Purchasing a secondhand iPhone 12 Pro Max can be a great deal—until you encounter issues like the phone not booting. A black screen with no response, even after charging, can indicate underlying motherboard problems. This guide will walk you through the troubleshooting process and repair steps to get your iPhone working again.
Model: iPhone 12 Pro Max
Announced: 2020, October 13
Customer’s Feedback
A customer purchased an iPhone 12 Pro Max from an online secondhand marketplace. Upon receiving it, they attempted to power it on, but the phone remained unresponsive with a black screen. Charging the device did not resolve the issue.
Troubleshooting
- Disassemble the Phone – Open the device to inspect internal components.
- Check for Missing Parts or Repairs – Noticed missing shielding covers and previous repair traces.
- Inspect the Motherboard – Found evidence of prior repairs.
- Test Boot Current with a DC Power Supply – The boot current remained fixed at 90mA, indicating a CPU issue.
Possible Causes:
- CPU not working due to poor soldering or damage.
- CPU working conditions not met.
Repair Process
Step 1. Remove the foam and sticker
- Remove the foam around the connectors
- Remove the black sticker tape on the bottom side of the motherboard
Step 2: Separate the Motherboard Layers
- Use a heating platform to carefully separate the two motherboard layers.
Step 3: Identify Previous Repairs
Inspect the motherboard and note previous repairs:
- The baseband and Wi-Fi chips were previously fixed.
- CPU had been repaired before.
Based on the boot current fixed at 90mA defect phenomenon, we can try to resolder the CPU to have a try.
Step 4: Resolder the CPU
- Remove the CPU and inspect under a microscope.
- Found two pads on the CPU chip had been fixed previously.
- Scratch out the pad carefully.
- Jump wires and form secure connections.
- Use a UV mask and cure with a UV lamp.
- Reball the CPU and solder it back onto the motherboard.
- Place the motherboard into a test fixture.
Step 5: Assembly Phone Phone
- Attach the screen and connect the DC power supply to trigger the boot sequence.
- The phone powered on successfully, and all functions were verified.
Key Takeaways
- A fixed boot current at 90mA often indicates a CPU-related issue, such as poor soldering, missing CPU pads, or CPU damage.
- Always inspect secondhand devices thoroughly before purchasing, and review the seller’s return policy to protect yourself.
Tools & Parts Required
- Heating platform
- Microscope
- DC power supply
- UV mask and UV lamp
- Soldering iron and reballing tools
- Jump wires
- Test fixture
By following these repair steps, you can successfully restore an unresponsive iPhone 12 Pro Max and avoid common pitfalls when buying secondhand devices.
