[POCO M3 Questions & Answers] goriath:
Hi everybody,

I would like to share my experience.

3 days ago one of my family members reported her POCO M3 suddenly turned off (about 50% battery) and then never turned on again. Pressing pw button for long time, combination with VOL-/+, all 3 buttons at the same time, nothing worked. Connected to the charger neither, no charging mode; totally dead.

I tried also to link the phone to a PC with the USB cable and in windows was detected as QUSB_BULK_CID:041C_SN:AD09F802 device but the phone display stayed black, nothing else.

Xiaomi service support confirmed that the phone was out of warranty (by only 3 months, meh!)

I opened the phone and put aside the back cover with fingerprint sensor, so I measured the battery and found it rock solid at 3.8V, so my guess the battery was good. I also measured voltage and current through a USB meter during the charging and @5V the current flow sticked to 80mA, so no charging at all. I found a video on youtube that suggest to detached the battery, try to use the charger again and then reconnect the battery. I did it, I detached the battery and connected the charger, the phone powered on this time, with the icon of a battery with a bolt inside only for a while, then the display turned black again. Then I reconnected the battery and I made sure that the ribbon cable coming from the USB port to the motherboard was conncted properly, so I disconnected and reconnected it.

It worked! The phone turned on again. I performed several reboot, switched off and on again, everything as usual. So after turning it off again I was happy to reassembly the phone, but after I finished to clean it as new, as soon as I turned it on again the phone didn't show any signs of life. Turned dead again!! Although I did again everything from the start, it turned out that the battery trick didn't work anymore...

I spent a whole day with the phone teared apart on my desk, no matter how much I tried, it never turned on again. So I found this thread which I read entirely, where the main hint is to remove the tiny shield from the back of the motherboard, since the big cap (actually it's an

inductor!

) underneath, heating itself, gets bigger and goes to touch the shield causing a short...

I was very skeptic about this explanation; how could it be as such design flaw?!

I watched several youtube videos in which removing the tiny shield was involved, but those indian technicians were also removing components too. Someone replaced the big cap inductor, other removed completely the two tiny caps...why? I can't understand.

Other videos involved reflow/reball the power management IC in the next shield in the middle (this made more sense to me, but I remembered that with the battery detached the charger powered on the phone with charging icon on screen, so that must have meant something - more likely a working IC in my case), even someone reworked ARM CPU and NAND flash in the third shield. I didn't know what to try, it was overhelming.

At last I decided myself to go for the @sarabbafrani suggestion, so I picked up my new hot air SMD rework station (never used one before, first time), my smd tweezers and silicon mat. I wrapped the motherboard with a safety shield made of aluminum kitchen foil and kapton tape (oh well it's about clear green PET tape, I don't think it's real Kapton since it was very cheap although it was advertised as such; in china they call it Koptan I think), keeping out only the affected area which I flooded with two flux drops (I made this one by my self at home).

Set the temp of the station to 380°C, the air flow to 40% and mounted 5mm nozzle - I kept the tiny lid between the tweezers while lifting slightly the PCB from the silicon mat, while heating the part it took me less than 10 seconds to remove it perfectly vertical while the PCB fall down on the mat under its own weight. It came off so easy and neat, no spread of solder, perfectly clean after some IPA.

I changed thermal pads with new high performance ones, reassembled the phone, cleaned it, shutted down and restarted it so many times that I forgot...

It - does - work, simple and plain (it's finally charging right now)

So at this point I would share some dimensional findings

I took my caliper and this is what I got:

Tiny lid height is 1.5 ~ 1.55mm

Tiny lid inside quote is 1.3mm (so its thickness is 0.20 ~ 0.25mm)

Cap Inductor height (big black one) is 0.95 ~ 1mm

PCB is 0.75mm - while the tiny lid + PCB (still soldered) whole assembly is 2.3mm overall

So room between cap inductor and the top inside the lid is about 0.35mm (MIN 0.3mm / MAX 0.4mm)

Now the question for @sarabbafrani is: could really an SMD component, a cap inductor in particular, under thermal environment grow so much to hit that metal part??

I think 0.35mm is a lot of space, isn't it?

I made also a different consideration; since 0.35mm is more likely mechanically enough, but still very close to the metal lid, it could be about arcing instead of touching?

I mean, the cap inductor can't grow enough to physically touch the lid (if actually does...), but it can come close enough to establish an electrical arc between itself and the metal lid...

Think of contacts switches or relay; while closing (or opening) breakdown voltage of the contacts gap is reached and exceeded, the air between electrodes is ionized, temp raise, a voltaic arc is started.

We might be dealing here with a capacitive inductive load, stored energy in the cap inductor which acts like a trigger; does it actually make any sense?

EDIT: found the diagram about the M3 PCB and discovered that SMD component supposed to hit the metal lid it's actually an inductor NOT A CAP; L3801 on schematics!