Re: Embedded Product ESD failure questions for the list

Chris_99 <cgruff@xxxxxxxxx> writes:

We have a product that is deployed around the world and is in the
human-ESD discharge path.
We are experiencing problems with an occasional CPLD shorting out
internally which, of course, requires
a field replacement of the entire controller. This plastic enclosure,
membrane keypad equipped
product has tranzorb protection on all keyboard lines, and the
tranzorbs are not bad on the
returning units. I cannot kill components on good controllers with
16kV spikes, even directly
to the membrane keyboard signals on the board. (although the processor
will reset essentially
every time)

Do you have series impedance as well as the transorbs on all lines)?
The transorbs clamp the voltage to a managable level (say 20-200V)
then the series impedance limits the current during the
spike. Resistors for signals, ferrites for power.


ESD ----------[R]--------> Capacitance or input protection diodes
|
TZ
|
=== Groundplane


Does the list have an opinion as to the voltage I should actually be
testing at? What voltage do
all of you test your embedded designs at? Is 16kV too low to test/
design keypad controllers at?

I would design to not crash at 16kV, then you can be reasonably
confident it wont blow up in the field. (Perhaps less true for designs
with a lot of distributed logic).

And does the list have an opinion on placing a grounded grid outer-
most on the membrane
keypad? Do any of you do that religiously? I am getting mixed opinions
to this question which
seems obvious to me (do the grounded grid).

In my opinion it should be possible to protect against even the
largest ESD events with enough filtering and the right layout / ground
plane. Having said that the grounded grid should help too. Make it
work without, then do the grounded grid anyway!


--

John Devereux
.

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