17 Replies Latest reply on Apr 13, 2019 7:44 AM by three-phase

    Can an ESD mat be tested without special tools?

    Bald Engineer James

      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?

        • Re: Can an ESD mat be tested without special tools?
          lui_gough

          I suspect some meters which cam measure in the tens of megohms can probably produce a reading on some carbon impregnated rubber mats. Then again there are other with a lot higher resistance rubber - I've never tried but perhaps an insulation resistance tester would be able to test those?

           

          Otherwise maybe a DC supply of several volts with the multimeter in series attempting to measure a low current range flowing through probes touched to sections of the mat might also work ...

           

          - Gough

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          • Re: Can an ESD mat be tested without special tools?
            jc2048

            I've tried it with the two kinds of mat I have. This is with 10V from a bench PSU and my 5.5 digit Fluke meter (I also put a 100 ohm resistor in series, in case I touched the probes together, though I  realised after that I could have just used the current limit on the PSU).

             

            Cheap, black, rubbery mat.

             

            a reading over a distance of 10mm of around 8uA

             

            Old, blue, 'Farnell' branded mat

             

            maybe something like 3nA (this is right down near the limits of what this particular meter will read)

             

            Neither is really a resistance. There isn't too much variation with distance.

             

            On a cheap handheld meter, the black one reads about half a megaohm. The probes are more pointed and dig into the surface a bit which is probably why it's less than the 10V/8u reading would suggest (1.25M).

            The professional blue one is over-range.

             

            The 3nA figure for the Farnell mat sort of suggests a rough equivalent resistance of 3.33  G ohms but it could be quite a bit different to that (the meter reads 2nA with the probes in mid air and the reading goes up to 5nA on the mat, so I'm not all that confident in it).

             

            Edited because somehow I stupidly wrote Tera rather than Giga.

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            • Re: Can an ESD mat be tested without special tools?
              shabaz

              Hi James,

               

              I just tested a couple of mats here, with a multimeter set to ohms, and the probes about 1 inch apart (near center of mat).

              The first mat was plastic-y, not rubbery. It measures around 12k ohm, although the resistance varies depending on the pressure I use. It's 12k with normal probe troubleshooting force.

              I don't actually use this mat for soldering or as a work-surface - I chopped it up and used it to line some drawers.

              The second mat is some 2-layer material, softer, and has a light-brown side and a black side (the light-brown side is the top surface). The black side has a resistance of about 10 Mohm and again that varies with probe force (more than order of magnitude reduction if pressed more firmly). However the light-brown side I cannot measure this way (I could apply an insulation tester to it, but have not done that yet), so it's at least 50M ohm, the limit of the multimeter, no matter how hard I press.

              Nether mat was expensive, the first one was maybe £25/$30 USD, the second is about $50, and both were larger than my desk (maybe 1500mm long, I have not measured).

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              • Re: Can an ESD mat be tested without special tools?
                dougw

                It might be interesting to measure the performance of various anti-static bags. The readings I get are all over the map.

                • Re: Can an ESD mat be tested without special tools?
                  rsc

                  ANSI / ESD S4.1-2006

                  Test Procedure According to ANSI / ESD S20.20-2014, an acceptable work surface will have a point to point and a point to groundable point resistance of less than 1.0 x 10^9 ohms. ANSI / ESD S 4.1-2006 provides test procedures for evaluating the electrical resistance of work surface materials. ANSI / ESD S20.20-2014 defines the control limits for work surfaces that are to be used in an ESD control program where ESD sensitive devices are handled.

                  Found this online.

                  https://www.elimstat.com/wp-content/uploads/2019/02/5300-Series-S4.1-Test-Data.pdf

                  http://documents.desco.com/pdf/TDtypeB2.pdf

                   

                  Scott

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                    • Re: Can an ESD mat be tested without special tools?
                      shabaz

                      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.

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                        • Re: Can an ESD mat be tested without special tools?
                          lui_gough

                          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

                          2 of 2 people found this helpful
                            • Re: Can an ESD mat be tested without special tools?
                              jc2048

                              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.

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                                • Re: Can an ESD mat be tested without special tools?
                                  Bald Engineer James

                                  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.

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                                    • Re: Can an ESD mat be tested without special tools?
                                      lui_gough

                                      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

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                                        • Re: Can an ESD mat be tested without special tools?
                                          shabaz

                                          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.

