[Nexus 5X Q&A, Help & Troubleshooting] Voicebox:
I'll tell you my experience, which isn't a direct response, but may help. My phone was dead and I tried an ice bath and putting in the freezer first, which would get it to actually start booting, but die quickly. However, I could get into recovery and ADB the OTA, which it hadn't done before the cooling.

So I tried a hair dryer on the motherboard since I didn't have a heat gun. That got it to start booting at room temperature, but it would stop at the very soon after the 4 colored circles started. So I bought a heat gun. It didn't include nozzle attachments so I was doing a blanket heating of the board. Though I did mask off the top of the board where the front facing proximity sensor is and some tiny chips or something, with foil. No one in the videos does that, but that sensor looks like it has some glass or clear plastic facing out, and I think the "chips" started rounding due to the heat. Basically they have sharp black squares on top and I noticed the edges in certain directions were rounded as if they started melting. I can not rule out that it had happened from the CPU or other chip heat itself as I didn't take a picture to begin with. Though I did take some later. I had just masked off the proximity sensor before heating, then after noticing the possible melting, moved the foil to cover more.

On the low setting, which is 750 degrees F/ 400 C, I did 2 minutes. The phone then booted all the way until I could unlock it. Then as soon as I swiped left, it froze, then died. Started it up again and it froze right after unlocking.

So I tried a mix of the high temp on the heat gun which is 1,000 degrees F, with lower temp for 2 minutes. I think I did 30 secs high and 1:30 lower. It then booted all the way and I pulled all my data off that I hadn't taken off before.

At some point it died. Somewhere along the way I did a cache wipe, factory reset, and loaded from scratch. It then died while I was a good way through restoring it. Can not recall if that was before I tried high temp for 2 minutes, but after doing full temp for 2 minutes, the phone acted normally. Since I had wiped it, I restored everything that I had backups for and put my SIM back in there and thought it was fixed. But after 9 hours of acting completely normally, it rebooted. Came back normal and then some time later it rebooted again. Then the cycle kept shortening up to a legit bootloop in a very short time. I mean like an hour after the first reboot. Going back at it with the heat gun again, and then again with also pressing down on the CPU with a wooden spoon didn't bring it back to life after that. It just bootlooped.

The connections are physically underneath the CPU. So it requires the heat to get to those connections. I do not know if there's enough room for the air to sneak under there or if the heat is transferring through the chip material itself. So although the melting temperature of the solder is likely way lower than what my heat gun puts out, it is not directly being heated by the air. If your setup is more precise than a paint stripping type hardware store heatgun that I have, or you're going to try an oven, it should take a lot lower temperatures. Since I did not have a nozzle that more precisely directs the airflow, I was constantly moving it around and have no idea what the effective temperature on the solder was.