A short circuit between the SIM_1V8 pin and GND in nRF9160

[Bartlomiej]

I ordered my second prototype board with the nRF9160, which includes small changes compared to the previous version. I encountered an issue with LTE connectivity. After some checks and hardware verification, I noticed that the SIM card is not being supplied with power. During system startup and while attempting to connect to the network, a few microvolts appear briefly on the SIM_1V8 pin for a few milliseconds. It seems like the microcontroller is trying to power the SIM card.

After further investigation, I found that the nRF9160 chip has a short circuit between the SIM_1V8 pin and GND. I desoldered the microcontroller and confirmed that the short circuit is within the nRF9160 chip itself; the PCB is unaffected. In comparison, the short circuit does not occur in the previous prototype version, which worked well and connected to the network without issue.

I received two assembled prototypes, and both exhibit the same problem. However, the previous prototypes worked correctly, and the chip was able to connect to the network properly.

I’m wondering if there’s an option in the microcontroller that needs to be enabled to supply 1.8V to the SIM card?

The difference between the chips used in the prototypes is as follows:

• Current prototypes: nRF9160 SICA B1 2230JF (with short circuit)
• Previous prototypes: nRF9160 SICA B1 2231JU (working properly)

I am using:

• SDK v2.5.1
• nRF Toolchains v2.5.1
• Modem MFW v1.3.6

[Asbjørn]

Hello Bartlomiej,

There isn't a setting for the 1V8 SIM supply that should have changed. Given that you are able to recover the short circuited ones by fixing the soldering between SIM_1V8 and GND I would suspect that something has changed on your layout of the board or something in the assembly procedure for the prototype run.

1. What did change between the two prototype runs with regard to layout? Anything related to the GND around SIM_1V8 or some of the SIM_1V8 pin itself?
2. Did the assembly house change some of the solder mask or solder flow between the two build runs?
3. You say the short circuit is within the nRF9160 chip itself, are you able to explain this a bit more? Are you saying there's solder on the chip/SiP itself short circuiting these two pins? Are you able to share pictures of this?

Best regards

Asbjørn

[RemcovH]

Hi Asbjørn,

Sorry for replying on an older chat. Only I have the same issue.

We had 2 production runs:

1. First prototype run of 4 fully assembled PCB's by a good production company. From these 4 PCB's had 2 issues with the SIM_1V8.

2. Second run of 16 assembled PCB's by the same production company (they also installed these PCB's in the final enclosure). From these 16, one had an problem with hte SIM_1V8.

Both issues, 3 PCB's in total, had the exact same behavior as is described by Bartlomiej.

• When NRF9160 is soldered on the PCB, there is a short (less then 3 ohm) on the PCB on the SIM_1V8 trace.
• When I remove the NRF9160, there is no short on the PCB SIM_1V8 trace.
• If I measre the NRF9160 between the SIM_1V8 pad and any GND_Shield pad.

Yes there were changes on the PCB, only those were not related to the NRF9160 or the eSIM to which the SIM_1V8 is connected. The production company we use, also regulary verify if there reflow process is correct. 

If they manage to produce 17 out of 20 correct PCB's I deduce there is not something inherently wrong with the production process. There most be something else that makes the SIM_1V8 very sensitive. I assume you would have had more quality issues from other customers. is this correct?