Cortex-M0+ in a 25-ball WLCSP — memory budget and package reality
The STM32L031E6Y6TR is an ARM Cortex-M0+ core running at 32 MHz, with 32 KB of flash and 8 KB of SRAM — the RAM ceiling is the tight constraint here, not the flash. The 1 KB of on-chip EEPROM (1K x 8) handles calibration constants or boot parameters without wearing the flash array. It comes in a 25-ball WLCSP (25-UFBGA, supplier package 25-WLCSP), which means the PCB fan-out is limited to the inner balls — a 4-layer board is typical for routing the 20 available I/O and the 10-channel 12-bit ADC. The package is the primary differentiator; most STM32L0 variants ship in larger QFP or QFN footprints.