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May 2009

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From:
Robert Kondner <[log in to unmask]>
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Date:
Thu, 14 May 2009 17:20:44 -0400
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Karen,

  There is no discharge event when you touch the charged plastic. 

  Where would a current flow along the plastic surface or through the
plastic. Glass is the same idea. These are insulators.

Bob Kondner

-----Original Message-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Karen Tellefsen
Sent: Thursday, May 14, 2009 4:13 PM
To: [log in to unmask]
Subject: Re: [TN] Bare PCB Packaging Requirements

An electrical field is a voltage (potential) gradient, and may be given is
volts/meter or newtons/coulomb.

If I rub a piece of plastic with wool so that it is charged and has an
electrical potential different from ground, then, while electrically
grounded, will I feel a shock if I touch the plastic? Probably, and that's
current.

If a chip is carefully placed on a uniformly charged plastic surface with a
uniform electrical potential, it will likely experience no current.  If a
bird perches on a single, live power cable, it will not be injured.  This
will not be the case if a squirrel simultaneously  touches the hot cable
and the return.  Likewise, if one of the connectors on an IC package is
grounded, possibly by somebody,s hand, and another connector contacts a
charged surface, it is possible for current to be passed through the IC.

A piece of plastic may not have a uniform electrical potential along its
surface, and hence an electrical field.

For more information:
http://en.wikipedia.org/wiki/Electrostatic_discharge

Karen Tellefsen - Electrical Testing
[log in to unmask]
908-791-3069

-----Original Message-----
Inge,

  Is Electric Field really the issue or is it more of a case of induced
currents?

  If I rub a piece of plastic with wool so It has a very high field, and if
I then set an IC on that plastic, will any current flow anywhere?

  If not then I do not see how the IC can be damaged.

  If I take a 1.5 volt battery and connect it to power and signal pins will
current flow? I think so and the part will probably be destroyed.

 So is it field or induced current levels?

Thanks,
Bob Kondner

-----
From: TechNet [mailto:[log in to unmask]] On Behalf Of Inge
Sent: Thursday, May 14, 2009 2:49 PM
To: [log in to unmask]
Subject: Re: [TN] Bare PCB Packaging Requirements

Electrostatic fields can be induced in high dielectrics and once there, may

be hard to get rid of. As little as 50 VDC can be a problem for IC chips
without enough I/O diode protections. We have had some issues which might
have been caused by such surface charges, but we could never prove it. To
be

sure , we demand ESD safe bags. Additional cost negligible.
Inge

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