[SI-LIST] Re: ESD failure - need advise

  • From: Douglas Smith <doug@xxxxxxxxxx>
  • To: Orin Laney <olaney@xxxxxxxxx>
  • Date: Wed, 24 Sep 2014 12:00:36 -0700

 HI Orin and the group,
One problem is that if that wire is a few inches long it may have a thousand
volts drop along it length from Ldi/dt drop leaving the surface that is
"grounded" as much as thousands of Volts above "ground" (other end of the
wire) during the fast parts of the ESD event, enough to still jump to nearby
circuits in some cases. If even 1% of that Ldi/dt drop couples through
Mdi/dtinto nearby circuits, problems can result.

Although some general rules can help, I find that each ESD problem needs to
be analyzed (through lots of current and/or voltage measurements) to insure
asolution is understood as to how it works. BTW, there are no voltage probes
commercially available from the big measurement companies than work around
ESD that I am aware of. The one that comes closest is a passive differential
probe from Fischer Custom Communications. My favorite current probe for ESD
work is the F-65 from Fischer Custom Communications.

Doug
On 9/24/14 12:38 AM, Orin Laney wrote:
Which is why I run a separate wire. -----Original Message----- From:
si-list-bounce@xxxxxxxxxxxxx[1] [mailto:si-list-bounce@xxxxxxxxxxxxx[2]] On
Behalf Of Douglas Smith Sent: Wednesday, September 24, 2014 12:00 AM To:
si-list@xxxxxxxxxxxxx[3] Subject: [SI-LIST] Re: ESD failure - need advise I
do not find simple general rules like this to be useful. On average, systems
with circuit and chassis ground strongly interconnected do best for ESD
immunity. I have confirmed this over hundreds of systems and thousands
ofmeasurements. There is always a possibility of an exception. That wire
carrying ESD to chassis can induce tens to hundreds of Volts/inch in nearby
conductors! Doug On 9/23/14 4:16 PM, Orin Laney wrote: One useful strategy
isto provided a trace, metal rail, etc. to intercept thezap and conduct it
directly to the chassis without sharing with circuit grounds. In other
words,it's a lightning rod. Somewhere else in the system the chassis and
circuit are presumably tied together, but not as part of thispath. Shunting
the zap works better than trying to prevent it with insulation and distance.
Orin -----Original Message----- From: si-list-bounce@xxxxxxxxxxxxx[4][1]
[mailto:si-list-bounce@xxxxxxxxxxxxx[5][2]] On Behalf Of Ramesh N Sent:
Tuesday, September 23, 2014 3:48 PM To: si-list@xxxxxxxxxxxxx[6][3] Subject:
[SI-LIST] ESD failure - need advise Hello Experts, Greetings!!! we are
havingESD fail (in regulatory testing) in one of our design. It's simple
wireless DAQ system with 2 boards [controller PCB + LED/Sensor PCB]. FPC
cable (5") is used as interconnect.Package is abs plastics. Light-pipe is
used for the for led indication. When +8kV is appliedthrough air
discharging,inadvertent LED trigger occurs . Randomly, system is also
hangingwhen multiple zapping applied (especially for 8kV voltage level). I
have tried to limit airgaps, added filter caps, etc but issue is not
entirelysolved. Is there any ESD simulation/analysis tool available for
addressing this failure analysis? Any help/debug tips related to this would
be very helpful. Thanks and regards, Ramesh
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