Here is a breakdown of what each term means and how the repair is performed:

The U2 charging IC

• What it is: The “U2” is a shorthand name for a specific integrated circuit (IC) on the iPhone’s logic board. In the iPhone 15 series, this is the SN2012027 chip. It is a charging control chip, also known as the “Tristar” or “Hydra” IC in older iPhone models.
• What it does: It manages all charging functions, including communication with the charging port, charging accessories, and power delivery to the battery.
• Why it’s replaced: A damaged U2 IC is a common cause of charging issues, battery drain, or boot loop problems. 

The EEPROM chip

• What it is: The “EPROM” refers to the Electrically Erasable Programmable Read-Only Memory, a chip that stores important data about the charging process. In the iPhone 15 series, the EEPROM is a companion chip to the U2, identified as 8N(Q4EE).
• What it does: This chip contains calibration data and other information specific to the charging circuitry. When the U2 chip is replaced, the data from the original EEPROM must be transferred to the new EEPROM.
• Why it’s programmed: Simply replacing the U2 IC and EEPROM is not enough. The new EEPROM needs to be programmed with the original data to ensure the iPhone’s charging functions work correctly and avoid errors. 

The repair process

1. Removing the old chips: The original SN2012027 U2 and 8N EEPROM chips are carefully removed from the logic board using hot air and soldering tools.
2. Reading and saving data: A specialized EEPROM programmer, such as the JCID or XZZ series, is used to read and back up the original data from the 8N EEPROM. Some programmers can perform this function without removing the chip.
3. Writing data to new chips: The saved data is then written to the new replacement EEPROM chip.
4. Installing new chips: The new U2 and programmed EEPROM are soldered onto the logic board.
5. Final testing: After assembly, the technician tests the charging function to ensure the issue has been resolved.