This is the heat sink and mainboard sitting side by side

The metal components on the mainboard are PCB shields. These are soldered into place.

The thermal pads for the lower two PCB shields are thicker, and sit in a divot in the shield.

The thermal pad for the upper PCB shield are thinner, and just touch the shield instead of sitting in a divot.

The thermal pad in the middle of the board is only for extra stability.