Beware cheap ESD mats...

[jpb]

Ages ago I bought an ESD heat resistant rubber mat with useful dips for putting screws in.
Today I finally got around to unpacking it and putting it on top of my grounded expensive ESD mat (bought from Digikey). (I've moved house 3 times recently so am still setting up my lab.)

I thought I'd check it was connected ok to ground.

I tested the resistance at 1000V on my Fluke 1507 and it comes to > 11Gohms (i.e. off the scale).
As a base line my normal ESD mat is less than 9G ohms. My other vinly ESD mat is less still.

I realise that this isn't a very scientific test but I have a strong suspicion that the "ESD" mat is just normal rubber but perhaps it is too high to be measured even with probes very close together (I pressed the probes in at about 2mm apart).

Anyway, I don't think I can trust it.

PS sorry for the dodgy screen protector in the photos - I really aught to remove it...

[madires]

A proper ESD mat has two layers. The top layer is dissipative and not conductive (very high resistance), the bottom layer is the conductive one. Pre-cut mats also have a button connector for grounding (via an 1M resistor) usually.

[wraper]

It's not an ESD mat, it's heat resistant working mat. So it's completely OK for it being static dissipative only without conducting layer. It does not have ground connection anyway.

I tested the resistance at 1000V on my Fluke 1507 and it comes to > 11Gohms (i.e. off the scale).
As a base line my normal ESD mat is less than 9G ohms. My other vinly ESD mat is less still.

Your "base line" is out of spec with what ESD mat normally should have. Sticking probes in the mat is not how it's done. You should have large contact area.
Watch from 2:00
https://youtu.be/gqcv58nAd70

[wraper]

Anyway, I don't think I can trust it.

You should not trust to your judgement in the first place. ESD mats are specified so resistance can vary 1000 times (10^6...10^9 ohms). You make improper measurement, then call alarm when get measurement which is unknown but might be only 23% higher compared to your base line  .

[Berni]

The top surface of the ESD mat is supposed to have very high resistance.

Proper mats always have two layers to them. The bottom layer tends to have resistances in the Megaohm range, this layer is usually black because it has plenty of carbon mixed in, this is the layer that collects all the static charge and guides it to the grounding connection. The top layer is the actual work surface and is designed to have very high resistance so that it does not mess with the operating on any circuits you might place on top, it tends to be so high in resistance its difficult to even measure with a normal multimeter, its job is only to dissipate its own charge down into the bottom layer over the thickness of it so the very high resistance is still enough.

If you want to test it out you can always place a metal object on it and zap it with a piezzo igniter a few times, if you then poke the object with a metal object in your hand and see a spark then it obviusly kept its charge, on a proper ESD mat the object should have its charge drained away pretty much instantly. Also the piezo igniter will love to jump a spark onto the surface of the ESD mat, but not to a non ESD safe completely insulated PVC mat for example.

The actual values can vary a lot for ESD mats, but in general the resistances are very high because that's still plenty enough to drain away charges as quickly as a human could reasonably produce at the work station. Yet so high that they wouldn't pose a electrocution risk (Perhaps if the ground becomes disconected) or to affect the operation of any circuits resting on the surface (Unless they are really really sensitive)

And yeah as suggested above there are standard procedures for testing ESD mats.

[jpb]

Thank you for the responses everybody (though the general assumption seems to be that I've no idea what I'm doing! )

Yes, I was aware that my measurement approach was not the official way to do it - I don't have access to such equipment. And I did say that perhaps it has a resistance (between two points) of > 11 Gohm but I would expect even a dissipative layer to have some (very high) resistance.

I replaced my probes with two squares of aluminium (around 10cm x 10cm) pressed down with the probes (yes I know - it is not the correct way but it is an approximation).

On the ESD mat (which has a surface dissipative layer) the resistance drops to around 14 Mohms.

The rubber dissipative only mat still reads as > 11 Gohms.

I tried fitting a aluminium sheet under the rubber mat and aluminium square above it. Still > 11 Gohms.

I found this reference :

https://www.gotopac.com/art-esd-resistivity

which states:

Dissipative materials have a surface resistivity equal to or greater than 1 x 10^5 Ω/sq but less than 1 x 10^12 Ω/sq or a volume resistivity equal to or greater than 1 x 10^4 Ω-cm but less than 1 x 10^11 Ω-cm.

In my effort to get a reading I found an Al block which is 10cm x 11.5cm x 6.5cm and has a fair weight (probably more than 5lb).
I placed an Al sheet (clip board) underneath.
I measured the thickness of the rubber mat as 3.1 mm using precision calipers (difficult to do because of a lip).

Still > 11 Gohm so the bulk resistivity is greater than 11E9ohms * 10cm * 11.5cm /0.31 cm = 4 x 10^12 ohm-cm

I also tested the meter on a series connection of 1% resistors 5G + 3G + 2G + 1G = 11G and the meter reads 10.8G which I think is in spec.

