Disassemble the iPhone 12 Pro with some LCD opening tools, first we need to test whether this problem is caused by a faulty flex cable. Keep the power flex cable connected and disconnect the other flex cables, connect the display screen and power cable.
After testing, the iPhone is still stuck on the Apple logo,and the boot current is stuck at about 240 mA. Now we're sure this problem is caused by a problem with the iPhone motherboard. Depending on the repair experience, this may be caused by a USB charging circuit failure, and we can try to fix it by replacing the iPhone USB charging IC.
Take off the iPhone motherboard, and place it on the MJ CH5-E preheating platform, and heat it with a hot air gun, pry the logic board with a pry bar while heating and remove it with tweezers when the logic board gets loose.
Place the iPhone 12 pro logic board on the Mijing Z20 holder fixture, apply some solder paste on the iPhone USB charging IC. Now heat it with a QUICK 861DW hot air gun, and then use tweezers to remove the iPhone USB charging IC.
Heating the solder pad with a hot air gun, and using a soldering iron and solder wick to clean it, then use PCB cleaner to clean the solder pad again.
Apply some solder paste flux to the solder pad, place a new iPhone USB charging IC to the right position, and then weld it with a hot air gun. Use the soldering iron to remove the tin from the pad and clean the tin on the signal board bonding pad. After that, clean the pad with the PCB cleaner.
Once done, we need to test the iPhone motherboard with a MJ C20 motherboard test fixture,connect the iPhone screen and plug the power cable, the iPhone can be turned on.
Attach the iPhone signal board to the Qianli Middle Frame Reballing Platform to reball it, once done, apply some solder flux on the bonding pad. Align the logic board with the signal board and install them well.
Test the iPhone 12 pro motherboard with a DC power supply, the boot current jumps normally. Assemble the iPhone 12 Pro, and press the power button, the iPhone 12 Pro is turned on successfully.
It is recommended to set the temperature to 200℃ when heating the motherboard with Heating Platform.
When the hot air gun is used to heat the motherboard, the recommended temperature is 330℃.
You can heat the iPhone USB Charging IC with a Hot Air Gun at 340 °C and airflow 3.