipative mat with 1M leak resistor to ground. The PCBs arrive in black conductive plastic racks, the kind with slots to keep the boards apart and to allow them to be carried around. Everything looks good. The damage was caused when the operator removed the board from the rack, they were grounded but the rack wasn't so whichever board they pulled first was damaged by the discharge to the front edge of the board where they gripped it. From then on, the rack was discharged and the remaining boards were fine. The fix was to add an additional 'ground--1M--alligator clip' and instructions for the operator to clip it to the rack before unloading any boards.

Brian.

 

[ads-ee]

A conductive mat is a bad choice because it conducts charge too quickly. The reason for using a static dissipative mat is so the charge can slowly move off the electrostatically charge object to leave the object at ground potential.

One of the procedures to avoid ESD damage to boards, where I work is a PCB must be in a ESD bag and it must be placed on the ESD mat work surface a few seconds before the ESD bag is opened. The quality group actually did measurements to verify that this would avoid problems with ESD damage. Walking across the lab with a PCB in a ESD bag could easily build up 20K of charge and pulling the PCB out of the bag and setting it on the table resulted in that 20K discharging though the PCB. Clipping on the ESD wrist band before opening the ESD bag would improve the situation, but it was determined that setting the ESD bag on the ESD dissipative mat (while putting on the ESD wrist strap, gives that few seconds of sitting on the mat) was the optimal solution.

 

[treez]

The damage was caused when the operator removed the board from the rack, they were grounded but the rack wasn't so whichever board they pulled first was damaged by the discharge to the front edge of the board where they gripped it. From then on, the rack was discharged and the remaining boards were fine.

Thanks, i think we will have to firm up our own ESD protocol. No doubt the operator wrongly thought that the PCB being in a conductive plastic rack, which was on top of an ESD mat, would mean it was safe from getting induced up to high voltage, but not so.
When i worked at one CEM we all wore ESD overcoats all the time, and also we used ESD footstraps. There was always debate about whether ESD footstraps were needed on one foot or both.

 

[ads-ee]

Thanks, i think we will have to firm up our own ESD protocol. No doubt the operator wrongly thought that the PCB being in a conductive plastic rack, which was on top of an ESD mat, would mean it was safe from getting induced up to high voltage, but not so.
When i worked at one CEM we all wore ESD overcoats all the time, and also we used ESD footstraps. There was always debate about whether ESD footstraps were needed on one foot or both.

I think you missed the point the rack was probably a wheeled cart thingy, which was not placed on the esd mat. It was rolled over to the workstation by someone else and the operator grabbed the board (not in an esd bag) from the rack and equalized the charge between the rack and the operator through the board. If they had grabbed the rack first or clipped the rack to the esd mat then the issues would not have occurred.

I get the impression you don't quite understand esd all the well.

 

[treez]

Thanks, though i think Betwixt is talking about one of these?
https://www.google.co.uk/search?q=p...hUD2xoKHdN6BBoQ9QEIRDAA#imgrc=6u9K0dd0rAPaSM:

 

[betwixt]

Actually one of these or one very like it:

They were loaded on 'low loader' carts and wheeled to the assembly station where they were unloaded and stacked on the floor beside the operator. The operator was probably at Earth potential but the rack and all it's contents were charged up, at least until touched!

Brian.

 

[treez]

Thanks thats interesting, i wonder if they would have benefitted from ESD floor mats. Floor mats on which to place the PCB racks...then maybe they wouldnt have needed to clip the alligator clip to the rack(?)

 

[betwixt]

Possibly but the managers didn't foresee the problem and the cost of retro-installing large floor mats might be prohibitive.

I designed an award winning ESD detector back about 20 years ago. It was in conjunction with a university who were looking for first year projects for students so they got the award not me :-(

Most industrial ESD detectors work on the principle of high impedance voltmeters but for this project I tackled it a different way. The device was very simple, a small neon lamp wired between the frame of a cart and a conductive strip trailing along the floor beneath it. The neon would flash at around 70V and discharge any static build up but the intention was not only to give that protection but to see how often it occurred. The neon flash itself was too weak to check optically but the magnetic field it produced was easy to detect and use to trigger a counter. So all it consisted of was a neon lamp, lots of turns of wire around it, an amplifier and a digital counter. When the neon gas ionized, the current induced a voltage in the coil that incremented the counter. It worked well and cost pennies.

Brian.

 

[treez]

Sounds interesting, reckon we could use one in our place...do a count now, then put in more ESD controls and see if we have improved it. We have had our behinds booted very firmly by a big ESD problem.
One thing that we would like to own, is a super-ESD-sensitive circuit board that breaks and stops working at the most minor incidence of ESD. Say just flash a LED but stop flashing it when its broken due to ESD. I wonder if having small signal FETs ( eg SOT323, and not tying their gates to ground with a 10k resistor) wouLD be good on it, because surely little FETs GS junctions are the things most sensitive to ESD damage?.......we could incorporate the sensitive component(s) into the circuit such that the LED stopped lighting when ESD damage had occurred.

What do you think are the components, or methods of connection, that render a component most susceptible to ESD damage?

Presumably tiny little (3mm x 3mm) 48 pin microcontrollers must be extremely ESD sensitive?

Also, some opamps dont have ESD protection diodes in them, but which ones?

