iPhone X Freezes after Entering Password

Today our repair course is about fixing an iPhone X that freezes after entering passcode.
The iPhone X freezes after entering passcode. Based on our experience, the issue is probably related to the NFC circuit.

Take apart the phone and disconnect the battery. Remove the display assembly. Take out the motherboard. Run cosmetic inspection of the motherboard. The motherboard is not water damaged.

The first thing we do is to separate the upper layer from the lower layer. Remove dust-proof sponge on the board. Set temperature of the heating platform at 160℃. Place the motherboard on the heating platform.

With the temperature of the platform reaching 160℃, take down the upper layer with tweezers and then the lower layer. The upper layer and the lower layer look just fine. For further confirmation, let's test the two layers with the test fixture. Before test, we need to clean the third space PCB.  Attach the lower layer to the PCB Holder. Heat with the Soldering Iron at 365℃. Clean pads around the edge of the lower layer with rosin soaked Solder Wick. Once done, continue to attach the upper layer to the PCB Holder. Heat with the Soldering Iron at 365℃. Clean pads around the edge of the upper layer with rosin soaked Solder Wick. Clean with PCB Cleaner afterwards. We find that S159 pad on the upper layer has come off the board.

S159 pad is the audio signal of the audio/speaker amplifier IC U5000. Coming-off of the pad means that NFC connecting pads nearby might have been pseudo soldered and therefore resulted in the frozen screen issue. Scrape with sculpture knife to get the circuit under S159 pad exposed. Then stick high temperature tape on components around.

Apply some mid-temperature solder paste to the missing pad. Heat with Hot Air Gun at 300℃, air flow 2. Once done, push the solder joint with tweezers carefully. Make sure the solder joint has shaped-up perfectly.

Then remove a complete pad from a spare iPhone X motherboard. Cover the pad face-down on the solder joint. Solder with Hot Air Gun at 300℃, air flow 2.

Tips: when soldering, press the pad gently with tweezers to ensure that the height of the pad is consistent with that of nearby pads.
Once done, apply some UV curable solder mask to the pad. And then solidify under the UV Lamp for 10 minutes.

Continue to scrape with sculpture knife to get the pad exposed. Clean with PCB Cleaner afterwards. Now we can test the upper layer and the lower layer with the test fixture. Attach the two layers to the test fixture. Get the screen connected. Connect the battery connector with the DC Power Supply.

Get the motherboard powered on with tweezers. The phone turns on normally. Enter the passcode. The phone enters the home screen normally. Touch function is also normal. We can confirm now that the fault is caused by pseudo soldering of the third space PCB. We can fix the problem by soldering the two layers back together.

Put the lower layer into the reballing mold. Get the BGA Reballing Stencil in position. Place the mold onto the base. The stencil is tightly attached to the lower layer because of the magnetic structure of the base. Smear some low-temp Solder Paste on the stencil. Wipe off excess solder paste with lint-free wiper. Then take the mold down from the base. Remove the reballing stencil.

Heat the lower layer on the heating platform at 160℃. Once solder balls have shaped up completely. Cool for 5 minutes. Apply some BGA Paste Flux to the third space PCB.

And get the upper layer in position. Power on the heating platform. With the temperature reaching 160℃. Turn the power switch off. Cool for 5 minutes. Then take down the motherboard from the heating platform. Again, let's assemble the phone and test. Fault cleared.