64 Kbit I²C EEPROM in a 4-WLCSP — the footprint drives the fit
The STMicroelectronics M24C64M-FCU6T/TF is a 64 Kbit serial EEPROM with an I²C interface, organized as 8 K x 8 bits. It is built in a 4-bump wafer-level chip-scale package (4-WLCSP, also listed as 4-XFBGA), which makes it essentially the size of the die itself — about 0.8 mm square. That footprint is the headline constraint: it saves board space in compact portable and IoT designs but rules out hand-soldering and most rework. Clock frequency is rated at 1 MHz, supporting Fast-mode Plus I²C throughput for parameter storage, calibration tables, or boot configuration reads.
Access time is specified at 450 ns. That is the time from the I²C slave address match to the first data bit becoming available on SDA. At 1 MHz (1 µs bit period), 450 ns is within the SCL low-to-data-valid window, so no extra wait states are needed. If you are replacing a slower EEPROM in an existing layout, verify the master's SDA setup/hold margins — the 450 ns figure is fast enough for standard I²C timing but not a zero-hold part.
Write cycle time and endurance
Page and word writes take 5 ms maximum. For a 64 Kbit device, that means a full-memory rewrite (all 8 K bytes in page mode) takes about 1.3 seconds if the master issues back-to-back page writes. The I²C bus is tied up during the internal write cycle — the device does not acknowledge new commands until the cycle completes. No endurance cycle count is in the listing, but the M24C64 family is typically rated for 1 million write cycles and 40-year data retention. Plan for wear leveling if writing the same byte more than a few thousand times over the product life.
The M24C64M-FCU6T/TF is listed as obsolete. Stock still exists in the spot market and on distributor shelves, but replenishment is not coming. The base product number is M24C64, and ST's current-production parts in this family (e.g., M24C64-FCU6T/TF) are typically available as active. Confirm the exact suffix and temperature grade against your system requirements before swapping.
Where this EEPROM fits — and where it does not
The 4-WLCSP package is the smallest available for this density, which makes it a fit for wearable, medical patch, and space-constrained sensor modules. The trade-off: no package leads to inspect, no rework without a hot-air station and underfill, and the board must be flat — no flex circuits without a rigid stiffener under the die.