Secondhand iPhone 12 Pro Max cannot boot issue repair process
Model: iPhone 12 Pro Max
Announced: 2020, October 13
Customer’s Feedback
Bought an iPhone 12 Pro Max from the online secondhand dealing platform. Try to power on to test once received, but the phone cannot boot, black screen and no response, try to charge but still does not work.
Troubleshooting
First, disassemble the phone to have a check. After disassembling, find some shielding covers are missing and there’s also some repair traces on the screen,
Take out the motherboard, obviously, it has been repaired before.
Let’s power with the DC power supply and trigger the boot process to check the current of the boot process.
The boot current is fixed at 90mA
Based on the repair experience, this kind of defect is normally caused by the CPU not working.
- CPU working conditions are not satisfied
- CPU poor soldering or gets damaged
Repair Process
Step 1. Remove the foam and sticker
- Remove the foam around the connectors
- Remove the black sticker tape on the bottom side of the motherboard
Step 2. Separate the two layers of the motherboard by the heating platform
Step 3. Check the motherboard carefully, and find below chips have been repaired before
- Baseband and Wi-Fi chip has been fixed before
- CPU has been fixed before
Based on the boot current fixed at 90mA defect phenomenon, we can try to resolder the CPU to have a try.
Step 4. CPU resoldering
After removing the CPU and checking under the microscope, we can see two pads on the CPU chip have been fixed before.
Carefully scratch out the pad on the CPU
Jump wires and make circles, then cure with the UV mask and UV lamp.
Reball the CPU then solder back the CPU to the motherboard properly
Put into the test fixture to have a test
Buckle with a screen and connect with DC power supply, trigger to boot. The phone turns on normally, and test all the functions are also OK.
Recombine the two layers of the motherboard and test again, everything works fine
Lesson Learned
- The current is fixed at 90mA when booting has a high chance is caused by the CPU problem （working conditions, CPU pads missing, CPU poor soldering, CPU damaged, etc.）
- When purchasing a secondhand product on the trading platform, pay attention to the trading terms to protect your rights.
