[SI-LIST] Re: Testing chips with system level specs
- From: "Pommerenke, David" <davidjp@xxxxxxx>
- To: <doug@xxxxxxxxxx>, <dcuthbert@xxxxxxxxxx>
- Date: Wed, 6 Sep 2006 08:04:19 -0500
Doug,
it is all a matter or likelyhood. If you use rounded electrodes (no =
corona discharge) and dry air you will get really fast discharges up to =
25k
V or so. We have measured such discharges usign the old Andy Hish =
simulator, but also with people.
If you look at the gemeotry of a memory card there are plenty of sharp =
edges. If we assume the card, or the cell phone or the camera is charged =
to 8kV and you plug the card into the reader then there will be some =
corona. This corona will initiate the discharge over a larger distance =
(e.g, Paschen length for 8kV is about 2mm or so) and the discharge will =
be soft. Without the corona the electrodes could approach further before =
they the discharge is initiated. In summary, I agree that for such a =
geometry the likelyhood of sub-nanosecond discharges at 8kV is probably =
low.
David Pommerenke
-----Original Message-----
From: Doug Smith [mailto:doug@xxxxxxxxxx]
Sent: Tue 9/5/2006 8:17 PM
To: dcuthbert@xxxxxxxxxx
Cc: doug@xxxxxxxxxx; Pommerenke, David; 'SI-List'; 'emc-pstc'
Subject: Re: [SI-LIST] Re: Testing chips with system level specs
=20
Hi David,
It is good to have lots of discussion on this. ESD is often only=20
considered when a gross problem surfaces. This discussion will help=20
bring the issue to the front.
What voltages were you using? Such fast events are common at low=20
voltages and rare at high voltages. I have personally measured edges=20
of <80 picoseconds at low voltages. For my chip example where slow=20
approach (say plugging in a memory card into a reader) is the norm, a=20
fast edge is probably not possible although I do not have=20
comprehensive data on this.
It seems one has to work at it to get a fast discharge at high=20
voltages, at least that is what I read through the lines. Hish et.al.=20
in a 1991 paper show three waveforms at about 10 kV, two slow and one=20
fast, but again they used a specific conical shaped tip with symmetry=20
that was required (that part was not spelled out clearly in the paper=20
but I was talking with a friend of Andy's). They said the likelihood=20
of the fast event was much smaller for a rounded tip.
Doug
David Cuthbert wrote:
> Doug,
>=20
> 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.=20
>=20
> 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.=20
>=20
> Dave Cuthbert =20
>=20
>=20
> LINEAR TECHNOLOGY CORPORATION
> =20
>=20
> Internet Email Confidentiality Footer=20
>=20
> This e-mail transmission, and any documents, files or previous e-mail
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> Thank You
>=20
>=20
>=20
> -----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
>=20
> Hi David and the group,
>=20
> You have presented good data which you and others have presented in=20
> the various standards bodies we have attended together. However, have=20
> 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=20
> measured a lot of discharges and none came close to that. Some had a=20
> vestige of the initial spike, but it was not much larger that the body =
> discharge after and its risetime was always much slower.
>=20
> If this concept was to be included in a standard, a lot of work would=20
> be needed to determine the right amount of filtering.
>=20
> But, to apply this waveform (8 kV contact discharge) to a solid state=20
> device like a flash memory card is not justified and will needlessly=20
> increase device cost. I do agree that an 8kV contact discharge has=20
> uses in system level testing which what the 61000-4-2 standard was=20
> intended for.
>=20
> There is a LOT of work to apply that standard to devices, much more=20
> 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=20
> completely determine the results (other pins grounded or ungrounded,=20
> if grounded how, and much more). If someone says their device passes=20
> this test, the statement is meaningless at this point unless the test=20
> method is documented.
>=20
> Doug
>=20
> Pommerenke, David wrote:
>=20
>>Group,
>>I like the idea from Doug to use a ferrite for reducing the risetime =
of a
>=20
> 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.
>=20
>>The current has two maxima, an initial peak caused by the charges on =
teh
>=20
> 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.
>=20
>>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
>=20
> 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 ().
>=20
>>In general the behavior is as follows:
>>
>> Fast rise times --- Slow rise times
>> =20
>> Fast approach slow approach
>> Dry air Moist iar
>> Clean surfaces Dirty surfaces
>> Oxid layer, or paint =20
>>=20
>>The effect of environmental conidtions on the discharge are very =
strong.
>=20
> 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.
>=20
>>To provide further evidence I attached a set of measurements that show =
the
>=20
> 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.
>=20
>>However, the likelyhood of having fast risetimes (e..g, less than =
200ps)
>=20
> 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.
>=20
>>Overall, I warn against changing the pulse parameters above some =
voltage
>=20
> without having strong evidence that the reduction in protection level =
is
> acceptable, the 0.7ns-1ns risetime is already providing only partial
> coverage.=20
>=20
>>Products that may see many ESDs or support critical functions should
>=20
> 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.
>=20
>>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
>>=20
>>Hi All,
>>
>>I have been writing and recording again, this time on applying system=20
>>level ESD tests to devices. If you are involved with either devices=20
>>that can be handled by people (for instance a USB thumb drive for=20
>>flash memory card) or the equipment they plug into you will find my=20
>>latest article and podcast of interest. Any standards people out =
there?
>>
>>This month's Technical Tidbit describes a method to simulate air=20
>>discharges at voltages above 4 kV in a repeatable way using a modified =
>>contact discharge. This method is especially useful in ESD testing of=20
>>solid state circuits using IEC 61000-4-2.
>>
>>Abstract: Contact discharge is used in ESD testing to improve test=20
>>repeatability, yet air discharge has significantly different=20
>>characteristics at higher voltages. A test method is described that=20
>>uses a modified contact discharge to simulate the characteristics of=20
>>an air discharge but with improved repeatability.
>>
>>The link to the article is the picture of the experimental test setup=20
>>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:=20
>>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=20
>>will then be able to play on most computers.
>>
>>Doug
>>
>=20
>=20
--=20
------------------------------------------------------------
___ _ Doug Smith
\ / ) P.O. Box 1457
=3D=3D=3D=3D=3D=3D=3D=3D=3D Los Gatos, CA =
95031-1457
_ / \ / \ _ TEL/FAX: 408-356-4186/358-3799
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| q-----( ) | o | Email: doug@xxxxxxxxxx
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