Radio Shack ESD mat doesn't meet spec
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Radio Shack ESD mat doesn't meet spec
 
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Well, I've convinced myself that the Radio Shack portable ESD mat, P/N
276-2370 doesn't work properly. The "ESD Association" http://www.esda.org has promulgated an industry-standard test for ESD mats, ESD S4.1. It is the standard specified by most commercial mats. I decided not to spring for the $70 to buy a copy of the standard, but other information I found on the web describes the test in general terms. It uses two circular electrodes, each weighted with 5 pounds, spaced 10 inches apart on the mat. The "Point to Point Resistance" is specified to be: At 40-60% RH: 10^6 - 10^7 ohms At 20-40% RH: 10^7 - 10^8 ohms At 10-20% RH: 10^8 - 10^9 ohms I don't know what the RH here in Santa Rosa was yesterday when I did the test, but I don't think it was very low since it has been raining recently and the ground is still damp. For sure the resistance shouldn't be below 10^9 ohms (1 gigohm) and probably more like 10^8 or 10^7 (100 or 10 megohms). I measured 2.5 x 10^10 ohms (25 gigohms), which puts the Radio Shack mat way out of spec. Test procedure: I didn't find a specification on the electrode size, but in the photo of a popular tester they look to be maybe 3 or 4 inches in diameter. For my test, the electrodes were two saucepans, each about 7 inches in diameter and weighted with 5 pounds. They were spaced 10 inches apart on the mat (3 inches edge-to-edge). I connected a 0.1 uF film capacitor between the two pans and charged it to 15V with a power supply. I set my ancient Simpson analog volt-ohm meter to 60 uA full scale. If I touch the leads across the capacitor immediately after charging, the needle momentarily jumps to about 6 uA (1/10 full scale) as the capacitor discharges through the meter. If I wait half an hour (1800 seconds) for the capacitor to partially discharge through the mat resistance, the needle jumps to about 3 uA. An R-C network discharges to 3/6 of original voltage in about 0.7 time constant. So the time constant must be 1800/0.7 = 2571 seconds. That implies the mat resistance is 2571 sec / 0.1 uF ~= 2.5 x 10^10 ohms. Al N1AL _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In a message dated 2/27/2008 9:54:12 A.M. Pacific Standard Time,
[hidden email] writes: Well, I've convinced myself that the Radio Shack portable ESD mat, P/N 276-2370 doesn't work properly. The "ESD Association" http://www.esda.org has promulgated an industry-standard test for ESD mats, ESD S4.1. It is the standard specified by most commercial mats. I decided not to spring for the $70 to buy a copy of the standard, but other information I found on the web describes the test in general terms. It uses two circular electrodes, each weighted with 5 pounds, spaced 10 inches apart on the mat. The "Point to Point Resistance" is specified to be: At 40-60% RH: 10^6 - 10^7 ohms At 20-40% RH: 10^7 - 10^8 ohms At 10-20% RH: 10^8 - 10^9 ohms I don't know what the RH here in Santa Rosa was yesterday when I did the test, but I don't think it was very low since it has been raining recently and the ground is still damp. For sure the resistance shouldn't be below 10^9 ohms (1 gigohm) and probably more like 10^8 or 10^7 (100 or 10 megohms). I measured 2.5 x 10^10 ohms (25 gigohms), which puts the Radio Shack mat way out of spec. Test procedure: I didn't find a specification on the electrode size, but in the photo of a popular tester they look to be maybe 3 or 4 inches in diameter. For my test, the electrodes were two saucepans, each about 7 inches in diameter and weighted with 5 pounds. They were spaced 10 inches apart on the mat (3 inches edge-to-edge). I connected a 0.1 uF film capacitor between the two pans and charged it to 15V with a power supply. I set my ancient Simpson analog volt-ohm meter to 60 uA full scale. If I touch the leads across the capacitor immediately after charging, the needle momentarily jumps to about 6 uA (1/10 full scale) as the capacitor discharges through the meter. If I wait half an hour (1800 seconds) for the capacitor to partially discharge through the mat resistance, the needle jumps to about 3 uA. An R-C network discharges to 3/6 of original voltage in about 0.7 time constant. So the time constant must be 1800/0.7 = 2571 seconds. That implies the mat resistance is 2571 sec / 0.1 uF ~= 2.5 x 10^10 ohms. Al N1AL _______________________________________________ Did the test standard you used tell you what the test device voltages should be. Also, most Static dissipative mats have a texture which can significantly reduce