Disclaimer: I'm sort of a noob, and this is a long post. Thank you all in advance!

summary: completely dead laptop with no signs of life whatsoever (suddenly, for no apparent reason)

Here's the deal: Lenovo Y470 (only a few months old with no water or shock damage). It stopped working suddenly (no lights, no sound, even when connecting adapter with or without battery).

I tried a different adapter (same electrical rating), but no luck.

I disassembled the thing completely, and tried plugging in the adapter and looking for signs of life with all different combinations of components installed (tried all combinations of RAM, CPU, USB power cords, screen, etc plugged in).

no luck. Then, I noticed (as I was plugging in the adapter to try for the millionth time) that there was a "spark" for an instant when I first connect the adapter to the power jack. The adapter's LED would then flash (indicating it isn't working or charging). So, I thought the power jack has a short of some sort (due to bad soldering or something). Scanned virtually every single component on the motherboard, and tested the power jack connections with a multimeter. No shorts or damage to anything on the entire motherboard.

Now I'm thinking I need to replace the motherboard.

But, my actual question: What does this "shorting" when connecting the adapter signify? (btw, the voltage across the power connections and current through it drop to virtually zero when the adapter is connected and "sparks", and they stay that way). The bewildering thing is that there are no damaged components, and the voltage across adapter terminals returns to normal after I disconnect it (so it's not damaged).

Please take a look at the pictures (of the motherboard's power connection and nearby components) and see if I'm missing something completely obvious...

Links to pictures and laptop and motherboard model:

pictures on DropBox

Motherboard model: LA-6881P

Laptop model: Lenovo IdeaPad Y470