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Why is VSIM 0V?

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davegravy:
Before I repeat this madness for a 3rd time could someone please check my design (attached, see schematic page 2) for the SIM holder connection? Comments on other aspects also welcome.

My design is a LTE-M cape for an SBC, it's based very closely on this sparkfun board: https://cdn.sparkfun.com/assets/d/7/b/c/3/lte_cat_m1_shield_schematic_v11.pdf

I had JLCPCB make and assemble 5 of my boards minus the Ublox Sara-R5 LTE-M module because they're very expensive and I can't afford to buy 5 of them so I opted to buy just a couple and hand-solder the module.

My first unit worked, powered up, AT commands over UART worked fine, but it wouldn't read the SIM card. I traced the issue to VSIM being 0V (if SIM is inserted or not). I double checked the schematic and layout and no obvious mistakes, so I figured it was a bad solder connection.

All the module pads are buried so I'm working blind. I decided to try and reflow, but then it formed a short between the main supply and GND. So I decided to desolder, clean up the module and pcb pads, and try again. Turns out it's very hard to use tweezers to grab the module and NOT the shield while it's molten. The shield coming off wrecked the module.

I soldered my second module on another board and same thing - everything worked except the SIM and again I measured 0V VSIM. This made me question that it could be a solder joint issue, what are the chances the same pad twice was the only one that failed to join?

FWIW I checked impedance of VSIM to GND and it's >1MOhm.  I also wrecked the 2nd module trying to desolder it, so now contemplating a third attempt as costs climb



This is the microSIM holder: https://www.lcsc.com/datasheet/lcsc_datasheet_2206302030_XUNPU-SMC-204-ARP6_C3446130.pdf

davegravy:
I'm thinking for a 3rd attempt rather than fully solder the module I'll just connect VCC, GND, PWR_ON and VSIM to the PCB via bodge wire and validate VSIM. Then only if that works I'll attempt another full surface mount solder

davegravy:
I just discovered that the VSIM supply is a negotiated level using serial communication to the SIM card per ISO-IEC 7816-3.

The SARA-R5 datasheet isn't clear how it handles cases of communication failure (e.g. due to connection issue or faulty SIM card). It may be disabling the supply to save power when it can't negotiate a supply voltage?

I guess I'll have to probe to see what's going on with SIM_IO, SIM_CLK, and SIM_RST.

davegravy:
There seems to be an error in the Sparkfun circuit I copied. The schematic shows a DT1042-04S0-7 for the ESD diode array, however that model needs pin 2 and 5 tied to VCC not GND as is done in the sparkfun schematic.

The actual manufactured product appears to use an ESDA6V1BC6 which will work with the GND connection.

Sigh.

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