the actual surface contact area affecting the readings considerably. I still think the best way to test is to fabricate a simple electrometer and connect the sensor (after charging) to the surface through contact. That will tell you very quickly if it is working. Point to this is the voltage of a transistor amplified Ohmmeter is very low these days. A standard VOM is usually 1.5 volts and many FET DVM is much lower because they are also used to test semiconductors and capacitors. ESD that is damaging is at least a few hundred volts. Most ESD workstations are certified to 1000 volts or more. Don't throw out the RS mat yet. The highest quality mil spec'd mats may cost you a lot more and not buy you anything. One big point - Mats do age and dry out becoming useless in a matter of a few years. Al WA6VNN **************Ideas to please picky eaters. Watch video on AOL Living. (http://living.aol.com/video/how-to-please-your-picky-eater/rachel-campos-duffy/ 2050827?NCID=aolcmp00300000002598) _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
K3 443 complete using a Radio Shack ESD mat
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I just completed K3 443 with the installation of the 100 watt amp. Everything went together well with the amp. The Radio Shack ESD mat ... well .... nothing happened as I built the unit. Works great!!!!
73 Lee - k0WA In our day and age it seems that Common Sense is in short supply. If you don't have any Common Sense - get some Common Sense and use it. If you can't find any Common Sense, ask for help from somebody who has some Common Sense. Is Common Sense divine? _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Re: Radio Shack ESD mat doesn't meet spec
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In reply to this post by AJSOENKE
On Wed, 2008-02-27 at 19:23, [hidden email] wrote:
> In a message dated 2/27/2008 9:54:12 A.M. Pacific Standard Time, > [hidden email] writes: > Well, I've convinced myself that the Radio Shack portable ESD mat, P/N > 276-2370 doesn't work properly. > <explanation deleted> > Al N1AL > > > _______________________________________________ > Did the test standard you used tell you what the test device voltages should > be. Yes, they specify it be tested with both 10V and 100V. I used 15V for my test since that was convenient. > Also, most Static dissipative mats have a texture which can > significantly reduce the actual surface contact area affecting the readings considerably. > I still think the best way to test is to fabricate a simple electrometer > and connect the sensor (after charging) to the surface through contact. That's basically what I did. I used electrodes similar to the standard ones specified in the standard. > One big point - Mats do age and > dry out becoming useless in a matter of a few years. Dry out? You mean they are wet? :=) Al N1AL _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Alan Bloom
A few months ago I reported here on some measurements I made on the
Radio Shack portable ESD mat (see below). Basically I found that it doesn't bleed off static charges as it is supposed to. (i.e. It fails the ESD Association resistance specification for ESD mats.) So I recently bought another ESD mat from Jameco. It's their 19.5x23.5-inch anti-static mat, P/N 10584, $16.45. Tonight I measured it using the same technique that I used with the Radio Shack mat. It's even worse! After a half hour, the capacitor was still charged to 75% or so versus 50% with the RS mat. So what's going on here? I can think of three explanations. (1) There's something wrong with my measurement technique. (2) Perhaps you're supposed to treat the mat with some kind of conductive material before use. (3) Cheap anti-static mats are a fraud and are worthless for their intended purpose. I can't figure out how it could be (1). As a sanity check I confirmed that the sauce pans I was using as probes are conductive and the capacitor is indeed 0.1 uF. I doubt it is (2) - I can't believe that the mats are supposed to be untreated as they come from the factory. I suspect (3). Years when when I was a components engineer at Hewlett Packard, I was measuring some of those rubber heat sink insulators and found they didn't even come close to meeting their thermal resistance spec. So I came up with Bloom's Law: "The harder a specification is to measure, the more likely it is to be a lie." If you buy a 1k, 10% resistor it will almost certainly be within spec since anyone with a DVM can easily measure it. Measuring surface resistivity by the ESD Association method requires an uncommon, expensive piece of test equipment that people who buy $16 ESD pads are unlikely