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a table dead-bug fashion and attempt to make discharges to
pins. Others put devices in sockets. Some use ground planes
while others don’t. In other words, there is no consistency and
no reason to believe any of these tests are valid, since they are
certainly far from being repeatable or reproducible.
In order to give some guidance to IC manufacturers, Working
Group 5.6 within the ESDA is developing a standard that will
define ways to use the IEC 61000-4-2 stimulus to perform
ESD tests on a device installed in a socket mounted on PC
board. IC pins are connected via track on the board to pads
that will allow using an IEC compliant simulator to zap pins
individually. Although not perfect, the standard will define the
test conditions and environment, and will help to put everyone
on a level playing field.
Because IEC 61000-4-2 simulators were never designed
for testing ICs, alternative simulators are being investigated
that are based on commercially available transmission line
pulse (TLP) testers currently in use by many semiconductor
manufacturers for investigating ESD protection structures
on devices. It seems like a natural progression, but there are
problems getting the IEC 61000-4-2 current waveform as
specified.
CDE – Cable Discharge Events – A Mix of Simulators
For some time, manufacturers of telecom equipment have
experienced problems with cable discharge events (CDE),
particularly concerning Ethernet cable ports.6 7 8 During
installation of an Ethernet cable network in a building, the
cable is pulled through walls and across overhead structures,
resulting in triboelectric charging of the cable itself. When
this now charged Ethernet cable is plugged into a piece of
electronic equipment, a discharge occurs to the Ethernet input
port and the electronics behind that port.
Conversely, let’s assume that the Ethernet cable is already
installed in a building, not moving and uncharged. A laptop
user is leaving a meeting and headed back for an office or
cubicle where they will likely reconnect their laptop to the
network via an Ethernet cable connection. As this person
moves through the facility, the person and their laptop will
accumulate charge – nothing too significant, maybe a couple
of thousand volts – too low to be felt as a discharge by the
user. When the Ethernet cable is connected, the charges on the
laptop and person holding it will equalize with the cable, and
ESD currents will flow as a result.
The only difference between this event and the case above
where the cable has the initial charge is polarity. It’s still a
cable discharge event.
The resulting currents from a cable discharge are complex.
Initially there is a very fast spike similar to an ESD event
from a person, followed by a ringing discharge the frequency
of which is dependant on the length of the cable. One might
expect the discharge from a charged cable to be the classic
square wave, but this is not the case. Reflections along the
cable length produce the characteristic ringing.
Working Group 14 of the ESDA is in the process
of developing a standard practice that will make
recommendations about how to test for the effects of CDE.
And, in order to get a test that is representative of a real
CDE, more than one kind of simulator is required. Although
some manufacturers have set up racks with very long cables
(hundreds of meters) which are charged via high voltage
power supplies and then discharged through relays into the
equipment being tested, this is cumbersome and difficult t
54 Conformity September 2008
o use for testing products. Not all manufacturers have the
facilities to construct such a system for testing.
The ESDA working group is recommending using two
different tests, either an IEC-compliant ESD simulator to
simulate the initial fast event, or a transmission line pulser
(TLP) to provide the long slow discharge that follows. Both
kinds of simulators are available commercially from a number
of manufacturers, and will provide an easy to accomplish
bench-top test for products with connectors to external cables.
Although we’ve been talking about Ethernet cables, the same
kind of event occurs from shorter cables as well. The charge
will likely be less because of the shorter length of the cable
(and lower total cable capacitance), but the effect is the same –
a CDE into a port of an electronic product. Possibly the most
widely used cables likely to be plugged and un-plugged into
a port by a user are USB cables, but any cable capable of
holding a charge will cause a CDE to occur.
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