How to fix iPhone 12 Pro Max Temperature Warning Issue

iPhone 12 Pro Max Temperature Warning Issue – Diagnosis and Repair

Issue Description

The iPhone 12 Pro Max displays a temperature warning upon boot-up, preventing normal operation. The device has a cracked screen due to a previous drop, indicating possible internal damage. To identify and fix the root cause, we need to conduct a thorough examination of the motherboard and its soldering points.

Step 1: Initial Inspection and Disassembly

1. Power On Test
   • Boot up the iPhone 12 Pro Max to confirm the temperature warning persists.
   • Check for any signs of overheating or unexpected shutdowns.
2. Disassemble the iPhone
   • Remove the screen carefully.
   • Disconnect all flex cables to isolate the motherboard.
   • Extract the motherboard from the phone body.
3. Check for Deformation
   • Under a microscope, inspect the motherboard structure.
   • The motherboard is found not deformed, ruling out severe physical damage.
   • However, pseudo-soldering (weak solder joints) is detected on the logic board.

Step 2: Separating the Motherboard

Since pseudo-soldering is identified, we must separate the motherboard to inspect and repair the bonding pads.

1. Prepare the Motherboard for Heating
   • Detach any foam and plastic parts surrounding the motherboard.
   • Place the motherboard on a 165°C heating platform to soften the solder.
2. Assist Heating with a Hot Air Gun
   • Use a Hot Air Gun at 330°C to help loosen the solder.
   • Gradually insert a Pry Blade to lift the logic board while applying heat.
   • Once the logic board loosens, carefully remove it from the motherboard.
3. Remove the Signal Board
   • Separate the signal board to fully expose the bonding pads.

Step 3: Repairing the Bonding Pads

Upon closer inspection under a microscope, some pads on the logic board are missing. We proceed with the following repair steps:

1. Clean the Pads
   • Use a Soldering Iron (380°C) and Solder Wick to clean any residual solder.
   • Apply PCB Cleaner to remove debris and flux residue.
2. Scrape and Restore the Missing Pads
   • Gently scrape the damaged bonding pads using a Sculpture Knife.
   • Caution: Avoid damaging the GND layer while scraping.
3. Re-Tin the Pads
   • Apply a thin layer of tin to the bonding pads using a Soldering Iron.
4. Install and Solder the Soldering Lugs
   • Position the Soldering Lugs in place.
   • Apply Paste Flux to ensure a strong bond.
   • Use a Soldering Iron to fix the lugs.
5. Apply Solder Mask and Solidify
   • Cover the bonding pads with Solder Mask.
   • Cure the mask using a UV Lamp.
   • After solidifying, remove excess solder mask with a Sculpture Knife.
   • Re-clean the bonding pads using PCB Cleaner.

Step 4: Reballing the Signal Board

To ensure proper motherboard function, the signal board needs reballing.

1. Attach the Signal Board to a Holder.
2. Align the Reballing Stencil on the signal board.
3. Apply Low-Temperature Solder Paste evenly across the stencil.
4. Remove Excess Solder Paste to ensure clean solder joints.
5. Detach the Reballing Stencil carefully.
6. Heat the Motherboard for Recombination
   • Place the motherboard back onto the 165°C heating platform.
   • Allow it to heat until the solder joints reflow properly.
   • Let the motherboard cool for 5 minutes before proceeding.

Step 5: Reassembly and Testing

1. Reinstall the Motherboard into the iPhone frame.
2. Reconnect the Screen and Flex Cables.
3. Power On the Device.
4. Verify the Issue is Resolved:
   • The iPhone turns on normally.
   • The temperature warning has disappeared.
   • The device is now functioning correctly.

Conclusion

The temperature warning issue on the iPhone 12 Pro Max was caused by soldering on the motherboard, leading to poor thermal and electrical conductivity. By carefully separating, repairing, and reballing the motherboard, we successfully restored the missing bonding pads, resolving the issue. The phone is now operational without temperature warnings.

For any further repairs, always ensure precise micro-soldering techniques and proper PCB cleaning to maintain durability and performance.

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