to have. So it's easy for the manufacturer to cut corners without getting caught. Al N1AL On Wed, 2008-02-27 at 09:51, Alan Bloom wrote: > Well, I've convinced myself that the Radio Shack portable ESD mat, P/N > 276-2370 doesn't work properly. > > The "ESD Association" http://www.esda.org has promulgated an > industry-standard test for ESD mats, ESD S4.1. It is the standard > specified by most commercial mats. I decided not to spring for the $70 > to buy a copy of the standard, but other information I found on the web > describes the test in general terms. It uses two circular electrodes, > each weighted with 5 pounds, spaced 10 inches apart on the mat. The > "Point to Point Resistance" is specified to be: > > At 40-60% RH: 10^6 - 10^7 ohms > At 20-40% RH: 10^7 - 10^8 ohms > At 10-20% RH: 10^8 - 10^9 ohms > > I don't know what the RH here in Santa Rosa was yesterday when I did the > test, but I don't think it was very low since it has been raining > recently and the ground is still damp. For sure the resistance > shouldn't be below 10^9 ohms (1 gigohm) and probably more like 10^8 or > 10^7 (100 or 10 megohms). > > I measured 2.5 x 10^10 ohms (25 gigohms), which puts the Radio Shack mat > way out of spec. > > Test procedure: > > I didn't find a specification on the electrode size, but in the photo of > a popular tester they look to be maybe 3 or 4 inches in diameter. For > my test, the electrodes were two saucepans, each about 7 inches in > diameter and weighted with 5 pounds. They were spaced 10 inches apart > on the mat (3 inches edge-to-edge). I connected a 0.1 uF film capacitor > between the two pans and charged it to 15V with a power supply. > > I set my ancient Simpson analog volt-ohm meter to 60 uA full scale. If > I touch the leads across the capacitor immediately after charging, the > needle momentarily jumps to about 6 uA (1/10 full scale) as the > capacitor discharges through the meter. If I wait half an hour (1800 > seconds) for the capacitor to partially discharge through the mat > resistance, the needle jumps to about 3 uA. > > An R-C network discharges to 3/6 of original voltage in about 0.7 time > constant. So the time constant must be 1800/0.7 = 2571 seconds. That > implies the mat resistance is 2571 sec / 0.1 uF ~= 2.5 x 10^10 ohms. > > Al N1AL > > > _______________________________________________ > Elecraft mailing list > Post to: [hidden email] > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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Alan,
Bloom's Law: I like it. Doug out my mat and, no, couldn't measure it with my DVM. It so happens that I do have a nice L/C meter (Almost All Digital Electronics model L/C Meter IIB) and I did discover I can measure capacitance: About 1 pf. Now, while thats not telling me the resistance, at least it's telling me that *something* is happening! If I have time, the sauce pans will come out... Where can I find a nice juicy capacitor... Regards, kurtt Kurt Pawlikowski, AKA WB9FMC The Pinrod Corporation [hidden email] (773) 284-9500 http://pinrod.com Alan Bloom wrote: > A few months ago I reported here on some measurements I made on the > Radio Shack portable ESD mat (see below). Basically I found that it > doesn't bleed off static charges as it is supposed to. (i.e. It fails > the ESD Association resistance specification for ESD mats.) > > So I recently bought another ESD mat from Jameco. It's their > 19.5x23.5-inch anti-static mat, P/N 10584, $16.45. Tonight I measured > it using the same technique that I used with the Radio Shack mat. It's > even worse! After a half hour, the capacitor was still charged to 75% > or so versus 50% with the RS mat. > > So what's going on here? I can think of three explanations. > > (1) There's something wrong with my measurement technique. > (2) Perhaps you're supposed to treat the mat with some kind of > conductive material before use. > (3) Cheap anti-static mats are a fraud and are worthless for their > intended purpose. > > I can't figure out how it could be (1). As a sanity check I confirmed > that the sauce pans I was using as probes are conductive and the > capacitor is indeed 0.1 uF. I doubt it is (2) - I can't believe that > the mats are supposed to be untreated as they come from the factory. > > I suspect (3). Years when when I was a components engineer at Hewlett > Packard, I was measuring some of those rubber heat sink insulators and > found they didn't even come close to meeting their thermal resistance > spec. So I came up with Bloom's Law: "The harder a specification is to > measure, the more likely it is to be a lie." If you buy a 1k, 10% > resistor it will almost certainly be within spec