CLK short circuit to GND BG95 M3

IoT Modules LPWA Module
1

Hello,
We prototype with a BG95 M3 module and have problems with a short circuit of the USIM_CLK line. We unsoldered the capacitor and cut open the connection to the SIM card holder to make sure none of these components are the reason for the failure.

I believe because of the short circuit the AT-Commands regarding the APP work fine (for example QPOWD command) but the commands regarding SIM connection (like CMGF, CMGL or CMGD) result in an ERROR - is this assumption right?

Is there any software reason why the CLK line is pulled down to GND - if yes how can I redo this pulldown?
I did a mistake in the layout by connecting the USIM_VDD with a 100nF capacitor to general GND and not to USIM_GND - could this somehow caused the CLK line short circuit?

This failure is on two PCBs out of an order of 5 PCBs so maybe it is an assembly mistake but I want to exclude any reasons. The other PCBs work fine with a resistance of around 55 k Ohm between CLK line and GND does that sound about right?

Thank you very much for any comments and advises to prevent this failure.
Bye Lukas

LayoutMistakes
LayoutMistakes895×601 85.4 KB

NoCapacitorOnCLKline
NoCapacitorOnCLKline605×825 111 KB

ShortCircuitonCLKline
ShortCircuitonCLKline1061×793 154 KB

2

The ERROR answers from the module and the cut of the CLK line to the SIM card holder
CutOpenTheCLKlineToSimCardHolder

AT Empfang
AT Empfang1758×1041 249 KB

3

You can use the command of AT+CPIN? to judgement the SIM card can be identified.
And according to the schematic ,it is recommended to add ESD protect devices for USIM circuits and pull-up resistor for USIM_DATA to prevent interference.
If the USIM_GND is connected to GND ,it is OK to connect GND to USIM_VDD.

4

image
You can refer to above.