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

[Bartlomiej]

This is the schematic corresponding to the SIM. The SIM bus is connected directly to the SiP. The U2 is the EMI filter for the SIM interface.

This is the first working prototype. The yellow line is the SiP power supply (3.3V), the blue line is the 1V8_SIM. The 1.8V is always present.

[Asbjørn]

Hi Bartlomiej,

I assume the resistance along the line from SIM_1V8 to VCC on U2 and VCC on J1 are all ok with the SiP removed? Ok as in you see infinite resistance/open circuit once the SiP is desoldered from the failing prototype? What sort of resistance do you measure along SIM_1V8 and VCC on U2/J1 on the failing prototype that still has the SiP soldered on?

The most likely cause is unfortunately something has shorted two or more pads on the SiP during the manufacturing process. How long were they running before you started probing the SIM_1V8 lines?

Best regards

Asbjørn