since anyone with a DVM > can easily measure it. Measuring surface resistivity by the ESD > Association method requires an uncommon, expensive piece of test > equipment that people who buy $16 ESD pads are unlikely to have. So > it's easy for the manufacturer to cut corners without getting caught. > > Al N1AL > > > On Wed, 2008-02-27 at 09:51, Alan Bloom wrote: > >> Well, I've convinced myself that the Radio Shack portable ESD mat, P/N >> 276-2370 doesn't work properly. >> >> The "ESD Association" http://www.esda.org has promulgated an >> industry-standard test for ESD mats, ESD S4.1. It is the standard >> specified by most commercial mats. I decided not to spring for the $70 >> to buy a copy of the standard, but other information I found on the web >> describes the test in general terms. It uses two circular electrodes, >> each weighted with 5 pounds, spaced 10 inches apart on the mat. The >> "Point to Point Resistance" is specified to be: >> >> At 40-60% RH: 10^6 - 10^7 ohms >> At 20-40% RH: 10^7 - 10^8 ohms >> At 10-20% RH: 10^8 - 10^9 ohms >> >> I don't know what the RH here in Santa Rosa was yesterday when I did the >> test, but I don't think it was very low since it has been raining >> recently and the ground is still damp. For sure the resistance >> shouldn't be below 10^9 ohms (1 gigohm) and probably more like 10^8 or >> 10^7 (100 or 10 megohms). >> >> I measured 2.5 x 10^10 ohms (25 gigohms), which puts the Radio Shack mat >> way out of spec. >> >> Test procedure: >> >> I didn't find a specification on the electrode size, but in the photo of >> a popular tester they look to be maybe 3 or 4 inches in diameter. For >> my test, the electrodes were two saucepans, each about 7 inches in >> diameter and weighted with 5 pounds. They were spaced 10 inches apart >> on the mat (3 inches edge-to-edge). I connected a 0.1 uF film capacitor >> between the two pans and charged it to 15V with a power supply. >> >> I set my ancient Simpson analog volt-ohm meter to 60 uA full scale. If >> I touch the leads across the capacitor immediately after charging, the >> needle momentarily jumps to about 6 uA (1/10 full scale) as the >> capacitor discharges through the meter. If I wait half an hour (1800 >> seconds) for the capacitor to partially discharge through the mat >> resistance, the needle jumps to about 3 uA. >> >> An R-C network discharges to 3/6 of original voltage in about 0.7 time >> constant. So the time constant must be 1800/0.7 = 2571 seconds. That >> implies the mat resistance is 2571 sec / 0.1 uF ~= 2.5 x 10^10 ohms. >> >> Al N1AL >> >> >> _______________________________________________ >> Elecraft mailing list >> Post to: [hidden email] >> You must be a subscriber to post to the list. >> Subscriber Info (Addr. Change, sub, unsub etc.): >> http://mailman.qth.net/mailman/listinfo/elecraft >> >> Help: http://mailman.qth.net/subscribers.htm >> Elecraft web page: http://www.elecraft.com >> > > _______________________________________________ > Elecraft mailing list > Post to: [hidden email] > You must be a subscriber to post to the list. > Subscriber Info (Addr. Change, sub, unsub etc.): > http://mailman.qth.net/mailman/listinfo/elecraft > > Help: http://mailman.qth.net/subscribers.htm > Elecraft web page: http://www.elecraft.com > > Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Alan Bloom
Here's some more information for the latest swirling controversy :-)
If you try to measure the resistance of an anti-static mat, you'll need an ohmmeter good to at least 1,000 megohms. Even my HP/Agilent 34401A 6 digit benchtop DMM can't measure that high! Why is the resistance so high if it is supposed to dissipate static? The answer is here < URL:http://www.3m.com/us/office/meetings/rg/pdfs/w4%20choosing%20the%20right%20mat.pdf > If you are convinced you need to measure yours, here is a place that sells test equipment for doing just that < URL:http://www.botron.com/ESD_product16 > 73, Lyle kK7PO PS _ I have no financial interest in any of the above companies. This is informational only. Use the information at your own risk. The websites linked may have all sorts of malicious stuff embedded in them. Etc. _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
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In reply to this post by Alan Bloom
Al, the explanation could be that the sauce pans are not machine flat, thus have a much smaller contact area than their measured diameter would indicate. A machined flat surface with the mat sitting on a very flat surface would probably yield much different results. Trying to make laboratory accurate measurements with jury rigged apparatus sometimes yields poor results.
