[SI-LIST] Re: Testing chips with system level specs
- From: "David Cuthbert" <dcuthbert@xxxxxxxxxx>
- To: <doug@xxxxxxxxxx>, <davidjp@xxxxxxx>
- Date: Tue, 5 Sep 2006 09:53:33 -0600
Doug,
I've done some measurements of actual human body discharges into a 2 ohm
current target. The rise time was less than 500 ps. I plan to continue this
work soon using a 2.5 GHz oscilloscope and a TDR. The human SPICE model I
developed is quite interesting and I'll be refining it.
I then built a circuit that quite accurately mimics the actual human body
discharge. It is much more complex than the usual ESD gun network.
Dave Cuthbert
LINEAR TECHNOLOGY CORPORATION
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-----Original Message-----
From: emc-pstc@xxxxxxxx [mailto:emc-pstc@xxxxxxxx] On Behalf Of Doug Smith
Sent: Monday, September 04, 2006 11:15 AM
To: davidjp@xxxxxxx
Cc: SI-List; emc-pstc
Subject: Re: [SI-LIST] Re: Testing chips with system level specs
Hi David and the group,
You have presented good data which you and others have presented in
the various standards bodies we have attended together. However, have
you ever seen an air discharge with a 700 ps rise time at 8 kV? If so,
what is the probability in the distribution of 8kV discharges? I
measured a lot of discharges and none came close to that. Some had a
vestige of the initial spike, but it was not much larger that the body
discharge after and its risetime was always much slower.
If this concept was to be included in a standard, a lot of work would
be needed to determine the right amount of filtering.
But, to apply this waveform (8 kV contact discharge) to a solid state
device like a flash memory card is not justified and will needlessly
increase device cost. I do agree that an 8kV contact discharge has
uses in system level testing which what the 61000-4-2 standard was
intended for.
There is a LOT of work to apply that standard to devices, much more
than the filtering I wrote about. There is no guidance in the standard
as to how to apply the discharge and how it is applied will almost
completely determine the results (other pins grounded or ungrounded,
if grounded how, and much more). If someone says their device passes
this test, the statement is meaningless at this point unless the test
method is documented.
Doug
Pommerenke, David wrote:
> Group,
> I like the idea from Doug to use a ferrite for reducing the risetime of a
contact mode ESD generator. However, I do not agree to the statement that
air discharge ESD will not show fast risetimes and high peak values at
voltages above 4kV. The reference event for the ESD standard IEC 61000-4-2
is the discharge between a hand-held metal part and a large metallic surface
(called "hand-metal ESD") in contrast to the IC-HBM standard that is based
on a discharge from the skin.
>
> The current has two maxima, an initial peak caused by the charges on teh
hand and on the metal part and the later body waveform. If the initial peak
will show up depends on the resistance of the arc as a function of time. If
the arc resistance drops quickly (let us say in less than 1ns) below the
source impedance of the discharging person (without going into details,
assume 100-300 Ohm http://web.umr.edu/~davidjp/paper/00478274.pdf ), then
the inital peak will show up. If the arc resistance drops slowly, let us say
it reaches 300 Ohm in 5 ns, then the initial peak will not show up, as the
arc resistance is too high during this phase of the discharge.
>
> So the quesion is: How fast does the arc resistance drop?
>
> This depends mainly on:
>
> - Voltage at the moment the discharge starts
> - Gap distance at the moment the discharge starts
>
> The smaller the gap, the faster the arc resistance will drop. The gaps
will in most cases not discharge over distances given by the Paschen-law,
but at smaller distance. This is a result of the speed of appraoch and the
statistical time lag ().
>
> In general the behavior is as follows:
>
> Fast rise times --- Slow rise times
>
> Fast approach slow approach
> Dry air Moist iar
> Clean surfaces Dirty surfaces
> Oxid layer, or paint
>
> The effect of environmental conidtions on the discharge are very strong.
Humidity dominates over all other influencing factors (I can email papers on
this topic on request). It is not possible to state: Above XYZ kV discharges
will not have an initial peak.
>
> To provide further evidence I attached a set of measurements that show the
peak current as a function of voltage having the arc length as parameter.
The data is from D.Pommerenke, ESD: Transient fields, arc simulation and
rise time limits, Journal of Electrostatics, 36, 1995, 31-54.
>
> However, the likelyhood of having fast risetimes (e..g, less than 200ps)
decreases above about 6-10 kV. Nobody knows the distribution of ESD
intensity in reality very well. There are a few studies, but they only help
to answer the question of voltage distribution, not of rise time
distribution or field strengths distribution.
>
> Overall, I warn against changing the pulse parameters above some voltage
without having strong evidence that the reduction in protection level is
acceptable, the 0.7ns-1ns risetime is already providing only partial
coverage.
>
> Products that may see many ESDs or support critical functions should
certainly not be tested at a different waveform. The 0.7ns - 1ns rise time
standardized contact mode waveform certainly does not cover the faster ESD
events.
>
> Regards,
>
> David Pommerenke
>
>
>
> -----Original Message-----
> From: si-list-bounce@xxxxxxxxxxxxx on behalf of Doug Smith
> Sent: Sun 9/3/2006 11:03 PM
> To: SI-List; emc-pstc
> Subject: [SI-LIST] Testing chips with system level specs
>
> Hi All,
>
> I have been writing and recording again, this time on applying system
> level ESD tests to devices. If you are involved with either devices
> that can be handled by people (for instance a USB thumb drive for
> flash memory card) or the equipment they plug into you will find my
> latest article and podcast of interest. Any standards people out there?
>
> This month's Technical Tidbit describes a method to simulate air
> discharges at voltages above 4 kV in a repeatable way using a modified
> contact discharge. This method is especially useful in ESD testing of
> solid state circuits using IEC 61000-4-2.
>
> Abstract: Contact discharge is used in ESD testing to improve test
> repeatability, yet air discharge has significantly different
> characteristics at higher voltages. A test method is described that
> uses a modified contact discharge to simulate the characteristics of
> an air discharge but with improved repeatability.
>
> The link to the article is the picture of the experimental test setup
> at the bottom of the home page at http://emcesd.com . Or just click on
> this link:
>
> http://emcesd.com/tt2006/tt090106.htm
>
> There is also an audio discussion of this article on my podcast site:
> http://emcesd-podcast.com where the direct link to the audio file is:
>
> http://emcesd-podcast.com/2006/september/2006-0904.mp3
>
> Can't download mp3 files? Download the following instead:
>
> http://emcesd-podcast.com/2006/september/2006-0904.dcs
>
> After download, change the extension from .dcs to .mp3 and the file
> will then be able to play on most computers.
>
> Doug
>
--
------------------------------------------------------------
___ _ Doug Smith
\ / ) P.O. Box 1457
========= Los Gatos, CA 95031-1457
_ / \ / \ _ TEL/FAX: 408-356-4186/358-3799
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| q-----( ) | o | Email: doug@xxxxxxxxxx
\ _ / ] \ _ / Web: http://www.dsmith.org
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