Explain baking the GPU in the oven concept

My laptop is a Dell D620, which has a reputation for GPU heat problems. I baked the motherboard in the oven (GPU is integrated), it's old, I don't care if it didn't work. But it worked. Screen was full of distortion and artifacts before I did it, now it's good as new. Also cleaned out the fan, ducts, cleaned out the old thermal paste, added AS5. Runs great, temps are nice and cool.

Two questions:

1.) From what I understand, this GPU overheating loosens some of the globules of solder or whatnot down there on the motherboard causing it to malfunction, and putting it in the oven melts these connections back together? Why doesn't the overheated GPU do this by itself - is it because it doesn't get hot enough to fuse them back?

2.) How long can the oven fix expect to last? I don't game on this thing, don't push it to the limit in any way. Will the cleaning out of the fans and new thermal paste affect how long the oven fix lasts?

Technically, that's four or five questions but who cares.

Bowtie, I've read various temps and times but I went with 390-400 F (200C) for 10 minutes. Apparently don't want to go hotter or longer than that, but I've never found a Grand Authority on baking your computer parts. I've read so many people who've done this and it's worked, so I said what the heck. And it worked. Don't know how long it will last, I can't imagine it's a long-term fix but who knows.