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                                            • Re: Can an ESD mat be tested without special tools?
                                              jc2048

                                              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.

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                                            • Re: Can an ESD mat be tested without special tools?
                                              Bald Engineer James

                                              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.

                                  • Re: Can an ESD mat be tested without special tools?
                                    Bald Engineer James

                                    lui_gough and shabaz , you both mentioned insulation resistance testers. I haven't used one of those before. Are they just high voltage multimeters?

                                     

                                    I'm trying to understand how they work. Seems like happly 1,000 volts to a DUT to measure its resistance would be a relatively dangerous measurement.

                                    • Re: Can an ESD mat be tested without special tools?
                                      lui_gough

                                      Insulation resistance testers are designed to measure leakage currents across insulation. They apply a current limited high voltage DC to measure high resistances (e.g. the Keysight U1461A I RoadTested can measure up to 260Gohm). They can also continually apply this voltage for a set amount of time to do dielectric absorbance tests, etc.

                                       

                                      The authority on this, no doubt, is three-phase as he's been reviewing quite a few units in his line of work.

                                       

                                      - Gough

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                                      • Re: Can an ESD mat be tested without special tools?
                                        three-phase

                                        I do not test anti-static mats, but do test insulating mats for when we do silly things like working on live apparatus, but I am looking for higher resistances than those expected for an ESD mat.

                                         

                                        The ESD mat will discharge the static voltage fairly slowly to prevent the user from feeling the shock, ESD clothing will also to that, but are also worn to prevent the build up to a degree. shabaz, yes antistatic clothing will have a micro fine grid woven throughout the fabric which is conductive. There is usually a couple of areas in the garment where the connection is made to the grid, as the grid has to touch the skin to be effective. You will see it more if you can look at a antistatic hi-vis coat, the grid will be very obvious then.

                                         

                                        It looks like the standard for ESD mats uses two test voltages, 10V and 100V. There are plenty of insulation testers that will have a 100V setting, a 10V setting is not so common and I suspect you would be looking at a quite expensive insulation tester aimed at the communications market, probably with a variable voltage function. You also need to read their specification carefully, as a lot of them will have a reduced resistance measurement at a lower test voltage, for the U1461AU1461A, that is 66 GOhms at 100V. Probably adequate for this application, but a cheaper insulation tester may only measure up to 100 MOhms with a 100V test voltage.

                                         

                                        As lui_gough as mentioned, they have a current limit, usually in the region of 1.5mA to 2mA for a handheld 'multimeter' style tester. Whilst this may make you jump, it isn't enough to kill, even on a 1000V test, so they are reasonably safe in the hands of a responsible person. There are two caveats to this, if you zap yourself with the insulation tester when on a ladder or in an unbalanced situation etc, the resulting fall could hurt you, secondly, and aimed at the timed tests, if you charge something up with the insulation tester, that apparatus can then hold that charge for a long while, and discharge it with a much higher current flow. That will hurt considerably more, and in some circumstances have the potential to be fatal. For this reason, the insulation tester should discharge the circuit itself after the test, but not all of them do, or do so very slowly.

                                         

                                        Timed tests are only carried out on insulation systems that show a polarising affect, I doubt an ESD mat can be polarised, to it is probably a short duration test, something like 10 seconds.

                                         

                                        The problem with measuring high resistances is that they are very prone to errors introduced by the setup, but these tend to result in a lower value rather than a high value.

                                         

                                        The tester doesn't measure resistance directly, it calculates it from applying a known voltage and measuring the resultant current. But at 100V and 100s of megohms, the current will be low and very susceptible to noise. It is also susceptible to leakage currents from the test leads and even yourself if you are holding the probes during the test.

                                         

                                        It looks like a lot of the ESD testers for shop use, seem to work on an LED go / no-go indication. You may be able to set up such a system with your power supply and a amplifier / comparator circuit comparing the voltage across a shunt, against a calibrated setting based upon the acceptable values in the standard. That may be cheaper than purchasing an insulation tester with the right capability.

                                         

                                        If you have access to an electrometer or source measurement unit, they usually have the right voltage ranges and can measure very high resistance values, but again are ridiculously expensive to purchase - unless you can persuade rscasny to do some road tests on ESD testers, insulation testers, or source measurement units!

                                         

                                        Kind regards.

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