Posted by
AJSOENKE on
Feb 28, 2008; 3:23am
URL: http://elecraft.85.s1.nabble.com/Radio-Shack-ESD-mat-doesn-t-meet-spec-tp462383p462384.html
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)
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