Willis 'Cookie' Cooke K5EWJ _______________________________________________ Elecraft mailing list Post to: [hidden email] You must be a subscriber to post to the list. Subscriber Info (Addr. Change, sub, unsub etc.): http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/subscribers.htm Elecraft web page: http://www.elecraft.com |
Testing anti-static mats - found one that works
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In reply to this post by Alan Bloom
It is important to use an anti-static mat to prevent damage to
electronic devices when working on solid-state equipment. Since Elecraft is heavily kit-oriented this is a topic of interest for Elecrafters. Quite some time ago I did some testing on the Radio Shack portable anti-static mat and found that its resistance was way too high to do a proper job of bleeding off static charges. I then bought another low-cost mat from Jameco and it also measured way too high. I began to doubt my testing methodology (described in a previous message, copied below). So now I have bought a third mat. This one is more expensive, but unlike the cheaper mats it has an actual data sheet that specifies that it meets ANSI specs for anti-static performance. I figured if it also measured bad then my testing must be in error. But it didn't. Here are the results: Radio Shack P/N 276-2370 ($22.99) - 26 G ohms (26,000 M ohms) Jameco P/N 10584 ($16.45) - 44 G ohms (44,000 M ohms) Digi-Key P/N 16-1121-ND, (Desco 66164) ($38.18) - 41 M ohms The Desco mat's resistance is about 1000x lower than the cheaper ones! This mat is quite large, (2 x 3 feet, 61 x 91.4 cm) so if you buy one you may need to cut it down to fit on your workbench. It comes with a "common point ground kit" (that you have to install on the mat yourself) that provides a long wire with solder lug to connect to ground and a two-socket connector for connecting one or two wrist straps. The wrist strap and cord have to be purchased separately. It looks like Digi-Key P/N SCP172-ND for $8.95 should work. So the total cost is about $47. But the cheap mats are no bargain if they don't work. I tried cleaning the surface of the mats with some wipes that are especially intended for cleaning anti-static mats (Digi-Key MTT20-ND) and they did help. The Radio Shack mat went from 26 to 6 Gohms and the Jameco went from 44 to about 12 Gohms. But those numbers are still way too high. The Desco mat's resistance also went down, from 41 to 20 Mohms. The Desco mat came with a small spray bottle of "Rezstore", their own brand of mat cleaner. By the way, the back side of the Desco mat is some kind of highly-conductive black rubber, apparently to keep the entire mat surface at equal potential. They warn that you should always use it blue side up because the resistance of the back side is too low. Sure enough, it was low enough to measure with my digital multimeter - about 80 kohms. So my recommendation is to buy a mat such as the Desco model that has a data sheet that specifies that it meets ANSI/ESD S4.1 or ANSI/ESD S20.20. If you already have the Radio Shack mat, then either replace it or at least clean it with a cleaner approved for ESD mats. (You're not supposed to use soap or detergent because it might harm the anti-static properties.) Gary KI4GGX recommends a product made by Techspray: http://www.all-spec.com/products/1733-QT.html Whatever you use, it is important to keep the mat clean. A word about my test procedure. Accuracy is not very good because I am trying to read the peak amplitude of a brief needle flicker. If I had an analog meter with a high-impedance input that would work better because the reading wouldn't change so fast. Also my sauce pans no doubt do not give the same answer as the ANSI-specified test probes. So overall, I doubt my measurement accuracy is better than +/- 50% or so. But for the purposes of this test that's good enough. In the test procedure below I had to substitute a 1.0 uF capacitor when measuring the Tesco mat in order to get a long-enough time constant to measure accurately. (Also, it's actually a Triplett meter, not a Simpson.) Alan N1AL On Wed, 2008-02-27 at 09:51 -0800, Alan Bloom wrote: > Well, I've convinced myself that the Radio Shack portable ESD mat, P/N > 276-2370 doesn't work properly. > > The "ESD Association" http://www.esda.org has promulgated an > industry-standard test for ESD mats, ESD S4.1. It is