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

[Bartlomiej]

Hello Asbjørn,

Thanks for replay.

To clarify my problem, I removed the nRF9160 chip and did the measurements (resistance measurements). The PCB without chip, has no short circuit issues, it means the resistance between SIM_1V8 and GND is infinite (no short circuit), also in other lines on SIM lines.

 

Asbjørn said:

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?

The only change in the SIM lines between the current and the last prototype is a slight offset of the lines. In my opinion, this does not affect the problem discussed.

Asbjørn said:

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?

I performed the resistance measurements only on the nRF9160 chip, which is removed from the PCB. The resistance between SIM_1V8 and GND is 0 ohm (short circuit). The chip/SiP does not show any solder shorting of the pins, it is only when measuring the resistance that you can see that there is a short circuit.

 

So if there's no setting for the 1V8 SIM, it looks like the actual chips are broken, but it's interesting why two chips have the same problem?

 

[Asbjørn]

Hello Bartlomiej,

If there's a short between the SIM_1V8 and GND on the package it indicates that the internal paths have broken down and the chips are broken unfortunately. 

You say that you could initially see a few mV on the SIM_1V8 pin when powering up the prototypes, but it only lasts for a short duration. It would indicate that the SIM_1V8 pin on the package is trying to start it up but there's a short and the regulator collapses.Did you capture this behavior on any tool? Have you removed the nRF9160 SiP from both of the two failing prototype boards?

Wihtout knowing more about the layout and changes, could there have been some changes to the soldering process and stencils used to mount the SiP on the latest prototypes that could have added more solder than intended or distributed it differently? 

Have you tried soldering a new SIP onto one of the kits from the second build run? What resistance are you measuring between SIM_1V8 and GND on a SiPs that haven't been mounted on any boards yet?

Best regards

Asbjørn

[Bartlomiej]

Hello Asbjørn,

Asbjørn said:

You say that you could initially see a few mV on the SIM_1V8 pin when powering up the prototypes, but it only lasts for a short duration. It would indicate that the SIM_1V8 pin on the package is trying to start it up but there's a short and the regulator collapses.Did you capture this behavior on any tool?

Yes, I have recorded it. The picture below shows the behaviour. The picture shows two different bords. The yellow line is the SiP power supply (3.3V), the blue line is the 1V8_SIM. About 1s after powering up, the 1.8V line spikes. In one case the voltage reaches 200 mV and in the second 6 mV, in both cases the spick duration is 244 ms.

Asbjørn said:

Have you removed the nRF9160 SiP from both of the two failing prototype boards?

Only one was removed. But the second board has the same behaviour and the resistance between SIM_1V8 and GND is also zero.

Asbjørn said:

Wihtout knowing more about the layout and changes, could there have been some changes to the soldering process and stencils used to mount the SiP on the latest prototypes that could have added more solder than intended or distributed it differently?

I don't know anything about it, the settings and parameters for production and assembly were the same as before. I think it will be difficult to get this information from the factory.

Asbjørn said:

Have you tried soldering a new SIP onto one of the kits from the second build run? What resistance are you measuring between SIM_1V8 and GND on a SiPs that haven't been mounted on any boards yet?

Now I don't have it, I have to order the new SIP. When the order arrives, I will check it.

It is worth saying that the other peripherals I use on the prototype in the processor work fine.

[Asbjørn]

Do you have any components between the SIM_1V8 pin on the SiP and the SIM card holder itself? Did you do any changes to the component selection between the two prototype runs?

I assume on the first prototype run the SIM_1V8 line goes up all the way to 1.8V and the duration, 244ms, is the same?

[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