ESD Protection for SWD Line

[hamit]

Hi to all,

I have designed a custom board with 3rd party module (RAYTAC MDBT50Q) which includes NRF52840. During debugging on the custom board, two modules were demaged and both of them have a short circuit between swdio and GND now. we think that we demaged the protection components on the module.

Despite the module have some ESD protection as below, do we need to add any extra protection to swd lines?

In serial production, should we tie the pins directly to ground?
Is there any suggestion or guide for ESD proof design?

Regards,
Hamit

[haakonsh]

If a GPIO (SWDIO) has short to GND then it is not unlikely that you have had an ESD event, but I think it's more likely that GND has been pulled below -0.3V, this will cause the ESD diode between the GPIO and GND to conduct current and due to it's low current capacity it will quickly burn out, and where the typical failure mode is a short. 

I suggest you investigate whether there's a voltage potential between GND of the custom board and GND of the programmer. 

How many boards were programmed successfully? 

[hamit]

Thank you for answer, I have checked the voltage difference between custom board's GND and debugger's GND and did not see difference.

For your question:

we produced 5 pcs of samples for our software team. The pcba was done manually and then modules are reprogrammed several times. Moreover, some of the samples was used for debugging. As the pcba done manually by heat gun, the solder inside the module might be damaged, so it is hard to detect the exact problem for us.

Just for the info: we realized that one of our linear voltage regulators was not stable due to a wrongly placed capacitor. It starts oscillating during a tiny oled screed opening. Since we placed the correct capacitor, then we did not see any defected module. we are using "Reg0 DC/DC and LDO mode disabled" as RAYTAC offered in user manual. It does not sense much but worth to consider for people who face such a problem later.

Now there is no problem.

Thanks very much,

Hamit