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Triophile 15th September 2011 06:00 PM

Anti-static mats
 
Hi guys.

Having potentially barbequed a few FETs in the past due to static, I thought I might as well invest in a mat, plug and wrist-strap. I bought all of these relatively cheaply off eBay, before wondering if there might be such a thing as a 'fake' anti-static mat (it only cost about 9 including postage for one about 50cms by 30cms).

The wrist-strap measures one meg in terms of resistance, from croc clip to the metal pad inside the band, so that looks OK. Is there any way to measure the mat? I tried on the highest resistance range on a DMM, one probe on the mat, the other on the stud, and from an initial '1' for overrange, the reading then dropped steadily. Does this sound right?

Maybe I'm being paranoid (I wouldn't imagine there's a huge market for fake mats), but I'd just like to be sure before I handle my next FET.

Cheers, jon.

Triophile 15th September 2011 06:01 PM

I should mention I was measuring from mat to stud without the mat connected to earth.

TerrySt 15th September 2011 09:36 PM

There are a couple of types of mats I have seen. One uses a bulk resistive material that is too high to read with a normal ohmeter and probes. If you use a couple of largish chunks of metal to increase the surface area of the measurement, you might get a low enough reading to show on an ohmeter, but it is doubtful.
The other type of mat I have seen (but haven't seen recently) used a carbon inner layer sandwiched between much less conductive layers. With those you could read the conductivity if you could contact the carbon layer. It was important that the ground snap touched the carbon layer, which didn't always happen.
I've also seen mats that were an all black almost cardboard looking material. Some of those were actually too conductive. You don't want it too conductive for a couple of reasons. One is safety. It would be a heck of a shock hazard. The other is that you can damage a part worse by picking it up off a conductive surface. The 1 meg resistance in the wrist strap cord is there for much the same reason.
Ideally, the mat should discharge the static charge in a few seconds, but not so quick that it can spark when touched.
If you can charge yourself up enough to draw a zap when you touch a grounded metal object (wear your fuzzy slippers or whatever it takes), try touching the mat. Ideally, you should not feel a zap when you touch the mat, but then after touching it for a couple of seconds, you should not get a zap touching a grounded metal object.

Terry

thaumaturge 16th September 2011 01:05 AM

Many of the Fluke meters have "Siemens" conductance ranges on their ohms functions. Siemens is the reciprocal of ohms (1/ohms). It is used to measure VERY high resistances. 1 nanosiemen = 1 gigaohm. You should be able to get a reading between the mat and the snap using the Siemens function.

What you want to see is SOME conductance. It doesn't take much to dissipate and bleed off a static charge as long as it has some conductive path to ground. The megohm (or higher) resistance to wrist straps and work surfaces serves a safety function.

Old static mats can be rejuvinated by spraying a mist of slightly soapy water onto them and letting it dry without wiping. Just misting spray cleaners like 409 over work surfaces and letting it dry will work just fine (as long as there is some part of the area leading to ground through a megohm resistor.)

Other tricks for dealing with static are; Dissipate any charge on yourself before picking up parts. When passing parts to others physically touch them first to equalize charges between you before touching parts. When you sit down at work area again dissipate charge on self before laying parts down. The trick is to never let the parts themselves make first contact with anything.

Doc

theAnonymous1 16th September 2011 02:16 AM

I have some 1/8" Velostat cross-linked polyethylene electrically conductive foam that I use as a mat (and any other use I can think of for that matter). I can measure anywhere on it with my meter and the resistance is less than 100k.

Scored a 24" x 50' roll of it on ebay for $10. Wish I would have bought a few more rolls after seeing what the retail price on this stuff was.


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