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Can an ESD mat be tested without special tools?

baldengineer

There are inexpensive continuity testers that let you verify a loop between you, your wrist strap, and ground.

 

Without investing into special weights (probes), is there an effective way to measure an ESD mat’s resistance? Or more simply, verify areas of an ESD mat are still effective?

 

Can it be done with “just” a multimeter and a power supply?

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  • in reply to rsc

    Hi Scott,

     

    Interesting information! I just tried measuring the top surface of my mat with an insulation tester, and now see that it is conductive slightly. I see about 3G ohm with typical probe pressure as if using multimeter test leads. It's in very good condition (unscratched etc, and just a couple of years old), I guess the standard measures it in a different way.

     

    Just out of interest I attempted a lab coat as well, given that it is supposed to be for ESD-safe environment. I think I can just about see some difference in the weaving, but I could be imagining it - I think I see faint rows of single strand of maybe conductiveness, about 5mm apart (horizontal strands in the photo below, about six of them across the photo). But I cannot measure the resistance no matter how hard I try with the positioning of the multimeter leads - also tried the insulation tester. The strands are so fine, I can't get contact with them.. I suppose I could soak two sections of the lab coat and place the probes there! Regarding strands, I don't think the entire surface needs to be conductive anyway. As I understand lots of static discharging can be (say) having a conductive brush very close to a surface. At least it works in my limited experiments.. if I wipe some plastic against some cloth to put on a charge, I can pick up tissue-paper fragments. But if I move a conductive brush close to the surface of the plastic (i.e. close but not touching the surface, and the brush is grounded via me), then the plastic is no longer charged and it cannot pick up bits of tissue paper.

    image

  • in reply to shabaz

    I wonder if the contact point was not a probe but more like a 10x10cm sheet of copper or something similar, whether the parallel paths would reduce the resistance to the levels expected.

     

    Then again, I suppose generating static is a way to test it ... perhaps we need a cat involved ...

     

    - Gough

  • in reply to Gough Lui

    The link that Scott gives talks of a "five pound" electrode, so it sounds like they've standardised on area and weight (to get a reproducible measurement).

     

    This seems to be the electrode used:

     

    https://www.electrotechsystems.com/products/electrostatic-instumentation/resistance-probes-clamps/surface-resistance-probe/

     

    That would explain Shabaz and myself seeing higher values.

  • in reply to jc2048

    Two electrodes and a specialized ESD meter were what I found when I first started looking. I've seen some other forums that have other ideas, but they are not working out so well either.

     

    I tried using ~2.5inch aluminum blocks as my electrodes. But even with adding some tape as insulation, I suspect I'm getting more leakage through that insulation layer (through me) than the mat itself when I apply a little pressure. I have two mats that I'm testing. I don't know the spec, and the other has a large range. They are both measuring in the 30-50 Mohm range. 

     

    The alternative test I tried is using a 10 Mohm series resistor and measuring the voltage drop across it. Then calculate the resistance as an unknown voltage divider. But at 64 volts, the most I can apply, I'm getting ridiculous values like 8 Gigaohm. The mat with a spec on it is supposed to be on the order of 10-100 Mohm.

     

    I had wanted to show a clever way to check if a mat is "okay," but I'm not sure I've come across that yet.

  • in reply to baldengineer

    To be honest, since the role is normally to discharge/dissipate ESD in a slow and safe manner, I think being able to measure anything in the megohms range is proof enough that the areas are workable. Most good insulators wouldn't normally be that low in resistance, so I probably wouldn't break a sweat about not achieving the datasheet numbers?

     

    On the other hand, perhaps electrode pads, bricks for weights ... no leakage through the body and high voltage might be a way to get the numbers desired as I suspect surface contamination might be artificially increasing the numbers as a uniform contact isn't achieved without sufficient weight ...

     

    - Gough

  • in reply to Gough Lui

    I agree with that too, because on it's own, the ESD mat isn't sufficient anyway, it is just one way of dissipating charge, and anyone with insulating shoes could still zap stuff even if it is on an ESD mat. I guess the resistance of the ESD mat is some compromise between slowly dissipating charge, and providing protection for the user and any powered-up circuit maybe.

    I remember with some FETs, they were so sensitive they didn't come with conductive foam, instead they had a removable conductive metal ring around the pins. I've RMA'd particular FET arrays from a distributor in the past, if they didn't come in the original tube (I can understand the distributor's desire to sell smaller-than-full-tube stock in foam and placed in a bag, but in this case I wanted the parts immediately transferred to another tube - they told me to junk them and I got replacement ones in the correct storage.

     

    By the way a conductive brush is a useful tool - it very quickly dissipates charge (not ideal near powered-up equipment of course, but useful if you're about to begin work near insulators). I use a record cleaning brush.

  • in reply to Gough Lui

    That's a really good point Gough. Thanks for grounding my expectations. (Pun intended.)

     

    I'm after a qualitiative measurement, so "some conductivity" is definitely qualitiative. I was just stuck on the idea of getting a number. 

     

    Thank you again.

  • in reply to shabaz

    I remember with some FETs, they were so sensitive they didn't come with conductive foam

    I think that dates back to the days before they developed conductive foam and other anti-static materials. I can remember dual-gate rf mosfets in cans with protection rings.

     

    When 4000 series CMOS logic was first introduced, they came in extruded aluminium tubes to keep them safe and it was a few years before they developed the plastic tubes. The original parts were very vulnerable, because they didn't have the protection diodes, but I imagine the change to plastic was actually driven by economics.