The older HP's used to have a problem with the graphics chip connection. For connection, read a bazillion solder points. Re-balling was a black art. Putting the motherboard in an oven was one last ditch possibility, having leapt over a lot of last ditches.
The Rivetess' HP was dead. I did some of the suggested things and on a quick rebuild it ran the boot sequence. Hooray. On the detailed rebuild, it was dead again.
Back in the UK one of my old neighbours fettles such kit. He'd had some success with cutting Coke tins as shields and using a hot air gun on the chip.
The writer takes no responsibility for thermally-distorted and otherwise molecularly altered hardware.