The 32-UFQFPN package with exposed pad measures 5x5 mm. The thermal resistance numbers assume that pad is soldered to a copper plane.
STMicroelectronics lists the STM32L431KBU6 as Active. The base product number STM32L431 is shared across density and package variants, so if a future board spin needs more Flash or a different pin count, the migration stays within the same family without a core architecture change.
26 I/O in a 5x5 mm footprint — layout and sourcing notes
The shipping medium is Tray, not Tape & Reel. For prototype or low-volume builds, tray quantities typically avoid the minimum reel commitment.
STM32L151CBU6A — a lower-speed, higher-I/O alternative
The STM32L151CBU6A runs a Cortex-M3 at 32 MHz and offers 37 I/O plus USB, compared to the L431's 26 I/O and 80 MHz Cortex-M4 with FPU. The L151 also includes capacitive-sensing touch support.