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ESD problem

Question asked by qubas on Nov 5, 2013
Latest reply on Nov 8, 2013 by mcwiggin.patrick
Recently I had a chance to do some quick ESD tests on my board, unfortunately time was very limited.

Some basic information:
-STM32F103T8U6 MCU,
-USB bus powered,
-USB is already protected with USBLC6-2SC6,
-Plastic enclosure (almost entirely occupied by PCB),
-no other connectors beside USB plug, no buttons / LEDs
-PCB space is very limited,

For 2kV and 4kV air-discharge everything was just fine. At 8kV my device stopped working. At first, after reset (unplug & plug) everything was back to normal, however after several attempts, I ended up with 2 dead boards. It turns out that in both cases only part that failed was the MCU.

1) How should I configure unused IO pins (power consumption is not important)?
2) What should I do with QFN pad? Currently it is left unconnected. Should I connect it to GND?
3) What about USB shield? I think about making a "guard-ring" along PCB edges, directly connected to USB shield, which will be connected to GND at a single point (via 4.7nF capacitor and 1M ohm resistor or alternatively via 0 ohm resistors).

I would also be very interested in any other tips/suggestions.

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