In mobile phone repair, screen failures caused by accidental drops are common, especially in flagship models with compact internal designs. In this article, Phonefix examines a Huawei P40 Pro black screen issue after a fall, analyzing the display power path, circuit structure, and fault origin. Through detailed chip-level diagnostics and repair steps, it provides practical guidance for technicians and enthusiasts.
I. Fault Description
· Failure Condition
After an accidental drop from a considerable height, the phone no longer displays anything upon powering on. The touchscreen is unresponsive, but system sounds indicate the device is booting normally.
· Initial Assessment
Black screen issues are most commonly associated with problems in the display module’s power supply, inductor soldering, or chip-level failures. Further testing is required to pinpoint the fault.
Tip: Failures in power chips are common after severe drops, often due to internal damage or microfractures. Thus, troubleshooting black screen issues should include a full check of the power delivery path, not just the display module and IC.
II. Fault Analysis and Circuit Tracing
Based on the Huawei P40 Pro's OLED display structure, the core power is controlled by a display IC. A black screen is usually linked to the following components:
1. Display IC (U1901)
· This chip controls three essential output voltages: ELVDD, ELVSS, and VREG, which power the OLED display. A failure in the IC could result in the screen receiving no power.
2. Power Management Chip (U9803)
· The power management chip supplies power to the display IC. If it malfunctions, the display IC won't receive the necessary voltage, resulting in a black screen. Using a Digital Oscilloscope to check the waveform and signal integrity of the output voltages, a stable input voltage with no output suggests a failure in the U9803 chip.
Main Power Path:
· LB1901 → VBAT_SYS_OLED → VBAT_SYS_OLED_ST → Display IC (U1901)
Voltage Testing and Key Node Identification
Using a digital multimeter and an oscilloscope, the following power paths were tested sequentially:
· VBAT_SYS_OLED: The input is normal, approximately 4V.
· VBAT_SYS_OLED_ST: No output, both in standby and after powering on.
· ELVDD (from U1901): 0V-This indicates the display IC is not operational.
III. Professional Disassembly and Preparation
To ensure operational safety and protect components during repair, the following tools and equipment are essential:
Hot Air Gun: For softening adhesive and removing the screen.
Screwdriver Set: For disassembling the mid-frame and motherboard.
Trinocular Stereo Microscope: To inspect solder joints and detect micro-cracks.
ESD Mat and Wrist Strap: To prevent electrostatic damage to the motherboard.
XZZ Repair Software: To access schematics and netlists, and quickly locate chip pins.
BGA Rework Station: For replacing or shorting components as needed.
IV. Repair Steps
Step 1. Disassemble Screen and Motherboard
Start by unscrewing the two bottom screws near the charging port. Use a hot air gun to heat the screen edges to around 80°C, softening the adhesive. Carefully use a suction cup and plastic pry tools to lift and remove the screen. Next, disconnect the battery and display flex cables. Afterward, remove the mid-frame and take out the motherboard, placing it under a microscope for a detailed inspection.
Step 2. Measure Power Path
Locate the U9803 chip on the motherboard, which is responsible for outputting VBAT_SYS_OLED_ST. Measure the input pin, which shows normal voltage coming from VBAT_SYS_OLED. However, measuring the output pin reveals no voltage, indicating that the chip may be damaged internally. Test the surrounding passive components to ensure there are no abnormal resistances or visible damages.
Step 3. Temporary Solution: Jumper Method
If the power supply chip isn't outputting voltage, a temporary fix is to short the input and output pins. Use tweezers to connect them and apply a high-temperature soldering iron or BGA Rework Station to create the jumper. This method should only be used when the power path is unregulated to prevent overvoltage damage to the display IC.
Step 4. Reinstall the Motherboard
Reinsert the motherboard into the mid-frame, reconnect the battery and display flex cables, and carefully reposition and secure the screen before fastening all screws.
Step 5. Power and Voltage Testing
Recheck ELVDD with a multimeter; it should show a normal positive voltage. Power on the device, and the screen should light up correctly with no flickering or artifacts, confirming the display IC is powered and the fault is resolved.
VI. Conclusion
When performing chip-level repairs, using the right professional tools and having a clear understanding of the circuit structure are equally important. If you're looking for high-quality mobile phone repair tools or want to stay updated with the latest repair knowledge, feel free to visit China Phonefix.