the standard > specified by most commercial mats. I decided not to spring for the $70 > to buy a copy of the standard, but other information I found on the web > describes the test in general terms. It uses two circular electrodes, > each weighted with 5 pounds, spaced 10 inches apart on the mat. The > "Point to Point Resistance" is specified to be: > > At 40-60% RH: 10^6 - 10^7 ohms > At 20-40% RH: 10^7 - 10^8 ohms > At 10-20% RH: 10^8 - 10^9 ohms > > I don't know what the RH here in Santa Rosa was yesterday when I did the > test, but I don't think it was very low since it has been raining > recently and the ground is still damp. For sure the resistance > shouldn't be below 10^9 ohms (1 gigohm) and probably more like 10^8 or > 10^7 (100 or 10 megohms). > > I measured 2.5 x 10^10 ohms (25 gigohms), which puts the Radio Shack mat > way out of spec. > > Test procedure: > > I didn't find a specification on the electrode size, but in the photo of > a popular tester they look to be maybe 3 or 4 inches in diameter. For > my test, the electrodes were two saucepans, each about 7 inches in > diameter and weighted with 5 pounds. They were spaced 10 inches apart > on the mat (3 inches edge-to-edge). I connected a 0.1 uF film capacitor > between the two pans and charged it to 15V with a power supply. > > I set my ancient Simpson analog volt-ohm meter to 60 uA full scale. If > I touch the leads across the capacitor immediately after charging, the > needle momentarily jumps to about 6 uA (1/10 full scale) as the > capacitor discharges through the meter. If I wait half an hour (1800 > seconds) for the capacitor to partially discharge through the mat > resistance, the needle jumps to about 3 uA. > > An R-C network discharges to 3/6 of original voltage in about 0.7 time > constant. So the time constant must be 1800/0.7 = 2571 seconds. That > implies the mat resistance is 2571 sec / 0.1 uF ~= 2.5 x 10^10 ohms. > > Al N1AL ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
Re: Testing anti-static mats - found one that works
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How are you measuring resistance?
Matt W8ESE On Wed, Apr 7, 2010 at 6:53 PM, Alan Bloom <[hidden email]> wrote: > It is important to use an anti-static mat to prevent damage to > electronic devices when working on solid-state equipment. Since > Elecraft is heavily kit-oriented this is a topic of interest for > Elecrafters. > > Quite some time ago I did some testing on the Radio Shack portable > anti-static mat and found that its resistance was way too high to do a > proper job of bleeding off static charges. I then bought another > low-cost mat from Jameco and it also measured way too high. I began to > doubt my testing methodology (described in a previous message, copied > below). > > So now I have bought a third mat. This one is more expensive, but > unlike the cheaper mats it has an actual data sheet that specifies that > it meets ANSI specs for anti-static performance. I figured if it also > measured bad then my testing must be in error. > > But it didn't. Here are the results: > > Radio Shack P/N 276-2370 ($22.99) - 26 G ohms (26,000 M ohms) > Jameco P/N 10584 ($16.45) - 44 G ohms (44,000 M ohms) > Digi-Key P/N 16-1121-ND, (Desco 66164) ($38.18) - 41 M ohms > > The Desco mat's resistance is about 1000x lower than the cheaper ones! > > This mat is quite large, (2 x 3 feet, 61 x 91.4 cm) so if you buy one > you may need to cut it down to fit on your workbench. It comes with a > "common point ground kit" (that you have to install on the mat yourself) > that provides a long wire with solder lug to connect to ground and a > two-socket connector for connecting one or two wrist straps. The wrist > strap and cord have to be purchased separately. It looks like Digi-Key > P/N SCP172-ND for $8.95 should work. > > So the total cost is about $47. But the cheap mats are no bargain if > they don't work. > > I tried cleaning the surface of the mats with some wipes that are > especially intended for cleaning anti-static mats (Digi-Key MTT20-ND) > and they did help. The Radio Shack mat went from 26 to 6 Gohms and the > Jameco went from 44 to about 12 Gohms. But those numbers are still way > too high. The Desco mat's resistance also went down, from 41 to 20 > Mohms. > > The Desco mat came with a small spray bottle of "Rezstore", their own > brand of mat cleaner. > > By the way, the