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ITThe Xiaomi Mi 11 Ultra, announced in March 2021, is a flagship smartphone known for its advanced features. However, issues such as moisture damage can cause the phone to fail to boot. This guide provides a detailed troubleshooting and repair process for a Xiaomi Mi 11 Ultra that cannot turn on due to moisture damage.
Model: Xiaomi Mi 11 Ultra
Announced: 2021, March 29
Defect Phenomenon
The phone was cleaned with alcohol, which caused moisture to enter the motherboard, leading to display issues and eventually, the phone not turning on. An official repair center diagnosed it as man-made damage, making it ineligible for warranty repair. The cost of replacing the board and screen was too high, so the phone was sent to us for repair.
Troubleshooting
Initial Inspection:
- Disassemble the phone and remove the motherboard.
- Inspect the motherboard under a microscope. While some areas showed minor corrosion, there was no obvious damage.
- Clean the corrosive parts with PCB cleaner.
- Inspect the small board at the bottom for corrosion; none was found.
Voltage Testing:
- Connect the motherboard to a DC power supply machine.
- No current leakage was detected.
- Attempt to trigger the boot process with tweezers. The boot current fluctuated between 0 and 200mA.
- Temporarily assemble the phone to test the charging function; the charging current was normal.
- However, the PMU (Power Management Unit) area was hotter than other areas.
Further Investigation:
- Reconnect the motherboard to the DC power supply machine; a current leakage of over 600mA was detected.
- Use the rosin inspection method to identify heat sources. The rosin on the PMU chip melted, indicating potential PMU damage.
Removing the PMU Chip:
- Cut off the shielding cover and remove the PMIC (Power Management IC).
- Clean the bonding pads on the PCB with solder wick.
- Measure diode values in the heat area with a multimeter, finding a short on a pin at the edge.
- Jump a wire on the capacitor side to apply voltage and identify shorted components.
- Apply 1.8 volts to the rail and find heat coming from the 5G signal board.
Inspecting the 5G Signal Board:
- Separate the two-layered 5G signal board.
- Find corrosive chips inside and remove them.
- Use the rosin inspection method again, identifying a melted capacitor. Remove the capacitor.
Repair Process
Reballing and Resoldering:
- Reball three corrosive chips and the PMU IC.
- Solder the reballed components back onto the motherboard.
Once the reballing process has been completed, test again to ensure that no leaks continue to occur.
- Recombine the 5G signal board with the mainboard.
Assembly and Testing
Assemble the Phone:
- Reassemble the phone with the original screen and parts.
- Press the power button to boot the phone.
Function Testing:
- Verify that the phone boots normally and all functions work correctly.
Summary
The Xiaomi Mi 11 Ultra’s no boot issue was resolved by:
- Removing the shorted capacitor.
- Cleaning and resoldering the corrosive chips related to the 5G signal function.
- Reballing the PMU IC.
This repair process requires precise work and careful inspection to restore the phone to full functionality without causing additional damage.
