[Elecraft] ESD mat measurements

Fri Dec 19 07:14:34 EST 2008

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
    kurtt at pinrod.com
    (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
>>
>>
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