ESD Testing Does Not Test EMC Susceptibility

Equipment designed for an industrial environment must recover from ESD events and be immune to magnetic coupling from motors and wires carrying current transients. 

In ESD testing, every connector port and accessible surface is shocked with an ESD gun.  The ESD gun is configured for 15kV air gap and brought close enough to discharge to the product.  All locations are hit again with an 8kV contact discharge.  Both tests are repeated with negative polarity. 

One would expect the transient currents inside the enclosure associated with the ESD testing would also test immunity to magnetic coupling.  If something can recover from the current associated with rapidly discharging a capacitor with 8kV through the device, you might think, it can recover from any reasonable pulsed current outside of the enclosure.  This intuitive reasoning turns out to be false.

A few years ago I performed this test on a product for use in an industrial environment.  Applications engineers reported that in apps testing interference from a motor being energized could cause the product to lock up.  How could this be if it recovered reliably from repeated ESD gun discharges? 

It turned out that a processor’s low-voltage core reset line was routed over a break in the ground plane.  One side of the split plane was connected to the product chassis.  The reset line, like most reset lines, was connected to an open-drain output on a reset chip and a weak pullup resistor.  A small amount of current flowing between the ground planes was enough to assert the reset.  A proper reset required two reset lines to be asserted..  The direct ESD discharges caused both reset lines to be asserted, hiding the potential problem of a partial reset. 

A stronger pull up increased immunity to magnetic transients.  Drilling through the trace and routing it with engineering wire, keeping it clear of chassis ground, completely solved the problem. 

A good source of magnetic transients is an ESD gun discharged to its ground with its cord wrapped around the device under test (DUT).  If I had done that test at the time of my ESD testing, I would have found the problem before my product went to apps testing. 

Lessons Learned:

1. Even though a reset line does not have high-edge rate signals that could make it an aggressor trace, high-impedance pullups make it susceptible to becoming a victim trace.  Trace routing has a huge impact on coupling. 
2. Whenver I’m doing ESD testing, even when pulsed magnetic field testing is not part of the test, I always discharge it the gun a few times directly to its ground to see if it affects the behavior of the DUT.