[SI-LIST] AW: Re: Testing chips with system level specs
- From: "Pommerenke, David" <davidjp@xxxxxxx>
- To: <doug@xxxxxxxxxx>
- Date: Mon, 4 Sep 2006 13:59:50 -0500
Doug,
The standard has many shortcomings, e.g., testing of PINs is only =
partially regulated, the selection of test points etc. You are right and =
the new version will only improve this a little.
On the 8kV air discharge:
We have measued a lot of air discharge and had many discharges as low as =
200 ps at 10 kV and up to 25 kV only a few hundred ps. The data is =
published in the Journal of Electrostatics. The correct question, as you =
mentioned, is knowing the propability distribution. There is no doubt =
that 8kV discharges are not that uncommon, and if the humidity is very =
low, 10% then one will certainly see discharges of extremly short arc =
length.=20
If anyone has problems obtaining a copy of the data, please let me know.
Now, if the consequence is just an upset, why worry for a consumer =
product? But for consumer products only 4kV contact mode testing is =
required. If a company goes beyond it is in most cases not a question =
regulations, but of some quality expectations.
David Pommerenke
-----Urspr=FCngliche Nachricht-----
Von: Doug Smith [mailto:doug@xxxxxxxxxx]
Gesendet: Mo 04.09.2006 12:15
An: Pommerenke, David
Cc: SI-List; emc-pstc
Betreff: Re: [SI-LIST] Re: Testing chips with system level specs
=20
Hi David and the group,
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,=20
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=20
discharge after and its risetime was always much slower.
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.
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.
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=20
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.
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.
>=20
> 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.
>=20
> So the quesion is: How fast does the arc resistance drop?
>=20
> This depends mainly on:
>=20
> - Voltage at the moment the discharge starts
> - Gap distance at the moment the discharge starts
>=20
> 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 ().
>=20
> In general the behavior is as follows:
>=20
> 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. 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 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) 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 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 =
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,
>=20
> David Pommerenke
>=20
>=20
>=20
> -----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,
>=20
> 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?
>=20
> 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.
>=20
> 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.
>=20
> 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:
>=20
> http://emcesd.com/tt2006/tt090106.htm
>=20
> 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:
>=20
> http://emcesd-podcast.com/2006/september/2006-0904.mp3
>=20
> Can't download mp3 files? Download the following instead:
>=20
> http://emcesd-podcast.com/2006/september/2006-0904.dcs
>=20
> After download, change the extension from .dcs to .mp3 and the file=20
> will then be able to play on most computers.
>=20
> Doug
>=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
/ /\ \ ] / /\ \ Mobile: 408-858-4528
| q-----( ) | o | Email: doug@xxxxxxxxxx
\ _ / ] \ _ / Web: http://www.dsmith.org
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