Ravinder,
I'd say that you are in pretty good shape now. EN 55024 (information
technology equipment), EN 61000-6-1 (generic devices in residential,
commercial, and light industrial environments), and EN 61000-6-2
(generic devices in industrial environments) all specify performance
criterion B for:
* +/-4kV contact discharges (including zaps to the horizontal and
vertical coupling planes).
* +/-8kV air discharges.
I like to test up to +/-8kV contact discharge and +/-15kV air discharge.
The effort we are willing to invest in trying to raise the ESD-immunity
level depends on:
* Where we are in the project.
* How much margin we have already.
* How much of a schedule crunch we are in.
* The expected "pain level" for the next round of ESD fixes.
* Whether my boss/client is starting to complain about the time I've
already spent in ESD testing, versus everything else he/she needs me
to do...
If we are ESD testing an engineering unit, with another board spin
planned-- and we have the time and opportunity-- I'll try to push the
ESD-immunity level as high as I can so that we can include the fixes in
the next layout pass.
If we are testing a Design Verification Test unit, with no more board
spins planned, I will shoot for at least 25% margin (meet performance
criterion B for +/-5kV contact and +/-10kV air discharges), with no
permanent damage for +/-8kV contact discharge and +/-15kV air discharge.
If we are running the final approval tests on my own or a client's
product, I grudgingly accept meeting performance criterion B for up to
+/-4kV contact and +/-8kV air discharges. But I'll want to get the unit
back into the ESD lab as soon as possible to see what happened to our
margins!
As always in engineering, we have to trade-off the time/effort to
perfect a piece of a design versus getting the entire job done. I
personally give ESD testing high priority, because it can help me find
and fix so many weaknesses in designs quickly. But the person paying
the bills has the final say, as long as we meet the legal and regulatory
requirements.
John Barnes KS4GL, PE, NCE, ESDC Eng, SM IEEE
dBi Corporation
http://www.dbicorporation.com/
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