ESD problem

jzawadzk · ‎2013-11-05

Posted on November 05, 2013 at 17:21

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.

#esd

Stm32User · ‎2013-11-08

Posted on November 08, 2013 at 12:43

I'd start by checking the decoupling on the power supply. Normally when you start popping devices its due to an over-voltage on the supply caused by the ESD strike; you need to shunt the energy to ground otherwise you can induce latchup. I found 100nf in parallel with 1nf was a good combination in sorting this type of problem.