Fix iPhone 11 Won't Turn On with Large Current

Hello everyone! Our course today is about repairing an iPhone 11 Pro that won’t turn on.

Press the power button. The phone won’t turn on.

Disassemble and Test

Then we disassemble the phone and test. Remove the two pentalobe screws at the bottom edge. Pull up on the small nub on the Suction Cup to lift the screen. Insert a Pry Piece when there is a gap and slide it along the gap.

After the screen is lifted, take the shielding covers off. Disconnect the battery and remove the screen. Connect the motherboard with the Power Cable. The ammeter shows a large current of 3.1A.

Disconnect the flex cables. Remove the SIM card tray. Detach the motherboard. Connect the motherboard with the Power Cable again. The ammeter still shows a large current of 3.1A. It indicates that the large current has nothing to do with other flex cables. The fault is on the motherboard.

Diode Mode Measurement

Run diode mode measurement of pin 1 on the J7010 of the battery connector. The diode value is 0. It indicates that the main power supply circuit is shorted to the ground. Since the circuit simultaneously supplies power for the logic board and signal board, we need to separate the motherboard to locate the fault.

Motherboard Separation

Remove foam on the motherboard. Heat the back of the motherboard with Hot Air Gun at 240 degrees celsius. Remove the shielding paper. Put the motherboard on the Heating Platform. To make it easier to remove the logic board after separation, drive a screw on the logic board. Set the temperature of the Heating Platform to 185 degrees celsius. Clamp the screw to remove the logic board when the temperature has risen to 185 degrees celsius. Remove the signal board.

Diode Mode Measurement

Run diode mode measurement of pin 286 on the signal board. The diode value is 0. It indicates that the main power supply of the signal board is shorted to the ground. Measure pin 286 on the logic board. The diode value is 363 which means the logic board is normal.

Open the bitmap. We can see there are many associated components on the main power supply circuit.

Rosin Detecting

Next, we need to locate the faulty component with rosin detecting. Remove thermal grease with a Sculpture Knife. Dip rosin with Soldering Iron at 365 degrees celsius. Smoke rosin on the signal board. Set the direct current supply to 4.1V. Connect the positive and negative anode of the direct current supply with multimeter probes. Connect the black probe to the ground. Connect the red probe to pin 286 on the signal board. We can see rosin on C1200 K has melted. It indicates that the capacitor is damaged.

Then we heat it with Hot Air Gun at 350 degrees celsius. Remove the capacitor with tweezers.

Clean rosin on the signal board with PCB Cleaner. Run a second diode mode measurement. The diode value returns to a normal value of 327.

Motherboard Recombination

Smear rosin with Soldering Iron and solder wick at 365 degrees celsius to remove tin on the bonding pad. Clean the signal board with PCB Cleaner. Remove tin and clean the logic board with the same method.

Attach the signal board to the Reballing Platform. Put the Reballing Stencil in position. To prevent the solder paste from flowing into the motherboard crevices, insert a metal plate. Apply a layer of low-temperature Solder Paste. Remove the Reballing Stencil.

Put the signal board on the Heating Platform at 185 degrees celsius to heat. After the solder balls are formed, turn the power off and cool the motherboard for 5 minutes. Apply some Paste Flux. Align the logic board with the signal board. Keep heating on the Heating Platform at 185 degrees celsius. When the temperature reaches 185 degrees celsius, keep heating for 1 minute. This step is to ensure that the logic board and the signal board fit closely.

Testing

Connect the motherboard with the Power Cable. Short to the ground of the J7700 pin 12. The boot current becomes normal. Put the motherboard on the phone. Connect the flex cables. Connect the screen and battery. Because the battery is dead. Plug the charging cable and the phone turns on normally.