iPhone 7 Repair REWA LAB

Fix iPhone 7 Won’t Turn On – CPU Repair

Fix iPhone 7 Won't Turn On - CPU Repair

What should you do if your iPhone 7 won’t turn on after being dropped? The problem may be caused by a software crash, or a complicated hardware issue.

This time, we received an dropped iPhone 7 Plus which can not turn on. The problem was finally detected as CPU being short circuited. Here we have detailed troubleshooting procedures.

iPhone 7 won't turn on

iPhone 7 won’t turn on

Diagnosis

Take apart the phone and disconnect the battery, then connect the phone with the DC Power Supply and press power button to power on. The current reading of the ammeter jumps to 350mA and then stays still, which is abnormal and indicates that the logic board is short circuited.

fix iPhone 7 plus black screen

disassemble the phone and detach the battery

Disconnect the power supply and take out the logic board. Run diode measurements on capacitors around power IC one by one. All the values are normal except  C1823 – It displays 0, which indicates that C1823 is short-circuited. From the schematics, we can see that C1823 is the filter capacitor of PP1V8-SDRAM, and it is directly connected to CPU SDRAM.

iPhone 7 CPU short circuit

diode measurements on capacitors around power IC

Repair

Attach the logic board to the PCB Holder, and take down the shielding cover by heating with Hot Air Gun, then remove CPU SDRAM with CNC Chip Grinding Machine. After chip grinding, attach the logic board to the PCB Holder and clean it with PCB Cleaner, then remove the black adhesive around with Sculpture Knife. After that, apply some BGA Paste Flux to the bonding pad. Align the new SDRAM to the right place(Don’t reverse the direction),  and then solder with Hot Air Gun. After cooling for 5 minutes, clean thoroughly with PCB Cleaner.

CPU SDRAM chip grinding

CPU SDRAM chip grinding

Testing

Connect the phone with the DC Power Supply, then short Pin 13 of J4504 to the ground with tweezer to trigger the power on system. This time, the current reading on the ammeter is normal.

connect the phone with the DC Power Supply

connect the phone with the DC Power Supply

Get the logic board assembled and buckle the display assembly. Connect the phone to the computer, then press and hold volume down and power button to get into DFU mode. Restore the phone via iTunes. Once done, insert the SIM card and activate the phone. After testing, All the functions work normally.

Current reading on ammeter

current reading on ammeter

For more details about iPhone 7 won’t turn on repair and iPhone repair videos, please check our YouTube video.

 

Tools Used:

DC Power Supply
Digital Multimeter
Integrated Mobile Phone Repair Platform
PCB Holder
CNC Chip Grinding Machine
Screwdriver
Solder Wick
Solder Wire

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