iPhone 6 IOS disappears Firmware Version after dropping

Restoring your iPhone or iPad can resolve a variety of issues, but sometimes you’ll experience issues, such as iPhone 6 firmware version cannot be found in IOS after dropping.
Nonlocal customer sent this iPhone motherboard. He said that the rear cover bended after dropping, but it can be used normally. After replacing the rear cover, I cannot find the firmware version in “about iPhone”. If you search on your iPhone or iPad that can't connect to a cellular network or cellular data, follow these steps.

Fix Process:
1. We save the teardown process as the customer sends the motherboard. Observing from the side face, I find that the board bends. Combining the description from customer, we presume that the baseband or the baseband power supply is pseudo soldering. Energizing it and turn it on to test the power supply. VREG_SMPS2_1V25, VREG_SMPS3_0V95 and VREG_SMPS4_2V075 are normal. Only VREG_SMPS1_0V90 has no voltage. The circuit diagram of these voltages is shown in picture 1-1.

2. The test points of VREG_SMPS2_1V25, VREG_SMPS3_0V95 and VREG_SMPS4_2V075 in bitmap are shown in picture 1-2. They are all measured at the inductors around the baseband.

3. The material objects of VREG_SMPS2_1V25, VREG_SMPS3_0V95 and VREG_SMPS4_2V075 are shown in picture 1-3.

4. According to experience, I cannot measure the baseband at CUP, which suggests that will cause no voltage on VREG_SMPS1_0V90. As the board bends slightly, we tear down the baseband and test it carefully, but I do not find any dropping pins. I continue to measure the diode data of I2C bus BB_EEPROM_SDA and BB_EEPROM_SCL that act as communication between baseband and baseband chip. The data is 650, which is normal. The circuit diagram of I2C bus is shown in picture 1-4.

5. Mount tin at baseband and assemble it to motherboard. Connect it to testing components and energize it to turn on. The problem is still the same. I still cannot find the firmware version in “about iPhone”. But after plugging the adjustable DC power supply, the display current is normal, we can be sure that there is no short circuit in at baseband. In order to be certain, I measure the diode data of all power supplies at baseband, but still I do not find any abnormal. Thus I can be sure that the communication between baseband and CPU is abnormal. Tear down the baseband again and measure the diode data that baseband flows to CPU signal circuits. The test points are shown in picture 1-5.

6. Finally, we know where the problems are. The diode data of the two yellow points are infinite in picture 1-6. They are the UART bus signals that run to CPU, named BB_CTS_L and BB_UART_RTS_L.
7. Since they run to CPU and are abnormal, I have to tear CPU down to make sure whether it is pseudo soldering or disconnecting. After tearing it down and measuring, I find that the two solder pads are connecting to the yellow points in picture 1-6. So CPU is pseudo soldering. After reassembling CPU, I can find the firmware version in “about iPhone”. Trouble is removed.

Remind readers: I made a detour here. I should measure whether the communication circuits between CPU and baseband are normal or not before assembling baseband.