Author Topic: Measuring surface resistance of ESD mat  (Read 13363 times)

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Offline cristoper

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Measuring surface resistance of ESD mat
« on: October 22, 2013, 01:07:27 am »
I'm in the process of putting together a little electroncis lab and thought it would be good practice to have a rubber ESD mat to protect my devices from ESD and my desk from solder. I bought and finally received this cheap mat from an eBay seller in Hong Kong. The specs on the eBay listing are given as:

Quote
Surface: resistance: 107-9?, color: green, gloss material
Bottom: resistance: 103-5?, color: black

I parsed that typography as 10^7 - 10^9? for the top, and 10^3 to 10^5? for the bottom.

I don't have a megger and my DMM has a maximum range of 50M? (no siemens range), and I live in a dry climate, so I'm not surprised I can't measure any resistance on the green top of the mat. However, if the bottom is really as conductive as the description suggests (100K?), I'd definitely expect to be able to measure it. But no matter where I measure or how hard I press with pointy probes, I get a measurement of infinite resistance.

My question: Am I measuring incorrectly, or did I buy a mat that's not suitable for ESD dissipation? It's not even very important that I have one (I tested this cheap one with my soldering iron, and it is heat resistant at least), but I'd feel sort of silly plugging my wrist strap into some completely insulating piece of rubber and thinking I had an ESD mat just because the eBay description claimed it was.

I found this thread from last year which has some useful information, but I'd still be interested in anyone's opinion or whether anyone has had experience with "fake" ESD work mats before:

http://www.eevblog.com/forum/beginners/esd-mat-selection-guidelines-help-please/
 

Offline alanb

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Re: Measuring surface resistance of ESD mat
« Reply #1 on: October 22, 2013, 01:24:37 am »
when measuring surface resistance it is normally given as ohms per square. The units of measure are immaterial but what you need to do is lay two equal length conductors on the surface and space them the same distance appart as their length. If you then measure the resitance between the two conductors it should give you a more accurate measurement.  Pressing two probe points into the surface will not give you a correct resistance reading.
 
 

Offline rolycat

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Re: Measuring surface resistance of ESD mat
« Reply #2 on: October 22, 2013, 02:13:32 am »
It sounds as though there is an issue with the conductive side of your mat, although it may be worth trying with a larger contact surface for your probes.

With the multimeter ground connected to one of the grounding terminals on my mat, I measured a resistance to ground of around 160 megohms (6nS) on the top surface and around 50 kilohms on the lower surface.

The measurement was made using a rounded probe tip (end of a crocodile clip). Variations in probe pressure have more effect than distance from the ground connection, and blunt probes give a lower resistance reading than sharp ones.
 

Offline cristoper

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Re: Measuring surface resistance of ESD mat
« Reply #3 on: October 22, 2013, 04:13:25 am »
Thank you both. I tried measuring again using two pieces of metal pressed against the surface instead of my sharp probes, and still get infinite resistance even on the black side which is supposed to be only 100KOhm/square. I'm obviously not going for an accurate measurement, but I'd expect something close to what rolycat's mat measures at, or at least something.

I'll contact the ebay seller and ask what's up, but I'm not holding my breath for a response.

Also, this mat has a strong plastic/almost-petrol-like odor even after setting it outside overnight. I don't really recommend this particular ebay item to any other cheapskates like me out there trying to save a few bucks on an ESD mat  :P
 

Offline Kjelt

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Re: Measuring surface resistance of ESD mat
« Reply #4 on: October 22, 2013, 04:42:26 am »
How big are the pieces of metal you used? I  have bought through an somewhat more official channel such a mat but thick rubber and 8 times more expensive and also had this issue in the beginning. How I finally measured it was connect the ground of the multimeter to the button on the mat and then connect the plus to a somewhat larger (such as 1x1 inch) metal (copper or at least very good conductive) plate  and press hard. You should reach under the 50Mohms then. If still not press the plate with a clamp somewhat tighter perhaps that does it. Still nothing hmmm then scrap it , I would say.
 

