nRF52840 stops working after replacing ANT pin passive components

[Superciuk]

Hello,

We have built a device that uses the nRF52840 reference design shown below. With the configuration shown the board advertises as expected. If we replace either of the components on the RF front-end (C4, L1, C13 and C14) using a soldering station, the board stop advertising and even after reprogramming the DUT become unusable. Has anyone experienced this problem?

Any suggestion?

Thank you,

Fab

[Priyanka]

Hi,

This does look like the PA (power amp) might be dead. The issue is that the ANT pad of the chip is basically the output of the RF PA with no on-chip ESD protection. A tiny ESD hit from tweezers / iron, or heating/handling that momentarily shorts or heavily mismatches the output, can blow the PA. Likely the MCU might be fine but the radio section might be dead, so then you might have to replace the SoC. 

-Priyanka

[Superciuk]

Hi Priyanka,

Thank you for your explanation. The lab and working bench are ESD protected and calibrated on a regular basis. What could momentarily short or heavily mismatches the output? Even if that happens, during rework the board has no power, the PA should not be affected unless ESD discharge happens.

Thank you,

S.

[Priyanka]

Hi,

Superciuk said:

during rework the board has no power

Even though the power is not on, a tiny static electricity or short during soldering can easily destroy the fragile radio output of the nRF52840. The antenna pin on the nRF52840 chip is super sensitive. It’s basically a tiny, fragile output that connects straight to the radio amplifier inside the chip, and it doesn’t have strong protection built in. So when you touch or replace the little RF parts, that pin is briefly left “naked.” And if at that moment any tiny static charge (even a few volts) jumps into it, it can quietly burn out that part of the chip. So the result is that your chip most likely still works, but the RF part is dead.

To prevent this in future, maybe you could first make sure to discharge the antenna part first, before touching it with anything, i.e. maybe the it to ground via a resistor etc so that it won't float. And make sure tat you don't touch it (either with hands, or tweezers or soldering iron) before you ground it.

-Priyanka

[Superciuk]

Hi Priyanka,

Thank you for your answer. According to the nRF52840 datasheet the device can withstand up to 500V CDM.

Thank you,

S.

[Priyanka]

Hi,

Could you send pictures of the board?

One,  of a board that you haven't modified and other one, a picture of a board that you have modified.

With close up of the section around the nRF52840, so that it is possible to see some details.

-Priyanka