EG91-EX random SIM problem

We are using the EG91-EX module (EG91EXGA-128-SGNS) in one of our designs, with SIM cards from Matooma.
Most modules have been working perfectly for months, but a few of them give us a CME error 13 from times to times.
When it happens, a software or hardware reset does not resolve the problem : the module reboots, but still gives a CME error 13 when we try to send data. The problem seems to solve itself after an hour, or after removing then re-inserting the SIM card.
Also, it tends to happen multiple times for a few weeks, then stop happening for a few more weeks.

Today, I have been lucky enough to be near a device with the problem happening. Here are a few oscilloscope screenshots:

Yellow: USIM1_CLK

Yellow: USIM1_CLK
Blue: GND (on SIM card connector)

All the signals have been captured on the SIM card connector.

The most obvious thing is the low level which is not 0V but approximately 250mV.
About an hour later, the problem had disapeared, and the low level was back to 0V !

Has anyone ever seen this behaviour ? Do you have any idea on what could cause this problem ?

Best regards,

I understand that it may not be a very common issue.
Here is a more precise view of the SIM CLK signal:

And the schematic of the connexion between the EG91 and the SIM socket:

Any feedback would be much appreciated !

Best regards,

We have a similar intermittent problem. Using a 6 pin SIM and withour a SIM_PRESENCE input units may respond with SIM Not Ready. I will check to see if the SIM_CLK is going to 0V.

Hi @johnnewell,
Actually we think we have solved the problem. We were using cheaper SIM cards, which were not gold plated and were subjected to corrosion. We have switched to more robust ones with gold plating, and it seems to work better so far. However, this issue can take time to appear so we cannot be 100% sure it is solved.

Is your device outdoors ?

Based on your scope traces I looked and found the module was not supplying a clock nor Vdd. Clearly not working. It had worked, so now I need the reason for failure. Benign environment and different SIM cards made no difference.

Ok, it seems like a different problem then.
I suggest looking for voltage overshoots if you have long traces, of maybe an X-ray analysis to check the solder joints.
Also, make sure your ESD protections are properly routed.
Good luck !