How To Troubleshoot A Dead iPhone - CPU Repair

Blake Apr 07, 2020

1073

The CPU of mobile phones functions just like the brain of our body. Which makes CPU repair the most complicated subject in mobile phone repair. Check our blog today, iPhone 7 won’t turn on caused by pseudo soldering of CPU and learn how to troubleshoot repair cases related to CPU.

Problem

Press the power button to turn on the phone. There is no response at all.Take apart the phone. Connect the phone with the DC Power Supply. Again, press the power button. The current pointer moves from 0 to 50mA-60mA and then stays still. Connect the phone with computer. The phone can not be recognized.
Press the power button to turn on the phone. There is no response at all.Take apart the phone. Connect the phone with the DC Power Supply. Again, press the power button. The current pointer moves from 0 to 50mA-60mA and then stays still.

Which indicates that USB IC or power supply of the logic board might have malfunctioned.

Diagnosis

First thing we do is to measure power supply of the logic board.

Take out the logic board. Connect the battery connector with the DC Power Supply. Press the power button on the Power Supplier. The system will then detect power button press.

Now, let’s measure the 16 rails of logic board power supply. The measured value is normal.
How To Troubleshoot A Dead iPhone - CPU Repair

So our next move is to replace the USB IC with a new one and see how it works.

Attach the logic board to the PCB Holder. Remove a small part of the shield plate first. Then detach the USB IC from the board with Hot Air Gun. Clean the bonding pad afterwards.

Solder a new USB IC and test again. The problem remains unresolved.

Troubleshooting

We can confirm now that CPU has pseudo soldered or damaged. Let’s re-solder CPU and see how it works.Remove the shield plate with Hot Air Gun. Stick High-temperature Tape on components around CPU. Then detach CPU from the board with Hot Air Gun. Clean the bonding pad and remove solder on CPU afterwards.
We can confirm now that CPU has pseudo soldered or damaged. Let’s re-solder CPU and see how it works.Remove the shield plate with Hot Air Gun. Stick High-temperature Tape on components around CPU. Then detach CPU from the board with Hot Air Gun. Clean the bonding pad and remove solder on CPU afterwards.

Now we need to reball CPU. Get the BGA Reballing Stencil in position. Smear Solder Paste evenly on the stencil. Heat with Hot Air Gun evenly so that all solder balls can solidify completely.

Once done, wait for the CPU to cool for 2 minutes. Apply some BGA Paste Flux to the bonding pad. Get the reballing finished CPU in position and solder with Hot Air Gun. Once finished, clean with PCB Cleaner.

Reassemble And Test

Now we can assemble the phone and test. Get the logic board and display assembly installed. Connect the battery and press the power button. The phone turns on normally. Run function test. All going well. Problem fixed.