 

[betwixt]

A floating gate tells you nothing because it could cause any conduction at any time and never see a difference from outside influences.
The trick is to use a device with extremely small input current and tie it to ground through a very high resistance, maybe 1G Ohm or more. It might be possible to add protection but obviously you need to be sure the protection circuit itself doesn't influence the readings.

Construction has little to do with ESD susceptibility and smaller components tend to suffer less because it takes a higher electrostatic field potential to be dangerous across a smaller distance. For the most part, it isn't the device itself that gets the discharge, it's the tracks and other connected components that conduct it to the device.

ESD diodes are generally present in op-amps between the input pins and negative supply and input pins and positive supply. They are there to dump excess voltage into the supply rails where presumably there is more circuit to absorb it safely. Devices with inputs that can exceed supply voltage generally don't have ESD protection, for example the INA117 which can handle 200V while running on a low voltage supply.

Brian.

 

[treez]

sorry i meant that the circuit board containing the FETs would be carried around the production area unpowered so as to try and give it ESD damage...then power it up so as to see if the ESD damage has occurred.
I definetely remember years ago people always said that CMOS devices in their old form were very highly susceptible to ESD damage.....if we could only get hold of some of those CMOS devices....we could incorporate them into the circuit in some way, such that if they were destroyed by ESD..the circuit would not work any more.

 

[betwixt]

You are proposing a sacrificial system, I wouldn't advise it. You have to rebuild the circuit each time and be absolutely sure you didn't damage it yourself in construction. Besides, you could probably throw a box full of unprotected CMOS devices across the room in a thunderstorm and they would still survive unscathed. The ESD warning is to help you avoid damage through mishandling, the devices themselves are pretty tough.

It is worth noting that in the UK climate where humidity is high all year, static build-up isn't too much of a problem. In countries where the air is dryer the risks are higher. In the US mid-West where I used to live and work, it was a serious problem, particularly in winter months. Perhaps you should locate here to one of the wettest areas in Europe. We have a saying "if you can see the mountains its going to rain, if you can't see them its raining already".

Brian.

 

[c_mitra]

... circuit board containing the FETs would be carried around the production area unpowered so as to try and give it ESD damage...

Try simple things; they are easier to debug. For example a neon lamp (just a bare lamp without the resistors). It is used in good old "line voltage" testers and works very reliably.

Humidifiers are not really that expensive; your work place is perhaps air-conditioned and THAT is the CULPRIT.

 

[treez]

A conductive mat is a bad choice because it conducts charge too quickly. The reason for using a static dissipative mat is so the charge can slowly move off the electrostatically charge object to leave the object at ground potential

Thanks, but this mat is an ESD mat and is just 10 kOhms per meter square...

https://www.farnell.com/datasheets/...MI6vvDrJ6J1wIVEBIbCh2irgg_EAQYASABEgIQxvD_BwE


...can Multicomp be wrong?

- - - Updated - - -

...this ESD mat...
https://www.farnell.com/datasheets/...MI6vvDrJ6J1wIVEBIbCh2irgg_EAQYASABEgIQxvD_BwE

.has a bottom layer resistance of 1e3 ohms, and a top layer resistance of 1e6 ohms...so you would think the bottom layer woudl kind of short the top layer out.
-Either way, there,s none of the gigaohms resistance here.

- - - Updated - - -

is it best to buy a highly conductive esd mat, (which look to be far cheaper), and then incorporate the 100Meg resistance in the earthing plug which connects it to earth?

 

[Kajunbee]

According to ANSI/ESD S4.1 worksurface standard a dissipating work surface should have a resistance of at least 1x106 but less than 1x109 ohms resistance. As far as your question on bottom layer shorting top layer. I guess you have to look at it as two resistors of different values in series and what effect this would have on max current flow.

 

[KlausST]

Hi,

It seems you still don´t understand how a ESD mat operates.

is it best to buy a highly conductive esd mat, (which look to be far cheaper), and then incorporate the 100Meg resistance in the earthing plug which connects it to earth?

No, no, no. This maybe is the cause why you were electrocuted.

-Either way, there,s none of the gigaohms resistance here.

No need for GigaOhms.

just 10 kOhms per meter square...

This is resistance per area, this is not what you siply can measure with an Ohmmeter.
You need an test configuration like given in ASTM d257.

--> is a rough rule: the smaller your electrodes, the higher the measured resistance. Surface resistance! Don´t pierce the mat with a sharp test prod!

And one square meter is a very big test electrode - not useful, only the unit is in Ohms/square_meter.

***
How an ESD mat should operate:
multilayer:
* the top layer is relatively high ohmic. It prevents high current flow when a PCB is placed on the mat with line voltage applied. --> no smoke
It also reduces ESD currents due to its high impedance.
* then there is a higher conductive layer. It is used to lead ESD currents to the earth connection. It also keeps areas of high potential small on the top layer (this prevents you from being electrocuted).
--> both layers help to keep ESD damage low: Low currents, relatively fast voltage decay, small areas, currents fed to GND.
* Maybe there is an additional layer (at the bottom) between this higher conductive layer and the desktop surface. It depends on the mat.

Klaus

 

[c_mitra]

...not useful, only the unit is in Ohms/square_meter

For bulk resistivity, the units will be Ohm.cm (or Ohm.meter) and for surface resistivity, the units will be simply Ohm.

For surface resistivity measurements, you will need two line electrodes (unit length) separated (parallel) by an equal distance (unit dist apart). The result will be in simple Ohms ...

That is how I am understand...