Under a microscope, apply a small amount of flux to the DAT0, CMD, CLK, and GND points. Solder short, equal-length jumper wires from the motherboard to your eMMC tool interface board. Keep wires under 10cm to avoid signal attenuation. Step 3: Connect to the Flashing Box
The STK-L21 dev board exposes programming pads or pins for the STM32L021F4P6. While manufacturer silk and schematics should be consulted for exact pad labels, the mapping follows STM32L0 reference pins: stk-l21 isp pinout