[One (M9) General] TheXev:
I just preformed the thermal paste mod a few hours ago, mostly because I was replacing the battery and USB port as well... figured it'd be a wise thing to do since I'd have the phone open anyway.

I'd like to show you my old battery. Here are a few pictures:

Let me explain to you why I replaced the battery (this will eventually be applicable to what you want to know). I was having a TERRIBLE time getting a full charge on this phone. I'd charge it to 100%, and after awhile it was only lasting 30mins IN AIRPLANE MODE. I thought maybe the battery needed calibrated (its battery life dropped suddenly, not gradually over time), so I tired that... no luck. After seeing batteries were only around $10, and seeing USB ports around the same, decided to install all of this stuff all at once. While I was searching for a ziped firmware and Lineage for the M9, I found this thread (somehow) and decided to do the thermal paste mod as well.

So, upon disassembly of the phone to install the new battery, I was trying to use a pick to remove the old battery... but around the outside of the battery, the pick was actually squishing INTO the battery! In the top pic, the old battery is on the left, and the new battery is on the right. What I believe this back tape around the battery is some additional protection encase the batteries covering is compromised and also to give the adhesive they used to glue the battery into place, a way of giving way instead of tearing the battery apart if it affixes to the casing too well. On my old battery, there is literally a squishy square line around all the way around the outside. The pictures don't really show how pronounced it is IRL. My new battery is rock solid. I do think part of the battery leaked through on one side from where the pick squished too far in, but that battery is getting disposed of.

Now, for my thermal results:

I chose to use Thermaltake TG-7 thermal grease. I had picked this up at Best Buy for around $8 USD or something to install a water cooler into my PC. Not the greatest stuff, I'm sure, but it is better then the 10 year old tube of regular Radio Shack thermal grease I had laying around and also better then the crap HTC put into the phone.

When I pealed the shielding back, I pealed further then the OP and put grease on the chip below the CPU, and just a dab on the tiny chip just above the cpu. I was able to click the shielding back into place and secure it (there is a pic in the below link to my Onedrive on how this looked after adding grease inside, but before I applied grease outside). I did GOB the thermal grease instead of spreading it thin (like I would on a PC CPU or GPU installation), since there wasn't any additional material to seal the gaps. I wasn't worried about adding any additional copper or other weird stuff like some of you did, as I knew this HAD to work batter then the garbage thermal compound the phone came with.

I don't have specific results for stuff under load because the battery was toast and the phone had to be tethered to a wall to do anything, but I do know what the idle temps being reported in TWRP were. Before the mod, the phone was idling in TWRP @ about 50C! I use this phone in rather extreme conditions as I have unlimited old school Verizon data (meaning no speed cap after 22GBs), so this phone sits in a window and takes the suns abuse. I was not surprised that the battery was shot (I accidentally powered the phone on after I started to disassemble it, only 9% battery even though it was plugged into the wall just before!) or that the phone wasn't cooling as well as it could stock. After applying the thermal mod, I can tell holding the phone that it heats up MUCH more evenly and it idles at 20C. This is a HUGE difference from before! I don't really care how it gets under load but I can see what it is sitting at now. Checking AIDA64 thermals, the battery is currently at 36C, but I wouldn't be shocked if it was a little hotter under load, since the CPU can dissipate heat so much faster after the mod (and it heats the whole phone rather evenly now).

I topped off my phone modding for the day my ridding myself of the stock firmware (which I had flashed in an attempt to fix the battery issues), and am moving from my old daily of Cyanogen (the last stable build that was for the M9) to the May 9th 2017 build of Lineage daily. All in all, a productive few hours.

Honestly, writing this post took me longer then what it did to install the battery, usb port, and do the thermal mod. I'm off to bed! lol

Link to complete One Drive pictures. I completely forgot to take a picture of thermal grease outside of the mobo, but I applied it nearly identical to the OPs on the outside:

https://1drv.ms/a/s!At4i227gUsi-kqsKva8o8CQU4K3Asg

PS: Yes, I know the battery is in backwards in a few of those pics. I figured it out rather quickly AFTER I reinstalled the motherboard. LOL