How do I fix motherboard issue not powering on

My friend has dell n5050 model laptop that doesn't power on. I did alot of troubleshootings like:

1) battery removed unplugged keeping power button pressed for 1min to reset.

2) I dismantled the laptop to check any part burnt and i tested the board with multimeter each and every component to see any voltage shortage and any part blown the multimeter shows all components fine and correct voltage coming.

3) I heated the BGA chipset to check whether video chip error and I Removed the bios chipset.

4)tried re-installing the bios firmware of this model using programmer device and checked still not working.

I would like to know what part might be the issue so I can solder and replace it?