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:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