Offline Thor-Arne

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Re: Measuring surface resistance of ESD mat
« Reply #5 on: October 22, 2013, 05:10:54 am »
I have a 60*120 cm 3 layer eds-mat from rs (stock #466-1643).

The blue layers are basically insulator, very difficult to get any resistance reading at all.
The black inside layer is really conductive, I measure approximately 5.2k ohms between the metal rivets in the corners, about 115 cm apart.
 

Offline edpalmer42

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Re: Measuring surface resistance of ESD mat
« Reply #6 on: October 22, 2013, 10:54:45 am »
Even with an ordinary DMM you can measure Gigaohms by using a voltage in the range of 50V or whatever works and setting up a voltage divider consisting of the DMM and the mat.  I used a couple of coins for contacts on my 3M dissipative mat and set up a series connection between the power supply, the DMM, and the mat.  I measured 0V38 on the 10 Mohm meter which calculates out to about 500 Mohms for the mat.  Just make sure that your meter doesn't automatically switch to very high impedance on low voltages.

Ed
 
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Offline cristoper

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Re: Measuring surface resistance of ESD mat
« Reply #7 on: October 23, 2013, 04:29:44 am »
With Ed's trick of using the dmm's input impedance as a high-value resistor in a voltage divider, I measured as low as about 50MOhm on both the top and bottom layers of my mat between a scrap of aluminum and the grounding snap.

Since that is the upper range of my meter, it emboldened me to get a larger sheet of aluminum and try again using my meter's ohm range. By pressing really hard I managed to measure as low as 43Mohm on the top (green) layer and 48Mohm on the bottom black layer.

So the bottom layer is definitely not as conductive as I'm pretty sure it should be. I can still use the mat as a heat- and cut-resistant work surface that might also help prevent ESD, at least.

How important is a conductive-ish layer is to the effectiveness of an anti-ESD mat?
 

Offline Old-E

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Re: Measuring surface resistance of ESD mat
« Reply #8 on: March 06, 2016, 09:34:28 am »
I was happy to find this thread on ESD testing.   Because I had serious concerns about my mat too.   Was unable to get any indication on top or bottom side with my 50 M ohm multi-meter.

Reading through this and thinking about it a bit triggered a thought.   Probably the easiest and most reliable way to test an ESD mat is with real world conditions as follows -

I happen to have a bench stool with very hot upholstery (static wise).   I don't like it, but assumed (hoped) that the antistatic hardware was taking care of it.   More than a few times, I have gotten bit when touching ground after sliding off the stool.   So the test consisted of first sliding of the stool and touching a grounded clip lead as a control - ouch! - Then sliding off the stool and touching the ground mat immediately before touching the ground wire.   Nothing!!!   I tried this back and forth many times, all with the same results, no shocks when touching the ESD mat first, even with only a small fraction of a second between touching the mat and then the ground wire.   The same test would be good for testing wrist straps too.   (I should do that)
 

Offline mmagin

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Re: Measuring surface resistance of ESD mat
« Reply #9 on: March 06, 2016, 12:51:09 pm »
I got one of these a little while ago to satisfy my curiousity: http://www.ebay.com/itm/272044615282

On my really nice rubber-surface (harder to burn) antistatic mat I finally bought a few months ago, pressed firmly against it, I get:
top: 10^8 ohms, bottom: 1000 ohms.
 

Offline Old-E

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Re: Measuring surface resistance of ESD mat
« Reply #10 on: March 07, 2016, 12:36:33 pm »
Very nice!   Didn't think there was anything near that low of a price.   I'll have to give that some thought.

Thanks.
 

Offline uncle_bob

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Re: Measuring surface resistance of ESD mat
« Reply #11 on: March 07, 2016, 01:17:23 pm »
I was happy to find this thread on ESD testing.   Because I had serious concerns about my mat too.   Was unable to get any indication on top or bottom side with my 50 M ohm multi-meter.

