LINCSEEK Infrared Thermal Camera is used to check the temperature of components on the phone to see if the PCB is short-circuited.
Part 1: Overall Testing
The phone has large current while the boot-up hasn’t been triggered. So it can be judged that the fault is on the main power supply circuit and related components.
Since there are lots of related components, the damaged component is hard to detect. An infrared thermal camera is typically used to quickly detect the faulty part.
Then, we use the thermal imager to fix an iPhone 11 Pro Max with large current.
Remove the motherboard. Using the multimeter to measure the diode value of the battery connector. The diode value is 314, which is normal.
Put the motherboard on the thermal imager. Connect the motherboard with a power cable.
Areas around the NAND are getting hot seriously. We need to separate the motherboard for further testing since NAND is not on the main power supply circuit.
Remove the logic board with Heating Platform. Because the phone has been badly dropped before, we can see that there are a lot of missing pads on the pads.
Next, connect the logic board with a power cable. The logic board doesn’t have large current. The fault is probably on the signal board.
Part 2: Further Testing and Repair
Using the probes of the multimeter power up the signal board separately. Large current appears on the signal board, so it can be confirmed now that the fault is on the signal board.
Put the signal board on the thermal imager and supply power to it. It can be seen that the temperature of areas around U5000 reaches 80 °C quickly.
There is a short circuit when measure the capacitors around U5000 with the multimeter. Additionally, a capacitor was found to be burnt out during measurement.
Measure with the multimeter again. The diode value returns to a normal value of 446.
There are still missing pads. Clean tin on the bonding pads of the signal board with Soldering Iron and Solder Wick.
Clean tin on the bonding pads of the logic board with the same method and then remove the thermal paste. Keep cleaning the bonding pads with PCB Cleaner.
Open Bitmap to determine the missing pads for repair. Since some missing pads are grounded, there is no need to repair them.
Scrape to show circuits of other missing pads with a Sculpture Knife. Apply tin to the bonding pads with a Soldering Iron.
Put the Soldering Lugs in position. Solder with a Soldering Iron at 380 °C. Apply some Solder Mask to the bonding pads that have been repaired. Then solidify with a UV Lamp for 5 minutes.
Then remove excess solder mask with a Sculpture Knife to show the pads. Next, we reball the signal board. Apply a layer of low-temperature Solder Paste evenly.
Align the logic board and heating with the Heating Platform. After recombination and the motherboard cools, connect the motherboard with a Power Cable.
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