NAND flash chip

• Purpose: The main storage for the device. The NAND chip functions like the hard drive of the iPhone.
• Capacity: NAND chips are available in various capacities, from 32 GB up to 1 TB or more, depending on the iPhone model.
• Replacement: For most users, this chip cannot be upgraded or replaced. For professional micro-soldering technicians, special tools are available to desolder and replace a faulty NAND chip. However, because of the encryption that ties it to the CPU, a simple swap is not possible. 

EEPROM chip

• Purpose: Stores a unique identifier (UID) that links the NAND chip and other key components to the iPhone’s CPU. This is part of Apple’s security architecture that prevents data from being read directly off the NAND.
• Size: The EEPROM is much smaller than the NAND, often referred to as “rice-sized”.
• Repair implications: The strong security pairing between the EEPROM and the CPU is why data on a logic board with a broken CPU is generally unrecoverable. For a repair to succeed, the original EEPROM chip must be intact and properly reinstalled, as it is required for the phone to boot correctly. 

Data security

• The data on the NAND flash chip is encrypted using AES (Advanced Encryption Standard), and the keys are generated by the hardware.
• The encryption process ties your CPU, EEPROM, and NAND chip together. If any of these unique components are damaged or replaced, the data cannot be decrypted and the phone will not function properly. 

Tools for technicians

• EEPROM programmers: Tools like the XZZ programmer allow technicians to read, save, and rewrite data for the EEPROM chip and the phone’s baseband (cellular modem) without having to desolder the chip. This is primarily for transferring data when replacing a logic board.
• NAND programmers: Some NAND programmers are available to expand the storage of older iPhones. However, the latest devices and their security features make this much more difficult or impossible.