iPhone 6 Plus boot screen is black or frozen repair

iPhone 6 Plus boot screen is black or frozen repair

Machine model: iPhone 6 Plus

Fault symptom: mobile phone is turned on screen is black or frozen
Customer description:
It is not displayed in normal use, but the VCC chip and power supply have been changed. Once in the test machine, it can be turned on and displayed for a while, and it will be screen is black or frozen.
Maintenance process:
Look, the first reaction is CPU Inveracious Soldering. For the sake of safety, it is still the test condition. Because it is a single board, directly button a test screen DC power supply to start up, and the current normally runs above 1A, that is, the screen doesn’t display, and the measurement has no positive or negative 5.7V VCC of PP5V7_LCM_AVDDH and PN5V7_SAGE_AVDDN. That should be because the working conditions of the screen are not met or there is a problem with the CPU. Unplug VCC of the 1.8V and I2C buses of the screen test interface.No problem found. Call diode value after power off.

iPhone 6 Plus startup screen is black or frozen repair Figure 1.

Compared with the normal motherboard, no abnormality is found, so if the positive or negative 5.7V VCC is not generated, it may be that the CPU displays the VCC chip.
Measure the resistance R0702 of LCM_REXT first. If there is something wrong with this resistance, the MINI mode of communication between the CPU and the display screen will not work (although this resistance problem rarely occurs on machines that have not done CPU and water damage).

iPhone 6 Plus startup screen is black or frozen repair Figure 2.

This resistor is connected to the CPU at one end of GND, and Measuring diode value 700 or so at the other end of CPU. Please observe whether there is Inveracious Soldering or not.
However, this resistance is normal, so remove the display VCC. After all, others are not at ease.

iPhone 6 Plus startup screen is black or frozen repair Figure 3.

PAD is very good after removal, without Short bridge and Inveracious Soldering. For small chips, I am used to typing the ground value of PAD after removal, and it doesn’t take much time, but sometimes it can save more time.Measuring diode value is also normal. Remove a display Parts motherboard from PAD again, and replace it with a boot test.switch lights Stuck on Apple logo.
After a few seconds of lighting,screen is black or frozen will be turned on, and screen is black or frozen will be turned on again after power failure. Now, two problems can be basically determined. One is that some points of VCC PAD are in a state of semi-off, and screen is black or frozen will be heated when the display chip is removed, and it will be abnormal after cooling, and there is CPU diode value.
Remove the chip displaying VCC again. After the fan blows Measuring diode value for ten minutes, it is found that Normal value, a little unwilling to give up. Prepare to test VCC for several important signals on PAD. When testing VCC, an abnormality was found. U1501 showed that C2#VCC of VCC chip kept current of change, and C2 # was reset _ 1V8 _ L. This is the reset signal of CPU. The board with normal current running on will definitely not keep VCC.There is only one possibility that there is something wrong with PCB layer. First, measure the test point of CPU reset signal RESET_1V8_L beside Nand.

iPhone 6 Plus startup screen is black or frozen repair Figure 4.

VCC is normal 1.78V, and the disconnection is determined. From the test point Jump wire to C2# of PAD, it can be displayed normally after half a day of startup test. At this point, the iPhone6 Plus mobile phone boot screen is black or frozen fault repair.
Summary: In addition to being able to determine the CPU problem by Measuring diode value, pressing U, etc, it is best to check other components that may cause similar failures before working on the CPU.