STM32WB55CGU6 custom board: ST-LINK cannot detect target, low current consumption, PA10 output weak
Hello,
I designed a custom 4-layer PCB using STM32WB55CGU6. After assembly, ST-LINK cannot detect the MCU.
ST-LINK connection:
- SWDIO connected to MCU SWDIO pin
- SWCLK connected to MCU SWCLK pin
- GND connected
- Target VCC connected
- ST-LINK reports target voltage around 3.25 V
- Error message:
"Unable to get core ID"
"No STM32 target found"
Main symptoms:
1. The board has 3.25 V on the 3V3 rail.
2. ST-LINK cannot connect to the MCU.
3. PA10 is configured as GPIO output high to drive LED1, but PA10 voltage is only about 2.18 V.
- R8 MCU side: 2.18 V
- R8 LED side: 2.15 V
4. The same firmware works on another known-good board.
5. I replaced the STM32WB55CGU6 with a known-good chip, but the issue remains.
The removed chip still works on another board, so the MCU itself does not seem damaged.
6. Current consumption comparison:
- Known-good board: about 23 mA
- Problem board: about 9.2 mA
- Known-good board while holding RESET: about 12 mA
- Problem board while holding RESET: about 3 mA
7. The non-GND side of C13, which is connected to the STM32WB55CGU6 VFBSMPS pin, is about 3.25 V on both the known-good board and the problem board.
8. SWDIO and SWCLK continuity from ST-LINK connector to MCU pins has been checked with a multimeter.
Known design issue found:
PB4 was accidentally connected to the battery/LDO input power net. I cut the trace between PB4 and that power input net, but the current consumption and ST-LINK issue did not change. Therefore I think PB4 is a design mistake, but maybe not the main cause of the current issue.
Things I suspect:
- STM32WB55CGU6 UFQFPN48 soldering issue
- exposed pad / thermal pad soldering issue
- possible solder wicking through vias under the exposed pad
- some VDD / VDDRF / VDDSMPS / VDDUSB pin not properly soldered
- SWD not working because the MCU is not starting correctly
Questions:
1. From the schematic and Gerber files, do you see any issue that would prevent STM32WB55CGU6 from starting or being detected by ST-LINK?
2. Is my power supply scheme for STM32WB55CGU6 correct?
3. Could the exposed pad vias or QFN soldering cause this kind of symptom: 3.3 V present, low current, PA10 weak output, and ST-LINK unable to get core ID?
4. Is connecting PB4 to the battery/LDO input net likely to permanently damage the MCU, or would it mainly cause leakage/back-powering/conflict depending on GPIO state?
5. What measurements should I perform next to distinguish between PCB design issue and assembly/soldering issue?
I have attached:
- schematic PDF