Help me save my smartphone

[Martini]

Hi, I know the board forum isn't really specialized in smartphone repairs but I had zero luck on the biggest phone repair Facebook group, so I thought I'd try here.

I need help to diagnose my Mi 11 (normal 11, not the Light, not the Ultra, not the 11T, not the 11i). Context: I'm an idiot but I understand Ohm's law, I have a decent multimeter, I can disassemble a phone and put it back together without ending up with extra parts, I'm reasonably handy with a soldering iron, I'm willing to learn how to use a hot air station (and buy one if need be), I suck at reading schematics.


I replaced the phone's battery in November with one from iFixit. I don't think this has anything to do with the current problem as it has worked flawlessly for 5 months since. More on the battery at the end.

One day in early April the screen froze and it restarted (and worked ok). Then one hour later, the screen froze again, it shut down and entered a boot loop.

I was able to enter recovery and Fastboot but it would not boot into the OS.
I wiped the phone, it made no difference. I installed Mi Assistant (for those who don't know, Xiaomi phones can access a Mi Assistant mode from the recovery to receive an update) and flashed my stock ROM the latest ROM with it, no difference. I tried flashing in Fastboot, I couldn't since the bootloader was locked (at that time: read further). I tried Mi Assistant again because I don't believe in Einstein's definition of insanity and.....the phone booted into the OS after the flash! I was able to configure it, restore the data from my Google account, everything. I reactivated USB debugging and linked my Mi account to the OEM unlock thingy to make it future-proof (again, if you're not familiar with Xiaomi phones: you have to do that and wait a week before unlocking your bootloader). Everything worked without a glitch for two hours. I then decided to restart it to exit safe mode (I don't know why it had started into safe mode). It "looped" a bit but ended up booting into the OS, I picked it up, unlocked the screen and poof it shut down and entered a long boot loop. Of course, I tried rebooting into safe mode, to no avail

There was one other symptom. Here is a picture of the boot screen during that bootloop phase:


(Sometimes it was perfectly clean but most of the time there were artefacts like these in random locations)


The only silver lining is that the timer before I could unlock the bootloader kept counting down.

A week later, I was able to unlock the bootloader. I flashed various ROMS many times with Mi Assistant as well as fastboot mode: it worked (the flash was successful) every time but it never left the bootloop. Then the symptoms worsened: now it would boot into fastboot directly (so not really a boot loop but somehow worse) and recovery was not accessible anymore (before I would go into recovery by holding volume up and power OR by sending the fastboot reboot recovery command, none of these worked). Then the symptoms worsened even further: it won't start at all, only show a low battery sign if the battery is low. No charge icon.
I tried a new battery just in case: I measured it at 3.8 V before installing it and the phone behavior has not changed. I measured the voltage of the old (5 months old) battery and surprisingly, it shows 0 V (yes I triple checked, the low voltage protection must have activated itself).

The symptoms made me think the Snapdragon 888 needed to be reballed (it's a very common issue on the Snapdragon 888 chipsets and the symptoms go from loosing WiFi to a complete dead boot, which my phone is in right now) until I saw the battery at 0 V: could it be a simple short?


I found the schematics but I can't read them by myself, there are dozen of pages I have no idea where to start or what to look for and I would need someone to point me into the right direction.
https://secyukle.com/dosya/uploads/schematics/Xiaomi_MI_11_Schematic_Diagram.rar

You'll see there is a file called Xiaomi Mi 11 resistance reference.pdf. I don't think it shows the resistance but rather the voltage drop in mV when checking with a multimeter in diode mode and I started doing that, most of it checks out, I put the outliers I found so far in red:




And then I see that Xiaomi Mi 11 PCB MB.pdf gives us the name of the connectors (such as J5401) and we can search for them in the schematics to check the pinout but then...what? I have no idea what goes where and what I should check :'(



Thank you very much for your helpful comments.

[quince]

Sounds like bad silicon or defective solder joints somewhere. You could try re-flowing, but these phones often have potting and underfill which will self-destruct in a reflow oven. I would just replace the phone.

A previous one https://hackaday.com/2017/03/21/fix-a-brick-fighting-the-nexus-5x-bootloop/