Reading through this and thinking about it a bit triggered a thought.   Probably the easiest and most reliable way to test an ESD mat is with real world conditions as follows -

I happen to have a bench stool with very hot upholstery (static wise).   I don't like it, but assumed (hoped) that the antistatic hardware was taking care of it.   More than a few times, I have gotten bit when touching ground after sliding off the stool.   So the test consisted of first sliding of the stool and touching a grounded clip lead as a control - ouch! - Then sliding off the stool and touching the ground mat immediately before touching the ground wire.   Nothing!!!   I tried this back and forth many times, all with the same results, no shocks when touching the ESD mat first, even with only a small fraction of a second between touching the mat and then the ground wire.   The same test would be good for testing wrist straps too.   (I should do that)

Hi

Ok so now for a little ESD theory:

The reason you get an "ouch" from your seat to ground clip experiment is that you get  high current discharge. Your charged body is pretty much instantly dumping some number of coulombs of charge at some number of kilovolts into zero ohms. You get a nice big womp of current and you feel it.

This is not the ideal case for protection.

The better thing to have happen is for a very large resistance to be present. Your kilovolts still are there. Your some number of (could be mili) coulombs are present. Instead of it all dumping in zero time, it dumps in a second or two. Instead of a great big burst of current, you get a much lower amount spread out over a longer time. There is less likelihood of damage from the lower current than the higher.

This is why you use high resistance mats for ESD protection. If all you wanted was a "good ground" an old style brass top bench is the best thing to use. Zero ohms, and low impedance into the GHz range is easy to get that way. It's not a good thing for ESD....

Bob

 

Offline Old-E

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Re: Measuring surface resistance of ESD mat
« Reply #12 on: March 07, 2016, 01:56:37 pm »
Thanks Bob!

Well understood.   Yes, my test of touching the ESD mat momentarily reduced the charge to below what I could feel when then touching the grounded clip.   My body still could have retained enough of a charge to damage sensitive electronics.   But when sitting at the bench with a wrist strap on (grounded through a 1 meg resistor), a static charge should have enough time to drain down near zero.

Yes, a brass bench top would be the optimum until someone happened to touch a high voltage source while resting his elbow on the bench top, or a high voltage source happen to touch the bench top.
 

Offline uncle_bob

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Re: Measuring surface resistance of ESD mat
« Reply #13 on: March 07, 2016, 11:33:41 pm »
Thanks Bob!

Well understood.   Yes, my test of touching the ESD mat momentarily reduced the charge to below what I could feel when then touching the grounded clip.   My body still could have retained enough of a charge to damage sensitive electronics.   But when sitting at the bench with a wrist strap on (grounded through a 1 meg resistor), a static charge should have enough time to drain down near zero.

Yes, a brass bench top would be the optimum until someone happened to touch a high voltage source while resting his elbow on the bench top, or a high voltage source happen to touch the bench top.

Hi

Damage to IC's is as much from current as from anything else. You *can* get direct voltage damage on a die, but on leaded parts it is mostly the current that kills them. The resistive stuff limits the discharge current and results in less likely hood of damage to an IC. It is a bit of a gamble, but you win far more often than you loose doing it that way.

Bob
 

Offline Old-E

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Re: Measuring surface resistance of ESD mat
« Reply #14 on: March 08, 2016, 04:21:47 pm »
Ah!   Good point!   Hadn't thought of it that way.

Thanks
 

Offline Old-E

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Re: Measuring surface resistance of ESD mat
« Reply #15 on: March 12, 2016, 03:15:14 pm »
mmagin -

I ordered the ESD, surface resistivity tester that you found on line at  http://www.ebay.com/itm/272044615282

Couldn't pass it up for $18.   Thanks for your suggestion.
 

