SOLVED: iPhone 6S current unstable and then died

The iPhone 6S accidently dropped into water and then pick it up, make it dry. After a while, we turn it on, the screen flashed then it turns black and can't turn on anymore.

Solution

1. Test the current. Connect iPhone 6S to DC power supply, check the current, it shows leakage 60mA, when we press the boot button, it shows 120mA then jump to 0mA,in the end, it repeats the jump process.

2. Tear down the CPU and shield cover of baseband. We can find that it have four eroded places, they are backlight power supply chip U4020, display power supply chip U4000, charging management chip U2300 and small audio chip U3700.

3. Clean the motherboard, then connect to DC power supply, the leakage current is getting smaller, about 20mA.After turning on, the current jumps from 70mA to 0mA repeatedly. Measure the power supply voltage of CPU U0600, finding no abnormality. Measure the voltage of I2C bus, finding that the signal I2C0_AP_SCL only has 1.1V. The I2C0 bus in circuit diagram is shown in picture 1. The I2C0 bus in bitmap is shown in picture 2. The material object of I2C0 bus is shown in picture 3.

4. Cut the power and use digital multimer to measure the diode data of the I2C bus, there are 404 and 430 which is normal. Viewing the eroded components photos, our preliminary judgment is that the display chip U4000 damaged, because the backlight chip is painted with glue, which is not easy to be damaged, so we remove it.

5. After removing the display chip U4000, energize it, but the current is still the same. While I2C0 bus voltage turns normal. This problem may be caused by power supply chip U2000 and backlight chip U4020.That the backlight chip U4020 causes the turn on failure is related to two groups of signal. One is I2C0 bus, the other is backlight brightness adjustment signal‒backlight chip DWI bus which runs to power supply and CPU. The DWI bus interface in schematic diagram is shown in picture 4.

6. Use multimeter to measure the diode data of R4020, we find that it is short circuit. Remove the backlight chip U4020 and energize motherboard to turn it on. The current jumps to 1A. After assembling backlight chip U4020 and display chip(VC468) U4000, the white Apple logo appears and iPhone gets into IOS normally. The iPhone 6S have wake up and can back to normal work.

Tips

1. As for the water-damaged iPhones, usually need take photos first for the watered places and then clean the motherboard. That will be beneficial, because if we find some problems during the repair process, we can refer to those photos to exclude badly eroded components.

2. The backlight chip DWI bus has SDI and SCK two signals. By measuring the diode data to ground, we can know whether backlight chip U4020 is workable or not. SCK can be measured at the resistor R4020 in picture 5-59 and SDI can be measured at the test point PP1002 in picture 5.

3. When dealing with the problems about the turn on failure after being watered, we can ignore the removed chips if they do not affect the turn on process. If the current jumps normally, we reassemble them.