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ITTwo iPhone 12 mini repair cases sharing.
Case 1. No Baseband and No Wi-Fi
The phone was drop-damaged, so there is a high chance the issues are caused by the pads between two layers getting lost or poor soldering.
First, separate the two layers of the motherboard with a heating platform.
After that, we can see there are a lot of pads missing on the middle layer.
Fix the missing pads then test with fixture, all functions back to normal. Then assemble the two layers of the motherboard with the heating platform, test again, and everything is fine.
The issue finally gets fixed by fixing the missing pads between two layers of the motherboard.
Case 2. No Boot
The client feedback: the phone still works fine before sleeping, and the next morning the phone just stopped working. There is no water damage or drop damage.
Check the phone status, as the customer said the appearance is good, and no damage traces.
- Press the power button no response
- Connect with charger, no charging
- Connect with PC, no connection
Then disassemble the phone and connect it with the DC power supply to test. After testing, there is current leakage caused by short damage. So let’s find out the shorted components first.
Step 1. Measure the diode value on the battery connector with the multimeter, the diode value is OK.
Step 2. Connect with DC power on the motherboard, touch and feel with left hand and find abnormal heat.
Step 3. Separate the two layers of the motherboard, and test the upper layer (logic board), still short. So the issue should be on the logic board.
Step 4. Apply rosin on the heating area, then apply with DC power. Rosin on the surface of U4200 gets melted, the issue should come from the U4200 or the relative filter type capacitors.
Step 5. No hurry to replace U4200, we can measure the capacitors around U4200 first. After measuring with the multimeter, a capacitor is shorted to the ground, which means this capacitor relative rail with short.
Step 6. Apply Rosin on the relative area, then connect with a battery, apply power to find out the shorted component
(ps. here we can jump wire then apply power, or use the multimeter + DC power supply to provide power)
Step 7. Find out the shorted capacitor, as it’s a filter-type capacitor we can just remove it directly.
After removing this cap, assemble the phone and test, the phone can be booted and all the functions are OK.
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