back side of the Desco mat is some kind of > highly-conductive black rubber, apparently to keep the entire mat > surface at equal potential. They warn that you should always use it > blue side up because the resistance of the back side is too low. Sure > enough, it was low enough to measure with my digital multimeter - about > 80 kohms. > > So my recommendation is to buy a mat such as the Desco model that has a > data sheet that specifies that it meets ANSI/ESD S4.1 or ANSI/ESD > S20.20. If you already have the Radio Shack mat, then either replace it > or at least clean it with a cleaner approved for ESD mats. (You're not > supposed to use soap or detergent because it might harm the anti-static > properties.) Gary KI4GGX recommends a product made by Techspray: > http://www.all-spec.com/products/1733-QT.html > > Whatever you use, it is important to keep the mat clean. > > A word about my test procedure. Accuracy is not very good because I am > trying to read the peak amplitude of a brief needle flicker. If I had > an analog meter with a high-impedance input that would work better > because the reading wouldn't change so fast. Also my sauce pans no > doubt do not give the same answer as the ANSI-specified test probes. So > overall, I doubt my measurement accuracy is better than +/- 50% or so. > But for the purposes of this test that's good enough. > > In the test procedure below I had to substitute a 1.0 uF capacitor when > measuring the Tesco mat in order to get a long-enough time constant to > measure accurately. (Also, it's actually a Triplett meter, not a > Simpson.) > > Alan N1AL > > > > On Wed, 2008-02-27 at 09:51 -0800, Alan Bloom wrote: >> Well, I've convinced myself that the Radio Shack portable ESD mat, P/N >> 276-2370 doesn't work properly. >> >> The "ESD Association" http://www.esda.org has promulgated an >> industry-standard test for ESD mats, ESD S4.1. It is the standard >> specified by most commercial mats. I decided not to spring for the $70 >> to buy a copy of the standard, but other information I found on the web >> describes the test in general terms. It uses two circular electrodes, >> each weighted with 5 pounds, spaced 10 inches apart on the mat. The >> "Point to Point Resistance" is specified to be: >> >> At 40-60% RH: 10^6 - 10^7 ohms >> At 20-40% RH: 10^7 - 10^8 ohms >> At 10-20% RH: 10^8 - 10^9 ohms >> >> I don't know what the RH here in Santa Rosa was yesterday when I did the >> test, but I don't think it was very low since it has been raining >> recently and the ground is still damp. For sure the resistance >> shouldn't be below 10^9 ohms (1 gigohm) and probably more like 10^8 or >> 10^7 (100 or 10 megohms). >> >> I measured 2.5 x 10^10 ohms (25 gigohms), which puts the Radio Shack mat >> way out of spec. >> >> Test procedure: >> >> I didn't find a specification on the electrode size, but in the photo of >> a popular tester they look to be maybe 3 or 4 inches in diameter. For >> my test, the electrodes were two saucepans, each about 7 inches in >> diameter and weighted with 5 pounds. They were spaced 10 inches apart >> on the mat (3 inches edge-to-edge). I connected a 0.1 uF film capacitor >> between the two pans and charged it to 15V with a power supply. >> >> I set my ancient Simpson analog volt-ohm meter to 60 uA full scale. If >> I touch the leads across the capacitor immediately after charging, the >> needle momentarily jumps to about 6 uA (1/10 full scale) as the >> capacitor discharges through the meter. If I wait half an hour (1800 >> seconds) for the capacitor to partially discharge through the mat >> resistance, the needle jumps to about 3 uA. >> >> An R-C network discharges to 3/6 of original voltage in about 0.7 time >> constant. So the time constant must be 1800/0.7 = 2571 seconds. That >> implies the mat resistance is 2571 sec / 0.1 uF ~= 2.5 x 10^10 ohms. >> >> Al N1AL > > > ______________________________________________________________ > Elecraft mailing list > Home: http://mailman.qth.net/mailman/listinfo/elecraft > Help: http://mailman.qth.net/mmfaq.htm > Post: mailto:[hidden email] > > This list hosted by: http://www.qsl.net > Please help support this email list: http://www.qsl.net/donate.html > Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:[hidden email] This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html |
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