This is a replica of 「KNMK-0001A “Fraduino”」, with a different microcontroller.
Shape reference: https://oshwhub.com/akikyoua/dong-fang-project_flandre-scarlet
Fraduino_ST microcontroller uses STM32F103Cx(C4/C6/C8/CB, STM32FEBKCx is also compatible).
Since the thickness of the USB plug is about 2.2mm, and the thickness of the PCB is generally unreachable, you can use a 2mm PCB (maybe expensive), or a 1.6mm PCB with a 0.5mm self-adhesive silicone strip, or a 1.2mm PCB with a 1mm self-adhesive silicone strip (10~12mm).
Fraduino_ST firmware uses Visual Studio Code and PlatformIO IDE, and the sample program can be directly burned.
The burning program needs to use the serial port, please ensure that the CH340 driver is installed.
Method to enter burning mode: Press BT (hold) -> Press RST -> Release RST -> Release BT.