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.
Obsolete — last-time-buy window closed
The M24C64M-FCU6T/TF is listed as obsolete. Stock still exists in the spot market and on distributor shelves, but replenishment is not coming. If you have an active BOM line using this exact order code, the safest move is to qualify a pin-compatible alternative from the same family — the M24C64 series shares the same I²C slave address and 4-WLCSP footprint across multiple speed and temperature grades. 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.