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The phone has been dropped before. After charging, the phone won’t turn on, gets stuck on the Apple logo, and keeps restarting.
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Next, we connect the power cable with the motherboard. The motherboard has no electricity leakage and large current.
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Then we connect the phone with the computer and flash the phone to see if the system is damaged. Get the phone into recovery mode with a Restore-Easy Cable. Plug in the USB data cable. Begin flashing. The phone can be flashed. But the phone still won’t turn on and keeps restarting after flashing.
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It can be judged that the motherboard has malfunctioned. Disconnect the flex cable and remove the motherboard. Remove foam on the motherboard and the rubber. Attach the motherboard to the holder. Attach the high-temperature tape to protect the connector. Remove the shielding cover with vertical Hot Air Gun at 380℃.
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Check inductances one by one around the power IC with Hot Air Gun at 280℃ to see if they are loose. If the inductances are loose, it is pseudo soldering. The inductances are not loose. Since the phone restarts itself after charging, then we try to replace the USB IC.
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Remove thermal grease with a Sculpture Knife. Cover the CPU with the shielding cover and fix with high temperature tape. Remove black adhesive around the USB IC with Hot Air Gun at 280℃. Pry up the USB IC with Hot Air Gun at 380℃. Apply Paste Flux. Remove tin on the bonding pad with Soldering Iron at 365℃.
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Use Hot Air Gun at 380℃ to heat the solder wick with soaked rosin to remove tin. Apply Paste Flux. Put a new USB IC in position. Solder with Hot Air Gun at 380℃. Clean with PCB Cleaner after the motherboard has cooled.
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Connect the motherboard with the power cable. Short to the ground of pin 10 on J4300 to trigger the boot-up. The boot current is normal.
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Then we install the motherboard. Connect the display and battery. The phone turns on normally.
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Conclusion
The iPhone XR restarts itself after being dropped and charged. The phone can be flashed normally but still keeps restarting. The inductances are found to be not pseudo-soldered after checking with Hot Air Gun. Since the USB IC is in the front of the charging line and has a high damage rate, our priority is replacing the USB IC. After USB IC replacement, the phone turns on and the fault is cleared.