RE: Embedded Product ESD failure questions for the list

May 23, 2008 2 Replies
C

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)

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?



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).



Thanks in advance;



Chris


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J

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

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).

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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M

We fixed a similar problem by putting 0.01 uF caps on all the lines that were subjected to ESD. We normally place ground planes (solid not hatched) around the keypad. We already had TVS diodes on the keypad lines but they weren't reacting quickly enough to the discharges.

Mark Walsh

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