Offline mmagin

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Re: Measuring surface resistance of ESD mat
« Reply #16 on: March 12, 2016, 03:28:23 pm »
mmagin -

I ordered the ESD, surface resistivity tester that you found on line at  http://www.ebay.com/itm/272044615282

Couldn't pass it up for $18.   Thanks for your suggestion.

I'm guessing it's copy of an old name-brand design or of some classic app-note design though I've been too lazy to open it up yet.  Presumably a FET-input logarithmic transimpedance amp followed by a LM3914 or something?

Edit: Looks like a CMOS dual opamp http://www.ti.com/lit/ds/symlink/lmc6042.pdffollowed by 12 stages of jellybean opamp (3 * LM324). I haven't tried to reverse engineer the schematic, but that set of 3 LM324s driving the LEDs reminds me a lot of the block diagram of something like http://www.analog.com/media/en/technical-documentation/data-sheets/AD8307.pdf  That pair of transistors are KSP10. 

That server is a harsh mistress, really made me scale down my images - oh, haha, I was only applying the jpeg quality change and not resize :)
« Last Edit: March 12, 2016, 03:50:07 pm by mmagin »
 

Offline uncle_bob

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Re: Measuring surface resistance of ESD mat
« Reply #17 on: March 13, 2016, 01:25:27 am »
mmagin -

I ordered the ESD, surface resistivity tester that you found on line at  http://www.ebay.com/itm/272044615282

Couldn't pass it up for $18.   Thanks for your suggestion.

I'm guessing it's copy of an old name-brand design or of some classic app-note design though I've been too lazy to open it up yet.  Presumably a FET-input logarithmic transimpedance amp followed by a LM3914 or something?

Edit: Looks like a CMOS dual opamp http://www.ti.com/lit/ds/symlink/lmc6042.pdffollowed by 12 stages of jellybean opamp (3 * LM324). I haven't tried to reverse engineer the schematic, but that set of 3 LM324s driving the LEDs reminds me a lot of the block diagram of something like http://www.analog.com/media/en/technical-documentation/data-sheets/AD8307.pdf  That pair of transistors are KSP10. 

That server is a harsh mistress, really made me scale down my images - oh, haha, I was only applying the jpeg quality change and not resize :)

Hi

Ok so now that you have opened it up ... does it measure anything useful? The only real thing you need is a "still looks ok" sort of output. Knowing that your mat is 123,456,789 whatever ohms per square is not really required.

Bob
 

Offline mmagin

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Re: Measuring surface resistance of ESD mat
« Reply #18 on: March 13, 2016, 04:35:14 am »
I'm confident it's not snake-oil, given the LMC6042 on the input, though I'm not sure whether it's a great design. 

I tried it with the optional clips (they switch off the back electrodes) on some newly acquired 100M and 1G resistors.  100M read as expected, but it read as insulating on the 1G.  But as a test of antistatic materials it seems like not a bad device -- it still serves the purpose of determing whether they work as they should.

My other attempts to read those resistors (50 volt supply in series with 10M-input DMM measuring voltage, and also a recently acquired antique fluke differential voltmeter) confirmed that Vishay certainly aren't lying.  :)  Personally, I suspect it's optimistic to try to resolve even orders of magnitude up around 10^9 to 10^12 with only a 9 volt supply.
 

Offline TechieTX

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Re: Measuring surface resistance of ESD mat
« Reply #19 on: March 18, 2018, 10:42:12 am »

I'm guessing it's copy of an old name-brand design or of some classic app-note design though I've been too lazy to open it up yet.  Presumably a FET-input logarithmic trans-impedance amp followed by a LM3914 or something?


Unless I miss my guess, it's a clone of this DESCO unit.  For 17 bucks shipped, I think I'll go for the Chinese copy.  DESCO wants $390 for the same thing, and theirs doesn't even include the test leads or 5 pound electrodes for that price; the electrodes are another $175. and the banana-to-banana leads for 'em are $16 for the pair.

We have a much more expensive combination megohmmeter at work, but for home use or small shops this would do the deed.
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