>In another life, I was working for a EMC Test lab and we always used the
>step by step procedure which was in the ESD Standard. We tested using this
>procedure for years and we did encounter some products who failed at low
>level ESD but had no problem at higher levels.
>
>We wondered what to conclude and had some hypothesis.
Benoit,
Our Electromagnetic Compatibility (EMC) Lab folks require us to perform IEC
801.2 Electrostatic Discharge (ESD) tests at all test levels up to the specified
maximum for the product for air discharge and contact discharge. For Horizontal
Coupling Plane and Vertical Coupling Plane tests, which are not required for the
CE Mark, they let us test at just the maximum voltage. During developmental
tests (we fix these before we release the product to manufacturing) we have seen
two forms of the effect that you describe-- that a product passes ESD testing at
low and high voltages but fails at intermediate voltages.
One set of symptoms was:
* Product worked perfectly despite low-voltage ESD hits.
* Around 8kV the product locked up occasionally.
* At 15kV the product did a power-on reset (POR) and restarted itself without
losing any data.
We found a couple of interrupt lines going to the microprocessor that were very
close to the back plate of the printer. At intermediate ESD voltages we put
enough of a spike on the interrupt lines that the microprocessor would start to
execute an interrupt, then hang because the interrupt went away. At higher
voltage we still hung the microprocessor, but would also trigger the POR circuit
starting the card up from scratch. Our fix was to
add 22pF capacitors on the interrupt lines at the microprocessor, and move them
well away from the edge of the card to make the card more immune to ESD.
For the other case, the product would work perfectly despite low-voltage or
high-voltage ESD hits, but would act up for intermediate-voltage ESD hits. This
was before I became involved with EMC/ESD testing, but the explanation I heard
was that the product was sensitive to the dI/dt of the ESD hits. At
low-to-medium voltages dI/dt was proportional to the voltage on the ESD gun. At
high voltages corona started carrying off the charge early, and effectively
slowed dI/dt below the susceptibility level of the product. This is akin to
bringing the gun in slowly, versus quickly (just below a speed that would damage
the product) as required by IEC 801.2.
John Barnes Advisory
Engineer
Lexmark International
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