Common reasons for a laptop to not power on at all would include:
- Failed battery (only without AC power supply)
- Failed AC power supply
- Failed motherboard component
- Damaged power socket
- Residual charge
You have eliminated residual charge being an issue by removing all power sources and holding the power button. Assuming your statement of
voltage to pins on main board
means you have tested the power from the AC adapter to the mainboard itself with a multimeter, you have likely eliminated the AC power supply and the damaged power socket from being an issue, and a failed battery is irrelevant if you've tested the machine without the battery, using just AC power.
It is still possible the AC adapter could be at fault even after testing power is getting to the mainboard, depending on what voltage is getting through (even then, some power supplies have a switching voltage making it hard to test whether it's actually faulty or not).
At this stage, the likely cause is due to a critical component failure on the motherboard. You can try some basics such as reseating any component or peripheral which can be reseated easily, such as RAM/HDD/DVD drive, although I wouldn't usually associate problems with these to the laptop not powering on at all.