Connect this troubled phone to USB and see if there is any response, there is no current. Also, it fails to boot up when we switch the power on.
Usually in this case, the first thing we are going to do is to pry the back cover with a pry bar after heating it. When the heat still remains we unbolt the screws, remove the bracket and disconnect the battery cable.Then we measure its voltage and see if it’s functional.
The reading is 4V, so the problem is about the motherboard.
Connect it to the main power and check out the current, the reading is 200MA, the indicator shifts swiftly, so shortcut.
Take down these connectors and the card tray, the motherboard. Observe the motherboard we just took off, there is no water-damaged trace. So now we can follow the repair idea of electric leakage.
Take down the shielding cover of CPU, measure the diode reading of VBAT_SYS，it’s 0, which indicates this is exactly the circuit that causes this phone’s no power-on. Remove the shielding cover of the charging chip on the backwards:
For the main power VBAT_SYS shortcut, we usually take out rosin.
Apply some rosin around the main power and the charging chip, then measure the voltage and see if there is any heating part. Here we measure the voltage of c1614 twice, no abnormality.
The rosin covering U4201 melts quickly, so here lies the problem.
330℃，40 wind, heat it for a few seconds and remove U4201.Clean off the bonding pad U4201 with some soldering paste
Take down the motherboard and connect it to the power, there is no leakage of electricity, so U4201 has been faulty.
Put the motherboard onto the PCB holder, we are gonna install a new U4201.330℃，40 wind, we weld U4201. Install the phone and see if can power on when it’s cool.
Fix the screws, power on.
Test the functions, all is well.