[Galaxy Note 4 Q&A, Help & Troubleshooting] mstoynov90:
Good news, people.

I managed to fix it in the weirdest way possible.

Maybe some of you have heard of this method!

Since I accepted the fact that the mainboard might be the culprit and I had no other options left, I decided to go for it.

What followed might sound pretty scary and illogical to some, but oh well:

- made sure all residual electric charge is discharged (removed battery and hold buttons for a minute)

- disassembled the phone.

- removed the mainboard from the frame.

- removed all the modules that allow removal (front camera, rear camera, earpiece, etc)

- soaked the mainboard PCB in window cleaner

- rinsed it with warm water, thoroughly

- wiped off

- left it under the air-conditioner to dry out for about an hour

- cleaned it up thoroughly with rubbing alcohol

- let it evaporate

(here comes the scary part)

- fired up my kitchen oven to 200C on a ventilated mode and let it heat up

- wrapped the mainboard in aluminium (kitchen) foil with the shiny side out

- made two rolls of the foil and put them on the oven rack

- set the wrapped up mainboard on the rolls

- closed the oven door and left it in for 5 minutes exactly.

- at the end of the 5th minute I opened the oven door and shut the heating off.

- I left the oven fan running and the oven door open for about half an hour until the wrapped mainboard SLOWLY cooled down to room temperature.

- unwrapped it, put the modules back on, connected all the wires and screwed it in the phone

- BOOM it booted up, without a problem whatsoever. :)

- I'm a happy camper :)

So, to summarize and explain -

This method is called reflowing. What it does is that it slightly melts the solder connections inside the PCB, and resolves microscopic cracks and bad connections, which might have been caused by extreme heat or cold.

(which turns out to be one of the main reasons for such bootloop issues, as many people just wake up to a looping phone, after it has overheated in the warm bed)

Professional electronic engineers have actual reflowing ovens that bring the PCB inside through some very specific temperatures, way more accurately than a kitchen oven, so it produces way more reliable results.

Of course 200C is pretty hot, so any plastic parts that can be removed, SHOULD be removed, or they might melt and you really don't want that. Good example for this could be the front camera, which while probably having a glass lens, it has a plastic housing which will probably go soft and render the camera useless, if not break it completely.

Reflowing should not be done to components you suspect might have just a software problem.

It is pretty much a russian roulette if you're gonna get anything out of the oven, that is not baked and dead.

I might have been lucky, and I probably am.

Therefore I want to say this out and loud:

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THIS IS NOT A SOLUTION OR A REPAIR GUIDE BY ANY MEANS. I DO NOT SUGGEST IT TO ANYONE, I AM JUST SHARING MY PROCEDURE.

THERE'S NO GUARANTEE THAT YOU'RE NOT JUST KILLING IT COMPLETELY BY ATTEMPTING A REFLOW.

YOU SHOULD ATTEMPT THIS ONLY IF THERE'S NOTHING ELSE TO BE DONE AND YOU'RE PLANNING TO THROW IT AWAY.

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So... now for the reflowing enthusiasts:

-aluminium foil wrap is there to spread the heat evenly and protect the components from direct heating

-aluminium foil rolls are there to keep it off the metal rack so it doesn't get a massive heat transfer from its contact with the rack.

-water/alcohol/windowcleaner rinsing is fine for electronics as long as they're not powered. (water leftovers can cause oxidation of metallic components, so make sure it dries out well)

-make sure the board is completely, completely, completely dry, before plugging it back in. Water drops could settle in the metallic hollow chip guards/heatsinks (i suppose that's what they are)

-do not just remove it from the hot oven, as you may just reintroduce new micro cracks by subjecting it to a sudden temperature change. Let it cool off on its own first.

-be careful as some components on some PCBs can emit some pretty poisonous fumes. Ventilate the area and give your oven a good run after that, to make sure it's all gone, before you decide to cook in it again.

- and MOST IMPORTANTLY - if you have any suspicion it might be a software issue, do not bother with this destructive method. You might break it even further or even kill it off, completely.

DISCLAIMER - I take no responsibility if you decide to go the reflowing road.

Any questions are welcome!