So despite my dodgy methodology, I persist in thinking that the dissipative rubber mat is not very dissipative. By my (admittedly flawed) calculation the bulk resistivity is at least 40 times the maximum for a dissipative material.
I admit I've not done a very precise measurement of surface resistivity because I lack the right probes.

Unfortunately I don't have a piezzo electric lighter handy to try that test.

I'm not trying to get any super accurate measurements here, I just feel a dissipative mat should register some sort of reading. Unfortunately I don't have any other purely dissipative rubber mats around to compare - my other mats are all of the standard multi-layer ESD variety.

[wraper]

It seem to have resistance specified. But that does not qualify even as static dissipative.

[Berni]

Yeah it is starting to look like that cheap ESD mat might be fake.

I suppose it still possible that its more like the pink antistatic foam and bags where they are not conductive at all, but made from a material that does not build a charge when you rub it. Not sure how easy that is to test out, perhaps requiring a electrometer voltmeter or maybe just a bench DMM with high Z mode.

[jpb]

Thanks wraper, I also have just found that number ("antistatic coefficient 1*10E12ohms") but I'm not sure what it means. I guess surface resistance between standard probes 10 inches apart or what ever was in the video.

So it looks like it might not be fake, just so high in resistance that I can't measure it (and probably outside the normal range for dissipative rubber mats).

Berni, thanks for your comments. I did try my experiment with a pink bubble-wrap bag and you're right, it too was measuring > 11G.

I really only started these measurements as a bit of fun having bought the 1507 cheap off ebay a while ago, but it does interest me that for hobbyists who don't want to spend money testing mats officially need to just take things on trust. Though having said that - for my standard mats I can measure the resistance between plates or to earth and it give me some re-assurance that things are connected ok.

I'm quite impressed with the 1507 as a meter, able to generate 1000 plus volts and measure up to 11 Gohms within a few per-cent from a small battery powered device.

[cdev]

Those silicone mats are not ESD mats. They are heat resistant soldering mats.

If you do ESD-producing things to them they WILL build up ESD.

[jpb]

Those silicone mats are not ESD mats. They are heat resistant soldering mats.

If you do ESD-producing things to them they WILL build up ESD.

I think you're right but they advertise themselves as ESD mats - it is printed on it. I suppose the argument is that they are a bit better than pure silicone rubber or other insulators.

Having invested money and effort in an ESD mat on the bench, I might just get a cheap dissipative mat for soldering - my ESD bench mat is probably ok for soldering on but it was quite expensive (the more costly rubber sort) so I'd like to keep it nice.

[Berni]

I did try mesuing about 4mm thick pink antistatic foam between two 4x12cm plates on my Keithley 617 and it came out to about 500 GOhm.(The instrument can measure up to about 1000 TOhm with a high voltage source)

[jpb]

I did try mesuing about 4mm thick pink antistatic foam between two 4x12cm plates on my Keithley 617 and it came out to about 500 GOhm.(The instrument can measure up to about 1000 TOhm with a high voltage source)

So a bit more than 11GOhm then !

Thanks.

[wraper]

I did try mesuing about 4mm thick pink antistatic foam between two 4x12cm plates on my Keithley 617 and it came out to about 500 GOhm.(The instrument can measure up to about 1000 TOhm with a high voltage source)

So a bit more than 11GOhm then !

Thanks.

But foam is mostly air. There is barely any material in between of the sides.

[Berni]

Well for comparison i also placed a 4x3cm pad (6mm thick) of black anti static foam between the same plates and instead got around 250KOhm

But yeah foam in general won't have really low resistance due to it being mostly air. Tho i can get the resistance to go as low as 1KOhm if i press down hard on the plates to squeeze out the air and make the plates get better contact with the rough surface of it.

I don't have any antistatic mats at home to test, but we did try measuring it at work when we got some new ones, we could mesure the bottom side in Megaohms but the top side was just too high resistance for normal multimeters and we don't have any electrometers or insulation testers there.

[wraper]

Well for comparison i also placed a 4x3cm pad (6mm thick) of black anti static foam between the same plates and instead got around 250KOhm

Black foam is not antistatic, it's conductive and made to short the leads of components.

[Berni]

Yeah it should be properly called conductive foam, but all of these foams are usually just put into the same "antistatic foam" basket, even by people who sell it such as Farnell and Digikey.

Just did it to show that foam can be just about as insulating or as conductive as you want it to be by mixing in the right stuff. Obviously for a ESD work surface you want it to be in the "just right" range somewhere in between. Too low resistance will mess with the operation of circuits on it and might conduct a ESD discharge in to other parts causing even more damage. Too high resistance and the charge doesn't bleed away fast enough, leaving objects on the work surface charged to a high voltage for a significant time.

[madires]

Pink plastic bags and foam are dissipative but they don't protect parts from an electrostatic discharge.