One way a LED TV dies.

[Housedad]

My daughters 55" LG 4KTV died one month after the extended warranty and of course, she brought it to me.

Turns on, no signal, backlight works, blue screen of death, no sound.  no lines.

Testing:  Power board works as designed. putting out the proper 190v and 13.2v DC.  No AC or ripple.  Motherboard getting full power power lights on board functioning without flashes,  All SMD caps. diodes and trans check for function and ESR if possible.

No shorts on the T-con board, Power working fine.  All components checked and good.  LCD board strip at bottom of display all good condition, parts check out, signal traces all clean and tested for continuity.

Now I started figuring it might be a solder point.  Started doing tons of traces on the motherboard, all OK.  Of course I end up saving the CPU for last.   

Pull the heat sink and lookie what I found below. The tape was so damn thin it barely touched the IC around the periphery.  Barely thicker than scotch tape.

Shoddy manufacturing?  Planned obsolescence? Some Chinese worker way overworked?  Bean counter saving coin in Korea? God knows.

I suspected after that a bad solder joint under the BGA and since it was dead anyway, I went over to a friend that had a rework station, added flux and did a reflow profile and seated the heatsink back with tape that was thick enough to fill the gaps and applied enough pressure to make if conform.

For right now, the miracle occurred: All is well in LG land. 
We'll see how long it lasts.  I'm not holding my breath.

[shakalnokturn]

Thin adhesive is not necessarily a bad thing, thin means less thermal resistance (if both surfaces are flat enough in the first place), in your case it does look like thicker is needed...
Did this BGA have a soft thermal pad between PCB and aluminum frame on the underside?
Pressing on BGA's during power-up has become one of my routine checks in flat TV troubleshooting...

[PKTKS]

IMHO These adhesives are crappy stuff.

I have found half dozens MOBOs with those adhesives just
spread underneath the heat sink  as if they had melted.

one MOBO rendered useless as the adhesive promoted
corrosion on the already crappy LGA damn small pins

when the adhesive melts and falls inside the LGA array
they turn that crappy thing a hell to clean..

RELIABLE THERMAL PASTE I NEVER *EVER*  HAD SUCH ISSUES.
Not even garbage old pastes...

2 cents.
Paul