Disagree.
Don W4DNR
Quoting MU 4CX250B <4cx250b@miamioh.edu>:
Speaking of high resistance mats, an interesting property is that the
resistance between any two points on the map is the same, no matter
the distance between the points. In other words, it doesn't matter
whether you put your test probes a cm apart or 10cm apart, the
resistance will be the same. That's why the resistance of a flat mat
is always specified in ohms, unlike three-dimensional materials whose
resistivity is specified in ohm-cm. In two dimensions, resistance and
resistivity are the same thing.
73,
Jim w8zr
Sent from my iPhone
On Apr 11, 2017, at 12:42 PM, MU 4CX250B <4cx250b@miamioh.edu> wrote:
Ah, Wise move on your part, Manfred. I wouldn't wear it either! Your
former boss needed higher level Technical Support!
Jim
Sent from my iPhone
On Apr 11, 2017, at 12:39 PM, Manfred Mornhinweg <manfred@ludens.cl> wrote:
Jim,
Manfred, I think you are worrying needlessly. A grounding wrist
strap connects to the mat, not to the device under test. ESD mats
typically have a resistance in the 10E7-10E8 ohm range. The mat
on my workbench has a resistance too high to measure with my
Fluke 87-V. The mats discharge static buildup, but neither they
nor the wrist strap pose any safety hazard.
Fine then. But the straps that one boss at the job wanted me to
wear were all metal. Indeed they connected to the mat - but to a
metal frame surrounding the static dissipative (highly resistive)
material, and that frame was grounded. In the end, that wrist
strap was grounded with a very low resistance, and I refused to
wear that, for safety reasons.
I have lost MOSFETS from not taking adequate ESD measures. Some
of the older devices, especially, are very easily burned out.
There are some that don't have the built-in zener protection -
those are indeed fragile. Laser diodes (or rather their built-in
photodiodes, I think) are also said to be very sensitive to
static. I have handled such devices with no more precautions than
the basic ones, and never lost any.
There's a reason all semiconductor distributers (Mouser, Digikey,
etc.) pack their components in ESD envelopes!
Yes, and that's actually a good thing to do, and I do it too, when
I ship something sensitive. My fundamental point instead is that
thoughtlessly used grounding straps and the like can CAUSE more
risk to the parts than they help prevent! I have seen people who
put on such a grounding strap, next to their static-safe
workbench, and then think that nothing bad can happen. Then they
reach over to a drawer and withdraw a MOSFET by the gate terminal,
and !ZAP!, they discharge the entire drawer through that MOSFET!
My practice instead is to first get hold of the drawer, to put
myself at its potential, then pick up the MOSFET by anything but
its gate terminal, then walk over to my desk, touch the desk, then
place the MOSFET on it. In doing so, I have already double safety
in it: By avoiding to touch the gate first, and by equalizing the
potential between myself, the desk, the MOSFET, and anything else,
in a safe way.
Most of this caution exceeds what's needed, but as you say, it's
smart to be careful. And I would add that it's good to be smart!
In the sense of thinking where static charges will form, what can
be charged relative to what, which items could carry significant
leakage current, and so on, and then acting accordingly. That's
much safer than using a mat, a strap, and stopping to think about
the matter, which is what I have witnessed some people doing!
Manfred
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